Posted By: Robert Nickels (ranickels)
Vintage Ham Radio 01/19/2023
I know many of us who enjoy restoring and repairing vintage gear look forward to winter when there is less competition for time and energy, and a chance to really make a dent in our "to be fixed" piles. A couple of years ago I set time aside for "Heathkit Singlebander Week" and went through every one of them I had, with the result that they're all working (or at least "were")... Likewise I had an R-390A party last year.
This year there was just a nearly overwhelming pile of "stuff that don't work anymore" and the winter project was to just dive in and fix it without getting too distratcted by other projects (a big temptation for me!).
First is the Heathkit KW-1 Chippewa which came to me in horribly hacked-up condition. Whatever problem the prior owner was attempting to solve involved replacing *every* wire in it with shielded cables with a .001uF bypass capacitor on BOTH ends. He used the old style cable where the shield was exposed which meant there were lots of potential shorting hazards wherever it was routed. Then he ripped out all the original screen and bias supplies and replaced them with a nasty rats-nest of solid state components, removed half of the plate tank coil and turned it sideways and added more to it (evidently to make it tune 160 meters) and other ham-fisted hackery. I decided to strip it to bare metal and rebuild it as a grounded grid amplifier. I had acquired the matching KS-1 power supply previously but the HV transformer had been replaced so the amplifier makes about 4-500 watts out but the nice thing is the 4-400As just loaf along.
Next came the Technical Radio aka "TecRad" LRR-6 receiver that I acquired from Lou W9LRS. This receiver worked but had no BFO, and having limited coverage of AM BC and two shortwave bands it wasn't essential but I wanted it to work, so out of the heavy cabinet the heavy receiver chassis came. The problem turned out to be a bad BFO switch! The toggle made the right noises but nothing happened and after attempts to clean and lube it I just replaced it with an identical one from my junkbox. The LRR-6 was an entertainment receiver that was sold primarily to the Navy, but which had to have very low local oscillator emissions per Navy specs.
While in the heavy receiver mode, I hauled the 51J-4 onto the bench and started troubleshooting it from the audio stages forward. This receiver has worked for many years but I noticed a drop in gain and most recently, it just about went totally deaf. Expecting to find a bad IF stage or something I was a bit frustrated to learn that everything seemed to work but just had low gain. I checked to make sure the B+ was correct, and on a whim put the meter on the filament pins - what a surprise it was to see the filament voltage was under 4VAC! This receiver came from the Cedar Rapids hamfest many years ago at a good price because it had a replacement power transformer that was causing other potential buyers to turn up their noses. I didn't care since it worked but now started wondering if the replacement was just inadequate to begin with and finally had failed with some shorted turns that reduced the output voltage. The 51J series draws 5 amps of filament current and patching in an external transformer brought it back to life. In response to my posting for a replacement, my friend John Poulton ("JP") K4OZY said he had one from a jumker and soon it was on the bench, ready to be installed. John is not only an all-round great guy but is responsible for saving all of the original factory documents from the TMC Corporation, and has spend untold hours scanning them and making them available for free online. Ham radio would be much worse without the contributions of JP, and I am just a recent beneficiary of his willingness to help others. The 51J-4 is working perfectly again!
All this was just a warm-up for hoisting the SP-600 onto the bench, again. I'd done the complete re-capping of this radio some years ago, including the nasty black beauties up inside the RF deck, but it had gone deaf and I was hoping for something simple like a tube failure. But it turned that I'd missed one of the IF bypass caps, which wasn't shorted but evidently had gone leaky enough to eventually burn up the series resistor. The SP-500 is too cool a receiver to remain on injured reserve for long, so I'm happy it's back in service. The world best band-scanner!
In the same category as one of my favorite radios, the Central Electronics 100V had been in need of attention after I noticed the sideband suppression was poor. I'd been through this transmitter before but have since learned a few tricks from Nick Tusa K5EF, aka "Mister CE" and that replacing the modulator capacitors with NEW rather than junkbox tantalums would make a difference. I also used the SDR to do the alignment and found that adjusting the carrier oscillator had a big impact that isn't documented in the alignment notes. In th end I was able to achieve the 50dB suppression Nick promised was possible with diligent alignment. The 100V is just a marvel of engineering and design, it's unfortunate the company never was able to make money selling it based on what it cost to build vs. what the ham market could pay.
Moving to the "lightweight" catagory, I wanted to get the bad news about my Swan 600T's loss of output, which I thought would translate to a new pair of 6LQ6s. I felt bad because I'd contributed to this problem by inadvertently leaving the transmitter turned on for a period of several weeks. It was unnoticed because I'd turned off the receiver, but neglected to do so to the transmitter. But upon examination of the key voltages I found the bias was raw AC! The extended burn-in evidently had caused the original filter caps to fail, which made it impossible for the PA to work. But that wasn't the only problem - there was no drive, either. Swan used both 6JH8 tube and solid state type balanced modulators in the 600T - mine had the tube which Swan gurus say is best - but there's a problem with the original circuit. The designers forgot to calculate the power in three resistors correctly with the result that they are being forced to dissipate power in excess of their ratings. For example R709 and R704 are 1/2 watt resistors yet the power thru them is .6 watts, and the 1 watt R710 is also right at its maximum power dissipation (and had gone open circuit). Overloading results in carbon comp resistors increasing in value to the point the circuit just doesn't work anymore. Replacing all three with 1 and 2 watt resistors brought the balanced modulator to life and the 600T now is making 300+ watts out again! (The photo here reminds me to find the proper glue to re-attach one of the knob inserts before it's lost!)
And that more-less clears off the "to be fixed" pile for the moment, but of course all I need do is look around the shack/museum to find additional candidates for the repair bench. And then there are the "new projects"....
In closing let me say that without the internet, and the friends across the country and even the world that I've been able to make via the various internet resources, it would be much more difficult to keep these old boatanchors afloat!