Heathkit HW-2036 VHF Transceiver Restoration - Part 3

 

In brief: This post is the final part of my 3-part restoration series on a Heathkit HW-2036A, a PLL-controlled, 10-watt, 2-meter FM transceiver from 1976 that I rescued from a Montreal flea market. In this part, I focus on restoring and aligning the receiver, improving its sensitivity from -105 dBm to -116 dBm, performing FM quieting measurements, replacing aging tantalum capacitors, and sharing a number of practical alignment tips that may also be useful when restoring other vintage FM transceivers.

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Aligning the VCO

After sorting out the problems on the transmitter side, I could finally turn my attention to the receiver section on the top side of the unit. As I mentioned earlier, the receiver was quite deaf. To put things into perspective, on my KDK VHF handheld from the same era, a -121 dBm signal can still be heard from the speaker, albeit barely. On this unit, however, anything weaker than -105 dBm simply disappeared into the noise. Naturally, I tried to fix the problem by tweaking the coils in the receiver section (L201 through L210). And, of course, because I took the shortcut, I ended up disappointed. Before taking the time to think things through, I actually made the situation even worse. :) Fortunately, I came to my senses afterward. Once I patiently followed the alignment procedure in the service manual, everything fell back into place.

Top view of the circuitry: The VCO is housed in its own shielded enclosure at the lower left, while the BCD switches are located at the lower right. The receiver board is at the upper left, and the synthesizer board is at the upper right.

Left: Synthesizer board. Right: Receiver board and component layout.

Since the service manual describes these adjustments in detail, I don't see any need to repeat them here. I'll just cover the general procedure and a few useful tips.

Receiver alignment begins with adjusting the VCO. Actually, I didn't want to touch the VCO because the transmitter frequency was already spot on (as you may recall, in this radio the transmitter frequency is the sixth harmonic of the VCO frequency). Still, I went ahead with the adjustment so I could honestly say I had followed every step in the manual. The adjustment is made by accessing the C511 trimmer capacitor and the L501 coil through the holes on top of the VCO shield. Interestingly, it's much easier to make this adjustment by monitoring the voltage at TP401 than by measuring the output frequency with a frequency counter. You'll need either an oscilloscope or a voltmeter with a 10 MΩ input impedance. The adjustment is performed once in receive mode, and then again in transmit mode by pressing the PTT.

Once the VCO is aligned, select your test frequency (the middle of the band is fine, for example 146 MHz) and move on to the receiver section. The antenna input should be left open, and the squelch should be fully open as well (with the volume turned all the way down, of course).

Aligning the Receiver Front End

Here's something that made me admire Heathkit even more. Realizing that not every amateur building this kit at home would have an oscilloscope or precision test equipment, Heathkit came up with a clever solution: it allows you to use the radio's own S-meter as a simple measuring instrument. To make this possible, the manual has you build a small RF probe using a few extra parts that were included with the kit. I decided to follow that method out of curiosity, and I really enjoyed it. Even so, I later verified the results with my oscilloscope and did a bit of fine-tuning with my own multimeter.

The RF probe as shown in the assembly manual

My version of the RF probe. 🙂

Once the probe is ready, unplug the connector from the P socket on the receiver board and connect the probe output in its place. Your S-meter has now become a measuring instrument. Next, back the cores of all the coils on both the receiver and transmitter boards all the way out (flush with the top). Then touch the probe tip to the C socket on the synthesizer board and adjust coils L402 and L403 for the maximum meter deflection. This sets the signal level going from the synthesizer to the receiver board.

The next step is to touch the probe tip to the G2 terminal of transistor Q202, then adjust coils L212 and L213 (the input filter) for the highest signal level. At this point, the RF probe has done its job. Reconnect the plugs you removed, and the front-panel microammeter becomes an S-meter once again. Now connect your signal generator to the antenna input (I started with a signal level of -50 dBm) and adjust coils L201 through L210 for the strongest indication. Each time the meter reaches full scale, reduce the signal level. Using this method, I managed to achieve enough sensitivity to hear the 1 kHz modulation at -116 dBm. The squelch started to open at -118 dBm.

Final IF and RF Alignment, a few Practical Notes

As I mentioned earlier, every detail of these adjustments is already covered in the assembly manual. I'm only giving a brief overview here. However, it's worth sharing a few practical notes I made along the way, since they can be applied to almost any FM receiver.

• During the first two steps, it's essential to check and optimize all the voltages with a modern high-impedance voltmeter. As enjoyable as it is to use the radio itself for its own alignment and watch the analog meter respond, a digital instrument lets you squeeze out an extra 0.1 mV here and 0.2 mV there, which adds up to a noticeable improvement. After all, the signal arriving at the receiver input is only in the microvolt range. The easiest way to do this is to connect a digital multimeter to the S-meter connection point (P socket) on the receiver board while adjusting L201 through L210. Following the same idea, you can also connect the RF probe to a digital multimeter instead of using the built-in meter. I tried every approach, and in my opinion the best method is to first follow Heathkit's procedure exactly as described, then use more advanced test equipment for fine-tuning and squeezing the last bit of performance out of the receiver.

