Gonset Communicator IV 6 Meter Version
A friend gave me one and I think he may have been laughing behind my back at the fun he knew I was going to experience. Aside from a couple of missing tubes in the RX and some suspect capacitors, the radio was in good shape. While waiting on the missing tubes to arrive from Epay, the fun began.
First of all, there is no 6 meter Communicator IV manual posted on the internet. There is a 2 meter version. The radios are very similar in most aspects other than the extra multiplication stage in both the RX and TX on the 144 mhz version so you can sort of go by the 2 meter manual to figure things out. While waiting on the tubes and a TX crystal to arrive, several paper and electrolytic capacitors were changed out. With replacement tubes installed and a 50.7 crystal, then time to begin. The TX worked great. 12-13 watts output. RX was a different matter. Although it had hiss, no signals detected.
It took most of an evening of head scratching, disassembly of the RX section to see what was under the shielded compartment and a pot of coffee to get an idea of how the 6 meter version deferred from the 2 meter one in the RX area. In my radio, the 6 meter receiver uses a 41.000 mhz crystal oscillator in the first mixer that mixes with the amplified incoming 50 mhz signal to produce a first If in the 9-13 mhz range. This is mixed in a second mixer where the VFO running at 6.7-10.7 mhz is injected and 2.3 mhz is the result. This is sent to a third mixer where a 2.755 mhz oscillator (not crystal controlled) is mixed and the 2.3 mhz is now converted to third IF of 455 khz. The 455 khz is then amplified a few times before sent to a detector, audio amp, and squelch circuits.
The worst thing about the Com IV was getting the RX receiving because the RF amp and input to the first mixer were tricky to adjust properly. It’s similar to the Swan 250 front end alignment because it will start oscillating if mistuned. I found the antenna coil slug adjustment had to be peaked toward the top of the chassis (nearest the tube socket) and not at the bottom, as some of the other coils that were adjusted. Once this was was properly adjusted, good RX sensitivity was quickly achieved through touch up of the rest of the receiver alignment. Calibration of the RX dial can be tricky also, it’s similar to the alignment of the 2 meter version with the exception of the frequency range. Again, the slug for the VFO oscillator was at the top and not the bottom of the coil form. Also the compression trimmers under the tuning capacitor required adjusting. These actually looked like screws that are part of the tuning capacitor frame and not the compression capacitor near the mixer stage.
Communicator IV – 2 Meter Version
Just picked up one of these. It’s missing the 6BQ5 modulator/audio output tubes. Waiting on new tubes. From Russia! 6P43P-E is EL84/EL86 tube with higher plate dissipation than a 6BQ5/EL84. I purchased 4 of them, NOS for less than the cost of 2 NOS 6BQ5/EL84. Unfortunately I find many of the NOS tubes sold in the USA to actually be “used” tubes. Reviews of the Russian tube were quite good, especially on the measured parameters of the tubes taken from a random lot.