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Ryan, first of all, it probably isn't your fault.

When one of these sets is purchased, most of the time we don't know when it was last used but when it was last used, it probably worked okay.

One filament much brighter than the other isn't a good sign. The filaments in the transmitters are run off a 12.6VAC filament transformer.IIRC, the finals are in series across the 12V winding. There could be something loading down one side of that winding.

Probably unrelated, but some parts do go bad whilst just sitting there, doing nothing. This is true of electrolytic caps, obviously, but can also happen with silver mica caps and even some resistors that can short or go very low resistance.

I would look at the resistance chart in the manual and meter every tube.
This may show you where the short is.

Chances are, whatever blew up is finished doing so but one way of finding it (when all else fails) is to flip the chassis over, remove the cover, power it up with an ability to very quickly cut power and see where the smoke is coming from. In the case of a shorted mica cap or similar, it's the resistors that are smoking but they will tell you where. Most of the components that aren't transformers are easily obtained.

Good luck,



Steve Wedge, W1ES/4

Time flies like an arrow. Fruit flies like a banana.

Sent with Proton Mail secure email.

------- Original Message -------
On Wednesday, February 8th, 2023 at 5:01 AM, K5DMA k5dma@... wrote:

Hello all. I recently acquired a T-4XC and R-4C that had been stored for years and were in unknown operational condition. Before attempting to bring the rig up, I did a visual inspection and pulled all the tubes from both the receiver and transmitter and tested them with a Knight 600A (sadly not my own, but used at an out-of-town shop; I don't have easy access to that machine). All the tubes checked out okay. Aside from some of the selector switches needing to be cleaned the R-4C seems to be working relatively well. The transmitter is not so happy.

I powered up the transmitter for the first time in the bias adjustment state described in the manual: the 160m band was chosen, and gain, plate, load, and preselector were set at their minimums. The receiver was powered on at this time, and I had the oscillator line between the two connected and the VFO selector on the transmitter set to Spot.

I flipped the power switch to on the T-X4C from off to SSB, and the transmitter powered up. All the tube filaments began to glow. I was waiting out the two minute warmup described in the manual before proceeding further with the bias adjustment. After half a minute, I heard the receiver jump to another frequency and a twist of the transmitter's VFO confirmed that the receiver had jumped to the transmitter's oscillator as expected. I noticed that one of the final tube filaments was glowing brighter than the other, something I attributed to the need to complete the bias adjustment I was just about to perform, but I decided to turn the transmitter power back off and physically rearrange things on the workbench first. I had the top cover off the transmitter and I wanted to make it so I could get to the bias knob on the power supply without reaching over the top of the transmitter. When I turned the transmitter off the receiver jumped back to its own VFO frequency. Pretty slick!

After moving things around for a minute or two I flipped the transmitter from off back to SSB. The receiver flipped back over to the transmitter's VFO, but after a second or less, it sounded like a string of Black Cat firecrackers were going off inside the transmitter. It emit a deafening series of irregular pops, a dozen or more in total, in the two seconds it took me to reach for the power switch, decide against touching it, and flip the power strip off instead.

There was a small amount of smoke coming from the front left area of the radio and a burning smell to follow. I still had the bottom cover on, and by time I had gotten it removed I could not discern where the smoke had originated. The smell seemed to be concentrated around the front left corner, perhaps around the slug-tuned inductors, the VFO selector, meter, or power switch. I can't say for sure.

I've spent, combined, probably close to two hours looking all over the radio with a flashlight trying to find some evidence of failure. I've carefully looked for things like a blackened/carbonized tube socket, a burned or cracked resistor, a melted wire, an errant blob of solder, a broken solder joint, a cracked tube socket, or an exploded capacitor, or any sign of arcing or localized excess heat. I can find nothing. I have no clue to point me to the source of the noise or smoke.

Since this happened, I've gone ahead and rebuilt the AC-4 power supply using the K0UYA / ES4AC-4 kit and acquired a variable autotransformer. I have yet to try powering the radio up again though. I'd feel much better if I could find evidence of the smoke and electrical arcing and analyze the affected circuit before I energize it, even at low voltage behind a variac.

Does anyone have a suggestion for where I might look or anything specific to test or check? I can say with some degree of confidence that the noise and smoke were not near the finals. I would place it within two inches of the front of the radio and three inches of the left side. I just cannot find a likely source.

A friend of mine suspects that the plate and load knobs and their associated control rods may be an issue. He thinks there should be some kind of insulator between the front rod and rear rode inside the bakelite coupler that prevents them from touching and fears a short may have occurred because there is in no insulator, and these rods seem like they'd likely touch inside the bakelite coupler. I had pulled these couplers apart and reassembled them with a strip of electrical tape along the coupler because the inside of the couplers is stripped smooth. Looking into this theory, I found that the finals side of the rod (connected to one set of plates on the air gap capacitors) is already grounded to the radio frame to begin with, and the control panel rod is actually isolated from the case by a plastic bushing. I think that this rules that hypothesis out, but I mention it in case someone can confirm that there should in fact be an insulator inside that bakelite coupler between the two rods (even if it isn't the cause of my current problem).

I appreciate any help or diagnostic suggestions. This is my first tube radio along with my first attempt at radio repair, though I do have some experience troubleshooting other transistorized electronics.

Thanks and all the best,
Ryan (K5DMA)
Texas, USA