• A coil can sometimes have two peak points: one with the ferrite slug near the bottom, and another with it much higher up. In such cases, Heathkit recommends using the peak with the slug closer to the circuit board.

• It's also important to remember that the ferrite slugs are fragile and brittle, and that the correct alignment tool should always be used (a fiberglass tool with a ceramic or other insulating tip that properly fits the slug). If a slug is difficult to turn, it can be backed out slowly and a small amount of PTFE grease applied. Despite being careful, I still managed to crack the slug in L209. Fortunately, I happened to have an identical replacement. Otherwise, I would have ended up having to wind a replacement transformer myself—a job I certainly didn't want to take on. Thankfully, I got away with only a minor mishap.

Coil L209 with the broken ferrite slug. Getting the broken pieces out was a challenge in itself!

On the right are the replacement ferrite slugs I bought about 15 years ago from the Onur 4 Business Center in Çankaya, İzmir. Funny how things work out—I never imagined they'd come in handy for this project. On my last visit, I was saddened to see that almost all of the shops were on the verge of closing.

• It's best to know from the outset that the alignment won't be completed in a single pass. The receiver reaches its best performance only after repeating the entire alignment procedure seven or eight times.

• Since the receiver and synthesizer boards, unlike the VCO, are not enclosed in RF-shielded compartments, I performed the final adjustments with the radio inside a "cover" I made from a disposable aluminum baking tray. I simply made a copy of the radio's original top cover out of the aluminum sheet, drilled small holes where the alignment coils were located, and then screwed it in place just like the original cover.

A disposable aluminum baking tray, waiting to be shaped.

The temporary cover installed in place and screwed, just like the real one

On the top, holes were drilled directly above the alignment coils using a full-scale (1:1) copy of the board layout as a template.

FM quieting test at different frequencies, with the temporary cover installed.

FM Quieting Test

Once the alignment is complete, it's a good idea to connect the radio to an antenna and spend some time listening to on-air stations. If possible, ask another amateur to help you evaluate the audio quality. On this radio, the audio can be improved even further by making small adjustments to the receive offset oscillator (using trimmer capacitor C438) and coil L210. I didn't spend much time tweaking them because I was already satisfied with the results.

Another test I performed to make sure I had achieved the performance I was after was the FM quieting test. As you may remember, one of FM's advantages is its ability to suppress background noise when a signal is present on the tuned frequency, effectively "quieting" the receiver so that only the desired audio comes through the speaker. The amount of quieting produced by a given signal level is, in fact, another measure of receiver sensitivity, and manufacturers usually specify this figure. Heathkit, for example, states that the HW-2036 provides 15 dB of quieting with a 0.5 µV signal at the antenna input.

Measuring this is quite straightforward. Disconnect the speaker from the audio output and connect a suitable load, such as 4 Ω or 8 Ω, in its place. Then monitor the audio level with a measuring instrument while applying an RF signal to the antenna input, and observe how many decibels the audio noise drops. At the moment, when I apply the specified 0.5 µV signal (-113 dBm) to the antenna input, I obtain 18–20 dB of quieting, which means the receiver is performing considerably better than Heathkit's original specification.

There is one important thing to keep in mind with this radio, though. Its sensitivity is not uniform across the entire 144–148 MHz band. That's why I keep referring to the alignment frequency. Unlike more modern receivers, which maintain nearly constant sensitivity over a wide frequency range, this design gradually loses sensitivity above and below the alignment point—roughly beyond ±500 kHz. It's therefore worth keeping this in mind when choosing your alignment frequency.

Replacing Aging -and Suspect- Components

Another thing I did was replace transistor Q204 and diode D201, since I suspected the S-meter wasn't behaving quite as it should. In fact, once the receiver had been properly aligned, the S-meter worked perfectly, but I wanted to be certain. Q204 was an MPF105, and I replaced it with an MPF102, whose characteristics are very similar and which I was able to obtain from a local supplier. I also substituted a 1N4148 for the original 1N4149 at D201.

More importantly, since I already had the receiver board out, I removed the synthesizer board as well and replaced every tantalum capacitor.

As you know, after forty or fifty years, tantalum capacitors have a tendency to develop leakage. I replaced all of them, mainly to ensure that the PLL loop would remain completely trouble-free. Modern multilayer ceramic capacitors would probably have worked just as well, but I preferred to stay faithful to the original design.

After replacing all of the tantalum capacitors (the yellow, chunky ones).

The last job—and one I consider purely cosmetic—was replacing the burned-out meter lamp. I could have used a yellow LED, but since power consumption isn't exactly a concern with this radio, and because I wanted to keep the restoration as faithful to the original as possible, I replaced it with the correct lamp instead (Grain of Wheat 11 × 470).

