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.
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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. |
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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.
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| The RF probe as shown in the assembly manual |
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| 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.
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| Coil L209 with the broken ferrite slug. Getting the broken pieces out was a challenge in itself! |
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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.
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A disposable aluminum baking tray, waiting to be shaped. |
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| 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. |
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| 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.
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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.