Just a note, if there was a VHF parasitic that might explain your odd frequency readings the other day. It might also have caused some tranmitter stage to run-away resulting in the smoke. Visual inspection and sniffing will likely turn up the toasted component (hopefully just a resistor). The cause may be harder to locate.

Take a good look at RFC5. If it was the 2E26 that "took off" it might well have taken out the plate choke.

One thing you develop a nose for (sadly) after a while is differentiating burned resistor, versus burned coils, versus burned transformers. It can be said you are not really a technician until you have let out your share of "magic smoke."

Curtis Eickerman

I don't think it had anything to do with the 2E26, which was not powered up at the time, S2 was in the Oscillator/Tripler position, which disables the final stage.

Is there any way a parasitic could produce excessive cathode current in the VFO? The VFO's cathode needs to be grounded on transmit, and I connected the VFO ground return to J4, in accordance with the instructions for the Gonset 3357 VFO. But I see on the schematic that J4 does not go directly to ground on transmit. Instead, it goes to ground only after passing through the secondary of the speaker transformer. I'm not an expert at smells, but that could have been a transformer smell. All the components under the chassis visually appear to be OK, no sign of anything burnt.

A parasitic seems to be a good suspect, could it have been the result of inadequate drive from the VFO?

73, Bill

I don't think a parasitic would happen in the VFO. I did see what you mean about the ground that happens via J4 through the switch and through the speaker transformer. I'm not sure what they had in mind there, but it might also be contributing to difficulty getting enough drive from the VFO. Unfortunately it might have also contributed to the smoke, but I hope not.

Curtis Eickerman

Two watts output may also mean the PA grid is starved for drive. Since the PA is running
Class C, and I assume is self-biased, a lack of drive not only would give you low power, but
it also might result in damaging the rig due to excessive plate current.

Regarding the VFO: besides shorting the choke, I'd also suggest keeping the coax length
between the VFO and Gonset as short as possible. You can try removing the cap between
the osc. grid and cathode, it was part of the Colpitts feedback circuit but in your application
it will shunt part of the VFO signal as well.

One remote possibility you might try is adding a 4:1 balun at the rig to boost the VFO signal
voltage level at the Gonsett. A small, good quality CATV balun should work at 8MHz.

Pete

I don't think it had anything to do with the PA because, at the time of the smoke, S2 was in the Oscillator/Tripler position, which disables the final stage. Good idea about the balun, although at this point I'm disinclined to risk further damage to the rig, think I'll trade the Johnson 6N2 for something more compatible.

By this time tomorrow, I'll be home and hopefully will know more about what happened.

73, Bill

Got home, disconnected the VFO, put a crystal back in, un-shorted the choke.

First applied power to the receiver and it is still working, guess the audio ouput transformer is OK.

Then applied power to the transmitter and turned it on with S2 in the Oscillator/Tripler position (no power to the final). The 12AV7 immediately began flashing with a purple, pulsating arc. Shut off the rig and put the 12AV7 in the tube tester, it was dead, no filaments.

Hopefully just a bad tube, but I wonder what made the old tube do that. And what was smoking, doubt it came from the tube itself.

73, Bill

Bill, I think you said there was smoke when everything went bad the other day. if so, a good visual inspectiong of everything conected to the 12AV7 is probably in order. I have had "smoked" resistors that were not all that obvious until I knew where to look. Of course if everything works find with a new (old?) 12AV7 then perhaps there is nothing to find.

Curtis Eickerman

Seems to be OK now, borrowed another 12AV7 and no sign of any smoke, I am able to transmit with crystal control. Closer inspection of the area around the 12AV7 revealed the smoking component, it was the 12AV7's 100 ohm cathode resistor, R5. It now reads 120 ohms, perhaps close enough, it would be hard to replace, as it is buried under 2 chokes. Output is now about 0.5 watts less than before, perhaps the result of increased cathode bias, or maybe the other 12AV7 was a little stronger.

Any idea what could have caused the problem, are we still thinking it was a parasitic?