Final Thoughts

As I mentioned in the first article, completing all of these repairs and modifications took about four months. During that time, I often had to stop and spend some time reading and researching topics I wanted to understand better. For example, I learned a great deal about how PLLs work, and I'm very glad I did. I've never hesitated to admit that things I simply buy ready-made give me only limited satisfaction, and that feeling fades quickly. Restoring the HW-2036, on the other hand, turned out to be an immensely satisfying project. It also added an interesting piece of equipment to my station—one with historical value, a great story behind it, and one that now performs beautifully.

I also documented the entire restoration in a four-part video series on my YouTube channel. You can watch it there if you're interested.

As always, if you have any questions, feel free to ask.

FAQ

How sensitive is the Heathkit HW-2036A receiver after alignment?

After carefully aligning the receiver, mine achieved a usable sensitivity of about -116 dBm. The squelch began opening at approximately -118 dBm, which is significantly better than the radio's original published specification.

Can the Heathkit HW-2036A be aligned without professional RF test equipment?

To a large extent, yes. Heathkit designed the radio so that its built-in S-meter can be used as a basic measuring instrument with a simple RF probe assembled from parts supplied with the original kit. However, a modern oscilloscope, digital multimeter, and RF signal generator make fine adjustment easier and allow better overall performance.

Why doesn't the receiver have the same sensitivity across the entire 144–148 MHz band?

Unlike modern synthesized transceivers, the HW-2036A is optimized around a single alignment frequency. Sensitivity gradually decreases as you move away from that frequency, so it's important to choose an alignment point that best suits your intended operating range.

Should the original tantalum capacitors be replaced during restoration?

In my opinion, yes. After nearly 50 years, tantalum capacitors can develop leakage that may affect circuit stability, particularly in the PLL synthesizer. Replacing them is inexpensive preventive maintenance that can improve long-term reliability.

What was the most challenging part of the restoration?

The receiver alignment itself was straightforward once I followed the service manual carefully. The biggest challenge was working with the fragile ferrite slugs in the alignment coils. One of them cracked during adjustment, and replacing it required finding a compatible spare.

Links:

My videos on the restoration of HW-2036, part 1: Click here to watch

My videos on the restoration of HW-2036, part 2: Click here to watch

My videos on the restoration of HW-2036, part 3: Click here to watch

My videos on the restoration of HW-2036, part 4: Click here to watch

Robert Sumption's video on the general modifications in HW-2036: Click here to watch

Heathkit 2036 Manual courtesy of W5RKL: Click here to download

Heathkit 2036 Manual (includes schematics) courtesy of Vintage Radio Info : Click here to download

Heathkit HW-2036 VHF Transceiver Restoration - Part 2

In brief: This post is the second of a 3-part series about the restoration of a Heathkit HW-2036A, a PLL-controlled, 10-watt, 2-meter FM transceiver from 1976 that I picked up for almost nothing at a flea market in Montreal. In this part, I cover the problems I found after bringing the unit home—from a stuck screw to low output power, a faulty DTMF keypad in the Micoder microphone, and unconfigured CTCSS tones—and how I diagnosed and fixed each one on the transmitter side. The next part will cover the receiver repairs.

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The bottom side of Heathkit HW-2036, while working on the receiver board.

Initial Inspection and Findings

In my previous post I had covered the features of the HW-2036 in general terms. Therefore, I will talk directly about the unit I bought from the flea market. After bringing the radio home, I first gathered the relevant documentation on the Internet. Thankfully, accessing detailed information about the HW-2036 was extremely easy. In addition to Heathkit's carefully prepared assembly and alignment manual, schematic diagram, and service bulletins, there were web pages or videos from amateurs who had previously worked on this unit. I collected these and noted down the information I personally found important.

Then, at a suitable time, I examined the unit. First, I looked for signs that could indicate other problems, such as impact marks or corrosion. I then opened the bottom section of the case and inspected the circuit board and components. I wanted to do the same on the top side as well, but I couldn't because one of the screws at the back was spinning freely. I then applied power and observed the unit's operation. My initial findings were as follows:

1. Since the case could not be fully opened, it was not possible to inspect the receiver and synthesizer boards. The first problem to be solved was this screw.
2. The unit was transmitting and receiving in both simplex and duplex modes, and there were no issues with the frequencies.
3. The transmit output power was far below what it should have been.
4. When listening from another radio, my voice sounded muffled as if coming from the bottom of a well. In addition, a strange tone was heard, resembling the noise of a discharging capacitor (reminding me of simple siren circuits).
5. The receiver was somewhat deaf. The squelch was opening at around -95 dBm, and it was practically "not hearing" signals weaker than -105 dBm.
6. The S-meter was not working, and the illumination lamp had burned out.

