Resurrection Radio

There I was minding my own business and up pops an item for sale on ebay that fell below my "Gag Price Level". It was something I owned before as a NIB item back in the late 1970's but was sold because of an impending job related move.

The current offering was in the basket case below tech special category but had a siren's call. So I made an offer that was below the Gag Level and it was mine. Now I should mention that I already have in the bins the main major ciruit board and two of the smaller circuit boards for this radio that were acquired earlier. So I had plenty of spare assemblies that might form the basis of a "radio resurrection".

Important disclosure: I paid $85 for this radio but reasoned that even if it did not respond to a Lazarus Resurrection approach, the Crystal Filter and the FB Yaesu Linear Analog VFO plus the Tubes and meter were worth more than the price. So the big prize is to get it working (which later you will see I did) to the second prize a valuable stash of parts !

This also afforded me an opportunity to use the Amazon Gift Card, a present from my daughter, to acquire a critical piece of shop equipment that would play a major role in the resurrection process. That tool is a Bosttich Professional Grade Air Compressor. It will be needed to clean the chassis and I may even take on sandblasting the rusted case.

I desperately need this project to balance the huge time sink involved with the caregiving of my XYL who is not well healthwise. I have a caregiver service come in 3 days a week for a four hour shift each day. This gives me some respite from the many jobs I now have as a part of my caregiver duties. I always like to place costs in a metric form so all understand. So far since the 1st of March the part time caregiver service equates to the purchase of an ICOM 7850 or two top of the line FLEX Radios. This is all out of pocket (my pocket).

Yes this puts me at the end of a very narrow pole but this is not the 1st time I have purchased a basket case. Several prior resurrections were undertaken including a Drake B Line and an earlier FTdx100. [See www.jessystems.com]

There has to be a road map of what needs to be done and in what sequence. Thus a plan is the very first step.

The above photos show the Before and After shots upon completion of the 1st level cleaning using the air compressor. Of note I found many Mylar capacitors that had cracked cases and replaced about 8 of those so far. I saw this same occurrence on some Heathkit gear --same type of cap. Guess they go south after about 40 years.

A "benchmark" comparison is to click on the two links and observe one benchmark item which is the metal tube shield around the 7360 Balanced Modulator. In the photo on the left the tube shield is totally rusted and in the photo on the right after a two day soak in Evapo-Rust you can see how nicley it has cleaned up. You can also observe the cover over the Power Amplifier cage assembly. Evapo-Rust works miracles but regrettably would do nothing insofar as enticing Mary Jo into the back seat of the 57 VW Beetle!

In the very upper part of the schematic are two dropping resistors that are connected to a supply rail greater that 100 volts and this is dropped down to around 15 volts. The tone oscillator a single transistor phase shift oscillator is shown in the lower part of the schematic. When the front panel selector switch is set to TUNE the output off of the Collector is routed to the Microphone input (Pin7) on a 12AX7.

Here was the problem --at turn on you could get the transmitter to TUNE, but after a bit of heating there was no Tone output (1500Hz) and of course no output in TUNE. But the transmitter would fully modulate in SSB.

This led to the conclusion that the Tone Board had an issue. There are 4 electrolytic capacitors on this board, and as a first step they were all replaced. Yet the problem persisted -- after a heat up no Tone. So, replacing the caps was a good idea but not the problem.

The next step was to conduct a more detailed examination of the circuitry. Essentially KB = 1 (oscillation) when cold but no oscillation when hot. My attention was drawn to the 10K pot that sets the Bias level on the Base of the Tone Oscillator transistor. It had a dab of paint on it to mark the factory setting, but I reasoned with component aging the Bias when hot was insufficient to sustain oscillation.

So, a bit of a test. I turned on the rig and tuned the rig and then waited until it was on for about 1/2 hour and attempted another tune-up. Nothing! A small adjustment of the pot and BOOM, we had a Tone and full tune up power output.

But we are not done! I let the radio sit overnight and then did a TUNE process just to be sure that the new setting would work COLD. It did. The change of the pot setting was very slight. We are good to go except for one more test. When it was hot, I reset the pot to the original location (matched the paint marks), and it stopped oscillating. I concluded we found the problem.