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. |
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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:
- 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.
- The unit was transmitting and receiving in both simplex and duplex modes, and there were no issues with the frequencies.
- The transmit output power was far below what it should have been.
- 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).
- The receiver was somewhat deaf. The squelch was opening at around -95 dBm, and it was practically "not hearing" signals weaker than -105 dBm.
- 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.
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| Heathkit HW-2036A's back side and the screw I had to immobilize with epoxy glue before drilling. |
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| 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).
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| Heathkit HW-2036A Transmitter Board. The bad screw is the one at top-right of the picture. |
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| Heathkit HW-2036A Transmitter Board - After replacing the electrolytical capacitors. |
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| 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.
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| 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. |
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| 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.
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| 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.
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| Notice the legs of the integrated circuit—there is some oxidation, even if slight. |
Also: I have explained the details of this restoration in 4 parts on my Youtube channel as well, you can watch it here.
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