Removing the Stuck Screw

Based on these findings, the first thing I tackled was the freely spinning rear screw. In order to remove it with a screw extractor bit, I first secured it with epoxy adhesive. Then I drilled a small cavity in the screw head with a drill, and finally removed it from its place with the screw extractor bit. This allowed me to access the circuit boards inside. Meanwhile, since I didn't want to replace just a single screw, I sourced screws of the appropriate diameter and length ("6#-32" in American measurements) and later replaced all the top case screws with new ones. When I removed the screw, I saw that the problem originated from the standoff inside—it had been spinning freely because it was not fully tightened from the other end. I applied a drop of super glue and retightened it.

Heathkit HW-2036A's back side and the screw I had to 
immobilize with epoxy glue before drilling.

Heathkit HW-2036A's back side - the bad screw is ready for extraction.

Transmitter Board Overview

Once the inside of the unit became fully visible, I examined the circuit boards and traced the circuit paths according to the schematic diagram, noting down the reference voltages marked on the schematic and comparing them with my own measurements. Since I did not see any major deviation in the values, I concluded that the components were generally in good condition. I also did not see any burned, deformed, or otherwise damaged parts.

After all this, I first focused on the board on the bottom side of the unit. This board contained the DC power input, audio (microphone) input and its amplifier, the transmitter, the 10 MHz reference oscillator, and the tone oscillator. The work I did regarding these circuits was as follows:

• First, I readjusted the 10 MHz reference oscillator (there was a drift of about 200 Hz). For this, we connect the frequency counter to test point TP108 and use trimmer capacitor C144.
• I replaced the electrolytic capacitors at the DC input (after 40 years of service, it was time for the old ones to retire).

Heathkit HW-2036A Transmitter Board. 
The bad screw is the one at top-right of the picture.

Heathkit HW-2036A Transmitter Board - After replacing
the electrolytical capacitors.

Layout of the board: The section with 5 ICs at the top left is the 10 MHz oscillator,
below it is the tone oscillator (and its trimmer resistors), and at the bottom left is
the microphone amplifier. The right side of the board is the transmitter.
The center is DC power section.

Microphone Repair (Micoder-1)

• I opened up the microphone (Micoder version 1) and ran some tests, and I found that when I cut the supply to the DTMF circuit inside, both that strange tone disappeared and I could hear my own voice normally from the other end. I removed the DTMF circuit and ran it independently. I took measurements and compared the results on its schematic with the reference values. During my tests, I realized that this circuit was producing tones correctly, but the flexible keypad used to command the circuit had gone bad. Because the bottom two rows of the keypad were in constant contact, the circuit was producing a corrupted signal when PTT was pressed. Since I had no chance of finding another copy of the same keypad, and I also wanted to preserve the original appearance of the microphone, I reinstalled the DTMF circuit with its supply point inside the microphone disconnected. We don't really need DTMF tones much these days anyway. No repeater is connected to telephone lines anymore; only repeater trustees use these tones to manage the equipment at the repeater site. So there was no need to insist on the repair either. I closed the microphone with the microphone capsule powered.

Working on the microphone (Heathkit Micoder-1). The tone circuit works normally when separated from the keypad and the contacts are shorted by hand. Unfortunately, finding a new or used copy of the same keypad is very difficult.

Heathkit Micoder-1 Microphone. The bottom two rows 
of the keypad you see here are short circuited. The
structure of the keypad does not allow repairs.

Powering the Microphone Capsule Without a Battery

• I did not want to use a battery to power the microphone capsule. I barely talk for 5 minutes once a month anyway, if at all. For that reason, I also didn't like the idea of leaving a battery that could leak inside the microphone. So, as suggested by W9RAS, I made the shield of the microphone cable the negative conductor and soldered it to a chassis point on the unit. I then connected the negative conductor in the cable to a 12V point on the unit, thus providing the required power to the capsule.

The black wire soldered to the slide switch inside the microphone now carries 12V;
the other end of this wire is soldered to the blue wire at the top left corner of the photo.

Transmitter Output Alignment

• After all this work, I readjusted the output stage to bring the output power back to 10W. Since the manual provides these adjustments in detail, I don't feel the need to repeat them; I will only give the main outlines and the key points below. First, I disconnected plug J102 going to the final amplifier at the output and connected a 50 Ohm dummy load, then measured the voltage at the sampling points from TP101 to TP104 and readjusted the adjustable coils from L101 to L107 (I also did this adjustment by looking at the signal amplitude at the final stage on the oscilloscope, but doing it with a voltmeter with high input impedance (10 MΩ) is more practical). Here, L105-L106-L107 in particular make things take a bit longer since they affect each other. Fortunately, Heathkit's manual explains in detail how all these adjustments are to be made... In the end, I reached 10 Watts of power at the transmitter's set frequency (147.010). Since 10 Watts is the specified power for this radio, I did not tamper with the output stage to try to get more.

CTCSS Tone Configuration

• After making sure the transmitter was working properly, I wanted to transmit with a tone to access the repeaters in my area. The tone circuit has 3 "legs" set up to produce 3 different tones, and each leg has miniature trimmer resistors that need to be adjusted for tone setting... However, I noticed that no signal was coming on any of the 3 tone options. After some troubleshooting, I realized that all of the fine-tuning potentiometers used to adjust the tones were in the "zero" position. The late amateur who assembled the unit had probably never needed a tone. As soon as the resistor settings were changed, I started seeing the tone on the frequency counter. I adjusted the 3 tones that would be most needed for me.

Cleaning Oxidized Contacts

• Let me also mention one procedure I performed not only on the transmitter board, but on all three boards. I lightly scraped the legs of all integrated circuits and the metal (contact) surfaces of all connectors with a utility knife, cleaning off the layers that had formed.

Notice the legs of the integrated circuit—there is some oxidation, even if slight.

I'm covering it all on a single page here, but what I described above took approximately from August to October. After taking care of the transmitter, it was the receiver's turn. You can read the rest of the article [here]

Also: I have explained the details of this restoration in 4 parts on my Youtube channel as well, you can watch it here.

Frequently Asked Questions

What are the common problems found in a used Heathkit HW-2036?

Common problems include low transmit output power, a deaf receiver with poor squelch sensitivity, a non-functioning S-meter, burned-out illumination lamps, muffled audio, and aged electrolytic capacitors. Mechanical issues such as freely spinning screws caused by improperly tightened standoffs can also prevent access to internal boards.

What screw size does the Heathkit HW-2036 top case use?

The top case screws are 6#-32 (American standard thread size).

How do you adjust the 10 MHz reference oscillator on the HW-2036?

Connect a frequency counter to test point TP108 and adjust trimmer capacitor C144 until the oscillator reads the correct frequency.

How do you adjust the HW-2036 transmitter output to 10 Watts?

Disconnect plug J102 going to the final amplifier and connect a 50 Ohm dummy load. Measure voltage at sampling points TP101 through TP104 and adjust coils L101 through L107 accordingly. A high-impedance voltmeter (10 MΩ) is more practical than an oscilloscope for this. Note that L105, L106, and L107 affect each other, so the adjustment requires patience.

What is a possible issue with the Micoder version 1 microphone on the HW-2036?

In my case, the DTMF circuit inside the microphone had a deteriorated flexible keypad, causing the bottom two rows to remain in constant contact. This produced a corrupted signal and a strange tone when PTT was pressed, also making the transmitted audio sound muffled. Disconnecting the DTMF circuit's supply resolved the issue. Since DTMF tones are rarely needed today, the circuit was left disconnected while preserving the microphone's original appearance. 

How can you power the Micoder microphone capsule without a battery? 

As suggested by W9RAS, the microphone cable shield can be repurposed as the negative conductor and soldered to a chassis point on the unit, while the original negative conductor in the cable is connected to a 12V point on the unit to supply power to the capsule. 

Why might the CTCSS tones not work on the HW-2036? 

The tone circuit has three legs, each with miniature trimmer resistors for tone adjustment. If the unit was originally assembled without configuring these tones, the trimmer potentiometers may be left at the zero position, producing no tone output. Adjusting these trimmers while monitoring with a frequency counter will restore tone functionality.

Links:

My videos on the restoration of HW-2036, part 1: Click here to watch

My videos on the restoration of HW-2036, part 2: Click here to watch

My videos on the restoration of HW-2036, part 3: Click here to watch

My videos on the restoration of HW-2036, part 4: Click here to watch

Robert Sumption's video on the general modifications in HW-2036: Click here to watch

Heathkit 2036 Manual courtesy of W5RKL: Click here to download

Heathkit 2036 Manual (includes schematics) courtesy of Vintage Radio Info : Click here to download

Heathkit HW-2036 VHF Transceiver Restoration - Part 1

In brief: This post is the first of a 3-part series about the restoration of a Heathkit HW-2036A, a PLL-controlled, 10-watt, 2-meter FM transceiver from 1976 that I picked up for almost nothing at a flea market in Montreal. In this part, I cover the history of Heathkit's VHF radio lineup and how the HW-2036A works under the hood. The following parts will go into the problems I found and how I fixed them. 

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Heathkit 2036 VHF Transceiver after the restoration

How I Found the Heathkit HW-2036A at a Montreal Flea Market

One of the active clubs in Montreal, WIARC, organizes a small flea market every fall where usually 15-20 people set up tables. The previous year I had arrived quite late and walked in almost at closing time. I spotted this unit on one of the tables there (in fact, two of them were stacked on top of each other). Partly because the seller was eager to pack up his boxes and leave, I bought it for what I'd call pocket change and tossed it in my bag. Let me be frank, I had bought it only because I was curious about its construction, and I might not have bought it had the selling amateur not offered such a low price. Good thing I did! As you'll read, it turned into a project that was both very enjoyable and very educational, spanning 4 months. 

Heathkit's 2-Meter VHF Radio Lineup: From HW-202 to VF-7401

Now let's get to know the unit a little (I don't feel the need to talk about the Heathkit company again, I had written about it at length before, you can read it in that post). The HW-2036 is one of the mobile VHF radios that Heathkit introduced in the late 1970s. Heath had released a series of radios for the 2 Meter band starting from the early 70s. The first of these was the crystal-controlled, 6-channel HW-202 (1973). It was followed by the synthesized HW-2026 (1975), but that turned out to be such a problematic design — especially in terms of spurious emissions — that for the first time in the company's history, a product was recalled from the market. The HW-2036, which came after it, was in a way the result of lessons learned from the 2026 (1976). Finally, the VF-7401 came out (1980). The 7401 was a more advanced unit featuring an LED display and scanning capability. Let me also note that all of these were sold as kits. Apart from using the same enclosure, there are also common points in their circuit designs, which you can see when you look at the boards. Don't assume they were cheap just because they were kits, either. In 1978, an HW-2036A was 269 US Dollars; the equivalent of that amount today is $1,911! 

The HW-2036 is the most common among these kit radios. There is also an HW-2036A version. The difference is that the first one covers 2 MHz of bandwidth on the 2 Meter band, while the second covers 4 MHz. However, so that those who had jumped in early and bought the HW-2036 wouldn't feel left out, Heathkit also released a modification kit — you could replace certain parts and convert your unit to the A version. The one I have is one of those units with this modification applied. According to the label on the bottom, the unit was completed on April 17, 1977. These labels came as part of the kits and carry the statement "I hereby declare that this unit I have completed complies with FCC standards." The amateur who finished building it would sign the label and stick it to the bottom of the enclosure. Therefore, each unit has its own serial number. 

Heathkit HW-202 - Crystal Controlled (1973)

Heathkit HW-202 from Heathkit 1978 Catalog

Heathkit 2026 - Synthesized (1975) 

The recall letter from Heathkit's president to radio amateurs. Heathkit entirely refunds you
 and pays your postage fee if you prefer to return your HW-2026. Additionally, you get
a gift certificate of 50 USD if you already finished the assembly,
if not, you get a 25 USD certificate.

Heathkit HW-2036 (1975)

Heathkit HW-2036 in 1978 Heathkit Catalog

Heathkit VF-7401 (1980) with digital display

HW-2036A Technical Specifications

Now let's take a look at the technical specifications of the HW-2036A: 

Operating range: 144 - 148 MHz 

Sensitivity: 15 dB quieting with -113 dBm (0.5 uV) signal 

AF output power: 1.5 Watts 

RF output power: 10 Watts Modulation: 

FM, adjustable 0 - 7.5 kHz 

Duty Cycle: 100% (at infinite VSWR) 

Tones: 3 adjustable tones between 70 - 200 kHz 

Transmit offset: Crystal controlled, 3 options: +600 kHz, -600 kHz, extra (left empty for adding a crystal with the desired frequency offset later) 

Heathkit HW-2036 Block Diagram

How the HW-2036A PLL and VCO Circuit Works

The heart of this transceiver is a voltage controlled oscillator (VCO). The VCO is also part of the phase locked loop (PLL) that determines the frequency the unit will operate on and keeps it on that frequency. You set the desired operating frequency using three mechanical BCD (binary to decimal) switches on the front panel. There is also a switch that sets the last digit to either 0 or 5 (Hz). A phase comparator divides the signal coming from the VCO by this number and compares the result with the signal coming from a reference oscillator, and if there is a phase difference, it corrects it by changing the voltage on the varicap diode in the VCO. The signal coming from the VCO is one sixth of the desired actual operating frequency, and the signal is multiplied by six in both the receiver and the transmitter. In the transmitter, the power is boosted to 10 W at the final output stage. The receiver mixes the signal arriving at the antenna with the sixth harmonic of the signal from the VCO, passes the resulting 10.7 megahertz intermediate frequency signal through an eight-pole crystal filter, amplifies it and then converts it to a 455 kHz intermediate frequency. This signal is separated by the detector and sent to the audio frequency amplifier to be amplified. As we can see, it may seem very simple compared to today's microprocessor-controlled radios, but it is by no means a "simple" unit (I'm saying this excluding software defined radio technology). What we have here is a PLL-controlled radio that can still do its job 40 years after its design... 

In the next part, we will look at the problems with the HW-2036 that came into my hands and how I fixed them. Also: I have explained the details of this restoration in 4 parts on my Youtube channel as well, you can watch it here.

FAQ

What is the Heathkit HW-2036A?

The HW-2036A is a PLL-controlled, 10-watt, 2-meter FM transceiver introduced by Heathkit in 1976. It was sold as a kit and covers the 144–148 MHz range.

What is the difference between the HW-2036 and HW-2036A?

The HW-2036 covers 2 MHz of bandwidth on the 2-meter band, while the HW-2036A covers 4 MHz. Heathkit also offered a modification kit to convert the original HW-2036 to the A version.

How much did the HW-2036A cost when it was new?

In 1978, the HW-2036A kit was priced at $269 USD, which is equivalent to approximately $1,911 today.

How does the HW-2036A frequency control work?

The transceiver uses a phase locked loop (PLL) with a voltage controlled oscillator (VCO). The operating frequency is set via three BCD switches on the front panel. The VCO output is one sixth of the actual operating frequency and is multiplied by six in both the receiver and transmitter stages.

What other 2-meter radios did Heathkit make?

Heathkit released four 2-meter radios: the crystal-controlled HW-202 (1973), the synthesized HW-2026 (1975), the HW-2036/2036A (1976), and the VF-7401 (1980) which featured an LED display and scanning capability.

Why was the Heathkit HW-2026 recalled?

The HW-2026 had serious problems with spurious emissions, making it the first product in Heathkit's history to be recalled from the market.

Links:

My videos on the restoration of HW-2036, part 1: Click here to watch

My videos on the restoration of HW-2036, part 2: Click here to watch

My videos on the restoration of HW-2036, part 3: Click here to watch

Robert Sumption's video on the general modifications in HW-2036: Click here to watch

Heathkit 2036 Manual courtesy of W5RKL: Click here to download

Heathkit 2036 Manual (includes schematics) courtesy of Vintage Radio Info : Click here to download

WB2CBA USB Sound Card Based Digital Interface and Vox for Digital Modes

In brief: The WB2CBA design CM108AH-based USB sound card VOX circuit enables the use of FT8, FT4 and WSPR modes on QRP radios such as the uSDX that lack a CAT interface. 

This article covers the installation of the WB2CBA (Barbaros Aşuroğlu) designed USB sound card VOX circuit with a CM108AH-based external sound card, and its use with the uSDX QRP radio in WSPR mode. This low-cost and easy-to-build circuit, intended for those who want to use digital modes such as FT8, FT4 and WSPR on radios without a CAT interface, automatically handles PTT control based on the signal level at the computer's audio output. The circuit schematic, parts list, elimination of ground loop and PTT voltage problems, and a WSPR performance comparison between 200 mW and 5 W are among the main topics covered.

WB2CBA Interface for Digital Modes as I built in on a perforated board

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WB2CBA's Sound Card-Based Digital Interface VOX Circuit

There is almost nothing I can say about this circuit — other than the fact that it works very well and is very easy to build. You can find extremely detailed explanations about both the construction and operation by its designer, Mr. Barbaros Aşuroğlu (WB2CBA), on the electronics projects pages of the Antrak website. The circuit consists of a USB sound card designed to be plugged into computers via USB, along with a few components to connect it to the radio through its audio input and output connectors. It is an extremely economical solution for operating in digital modes with radios that lack CAT control — such as the uSDX or your own homebrew QRP radio.

I am not particularly fond of digital modes myself — especially modes like FT4 and FT8, which cannot be considered real two-way conversations, bore me. It is a matter of taste and preference, but these techniques feel to me like they require no operating skill, and I feel useless. As I have noted in other posts, if something doesn't require work, learning, and effort, it holds no interest for me. So I don't use these modes for actual communication — with one exception: WSPR. When I built this circuit, my intention was not to do FT8, but to operate WSPR with the uSDX.

I had actually been running my WSPR transmitter on and off for a few years, but there were two things I was curious about: what would happen if I increased the output power, and if I also received instead of just transmitting, how many stations would I be able to hear. I decided to build this circuit to connect to my uSDX and try it out. I was curious where I could reach by going from 250 mW up to 5 W. And how many of the stations that heard and decoded my signal would I in turn be able to hear?

Also designed by WB2CBA, this is my WSPR transmitter.
It connects via WiFi to my home modem, syncs the time
 from a server over the Internet, and transmits a message every 4 minutes

Components and Construction of the VOX Circuit

There isn't much to say about the construction. If you look at the schematic above, you will see that the components are extremely easy to source. The only part you need to specifically seek out is the USB sound card built around the CM108AH integrated circuit. I used the type whose photo you will see below — you can get this online for 3–4 US dollars including shipping. Mr. Barbaros has also designed a PCB to mount this sound card on, but I made the connections on perforated board in a layout similar to his. It obviously didn't turn out as elegant as a proper printed circuit board, but it does the same job.

WB2CBA Digital Interface Schematic

The CM108 USB Sound Card I used for this project

WB2CBA Interface for Digital Modes as I built in on a perforated board

Post-Build Operation, Tips, and Solutions to Some Possible Problems

What the circuit does is cause the radio to switch to transmit when a signal of a certain level is detected at the computer's audio output. It does this by amplifying and rectifying the signal and applying it to the gate of a FET transistor. The transistor, connected to the radio's PTT input, short-circuits the transmit and ground lines when it conducts. A red LED lighting up at the same time shows us that "PTT has been pulled to ground." In the other direction, the audio signal coming from the radio passes through a variable resistor before entering the computer. It sounds simple, but after assembling it I couldn't get it working right away and struggled for a while. Let me summarize the reasons:

• The positive voltage at the uSDX's PTT pin could not be pulled fully to zero for a reason I couldn't understand (it was staying around 0.2 V) and the transistor couldn't conduct. I couldn't resolve the problem until I added a diode to block the voltage coming from the radio into our circuit. Note: this diode is not in the original schematic, and when I asked Mr. Barbaros about it, he said he hadn't needed to make such an addition. So the problem may be specific to my circuit or my connections to the radio.
• After that problem was resolved, another situation began to emerge: when I plugged in the connector coming from the circuit into the radio's audio output, the transistor would start conducting and put the radio into transmit. I resolved this too by adding a diode to the end of the cable coming from the radio. I believe the problem is ground-related — probably a ground loop forming, with current flowing somewhere it shouldn't. The cleanest solution would be to use an optocoupler between the VOX circuit and the radio to break the electrical connection, but it must be remembered that this circuit was designed to be simple by nature. If we can get it working with two diodes, we'll stick with that. :)
• The third problem I encountered was on the software side. Through trial and error I realized that the level of the audio signal coming out of my computer wasn't sufficient to drive the transistor into conduction. No matter how much I adjusted the computer's audio settings, I couldn't solve the problem. I eventually realized that in the WSJT-X software I was using, it isn't possible to raise the level sufficiently using the vertical slider on the right side of the interface without adjusting it. I left that setting at the lowest point at which the radio switches to transmit. I should also mention something about the receive level setting: it should be set to 30 dB with no antenna connected to the radio.

The diode I had to insert between the transistor's drain pin and the radio

Finally in operation - stable now

The input level on the left needs to be set to 30–40 dB, and the output level
on the right should be left at the lowest point at which the radio switches to
 transmit when pressing the 'tune' button just above it.

Conclusions and Some Observations

I used the circuit I built this way in WSPR mode for a while (and even did a bit of FT8 and FT65). What I found:

• Despite the large 14 dB difference between 200 mW and 5 W output power, I didn't see a very large difference in the distance of the stations that heard me. To give an example from the eastern direction: at 200 mW I was heard at most from Austria. At 5 W I was heard from Italy, the Balkans, and once from Turkey.
• The number of stations I could hear was, as I expected, far fewer than those I could reach. I wasn't expecting more than that anyway, with a 10-meter wire strung from a first-floor window to the garage awning.

These results once again confirm what is already known:

• Unlike modes such as SSB and AM, with CW or — if you like them — digital modes, you are always heard from farther away.
• Instead of higher power (100 W and above), it is better to invest in a better antenna.

One final point: to operate in the digital modes mentioned above, your computer's clock must be accurate. If you use WSJT-X as I do, you will also need software that periodically connects to a server over the Internet and re-synchronizes your computer's clock. I used BktTimeSync for this.

A wonderful circuit for those who want to get into digital modes with an inexpensive and enjoyable build. I wish success to the amateurs who try it, and once again extend my thanks to Mr. Barbaros.

FAQ — Frequently Asked Questions

1. What does the WB2CBA USB Sound Card VOX circuit do? 

Simply put, it allows the computer to communicate with the radio. When the audio signal from the computer reaches a certain level, the circuit automatically switches the radio to transmit. It stands out as one of the most practical and inexpensive ways to use digital modes such as FT8, FT4, or WSPR on radios without a CAT interface — such as the uSDX or a homebrew QRP radio.

2. Which USB sound card should be used? Is the CM108 mandatory? 

The design is based on the CM108AH chip, and cards with this chip can be easily found online at very low cost.

3. The PTT voltage won't go to zero and the radio won't switch to transmit — what should be done? 

This problem is encountered particularly with the uSDX: a residual voltage of around 0.2 V remains at the PTT pin and the transistor cannot conduct. The solution is surprisingly simple — adding a single diode between the radio and the transistor's drain pin is sufficient. This diode is not in the original schematic, but it has been found to work when this voltage problem is encountered.

4. The radio switches to transmit as soon as the audio cable is plugged in — what causes this? 

Most likely a ground loop problem is occurring. Current is flowing somewhere between the radio and the computer where it shouldn't be. Adding a diode to the end of the cable coming from the radio resolves the problem. For a more permanent and cleaner solution, using an optocoupler could be considered; however, since simplicity is at the heart of the circuit's design, sticking with two diodes is reasonable if that gets the job done.

5. The audio level in WSJT-X is never sufficient — what should be done? 

Opening the horizontal slider in the interface all the way usually isn't enough. The vertical slider on the right should be left at the lowest point at which the radio switches to transmit. For the receive level, 30 dB with no antenna connected is a good starting point. It requires some trial and error, but once found it stays stable.

6. Which radios does this circuit work with? 

It is expected to work with any HF radio that has a PTT input. The primary target audience is radios without CAT control — uSDX, homebrew QRP radios, and similar models. If a commercial radio with a CAT interface is being used, controlling PTT through software is a more practical approach.

Links

WB2CBA's original article: Click here

BktTimeSync download: Click here

WSJT-X download: Click here