73, Bill

I can't imagine a dual triode of that size being able to generate enough RF
to damage anything. If the cathode resistor smoked it sounds like a grid
to plate short took place, or B+ got on the control grid somehow.

Pete

Could that have been the result of inadequate drive from the VFO, or otherwise related to the VFO? Seems to be working OK with the crystal.
73, Bill

Bill I don't have the schematic available at the moment, but it is possible that inadequate drive was the problem if the 12AV7 is depending on drive to develop proper grid bias. If so, the absense of drive might have allowed excessive cathode/plate current and smoked the resistor.

Maybe someone that can look at the schematic can check on that.

Curtis Eickerman

Thanks, schematic is here:
http://qsl.net/nz5n/Gonset%20Communicat ... 20list.pdf

Parts list and voltage test points here:
http://qsl.net/nz5n/GonsetCommunicatorI ... 0chart.pdf

My unit is different from the published schematic in at least one respect, and it directly relates to the 12AV7 grid section -- the rotor of C4 is grounded, it does not go to the R5-C5-L1 junction. When I first got this Gonset, it was not transmitting, and I got the TX working by replacing the 12AV7, which tested bad, and this was long before I had the VFO. Now I'm wondering if perhaps the VFO was not totally at fault.

73, Bill

The cathode resistor should provide some self-biasing. They may well be pushing the tube
ratings, however.

And yes... we used those rigs back in my CD days in the early 1960s. They did run very
hot!!! There are a lot of tubes in a small area.

Pete

I'll say, Pete,you can burn your finger by touching the chassis after it has been on for an hour or more. I have see that some people put fans on the back.

Anyhow, things are not looking good, power output drops further after the rig gets warm. When that happens, plate and screen voltage drop as well. Maybe it is just the smoked R6 going higher in resistance when it gets warm, but this rig is definitely not back to normal. Maybe some other component was damaged, how could R6 affect plate voltage?

73, Bill

"Sometimes the long way around is the shortest way home."
Take an evening and soldering equipment. Take digital pictures and remove parts down to the crispy-critter resistor. Then swap it and check all the opther parts values before restoring them.
Then use the digital pictures to recheck again before replacing the cabinet.
Been There,
Pat

Roger, Pat, I have resigned myself to the fact that the smoke resistor needs to be replaced, it is now reading about 400 ohms instead of100. Hope that is the only problem.

73, Bill

Well, I replaced the smoked resistor today and checked other resistors in the area. All within tolerance.

Output is still low, just 3w (better than the 2w of yesterday but worse than the 5w of last week.

Voltage readings indicate low bias on the 12AV7 and 2E26. B+ and screen are normal, even a bit high (a prior owner replaced the tube rectifiers with diodes, and this resulted in higher voltage.

Test point, today's reading, spec sheet (location)

A, 1.2, 1.6 (6CL6 cathode)
B,150, 136 (6CL6 screen)
C, 295, 255 (6CL6 and 12AV7 plate)
D, 1.5, 4.0 (12AV7 cathode - location of replacement for smoked resistor)
E, -39, -90 (12AV7 grid)
F, -16, -55 (2E26 grid)
G, 165, 165 (2E26 screen)

Any thoughts on what may be wrong?

73, Bill

A spec sheet for a 12AV7 lists the maximum grid voltage as -50. Seems odd that you are seeing -39 or -90.

Curtis Eickerman

Good news, it is now working better than ever. It turned out that the 12AV7 that I used to replace the one that was arcing was not up to par. I replaced it with one that tested strong and am now getting over 7 watts out, previous best had been 5.5w. So I am happy tonight.

Other news is that I purchased a Gonset 3357 VFO that was specifically designed for use with the Communicators. Hopefully it will arrive here soon in working condition. I will put the Johnson 6N2 VFO up for sale. It is a good, stable VFO but it just seems incompatible with the Gonset. Guess we'll never know for sure what caused those weird frequency readings and the arc/smoke.

Curtis, as always thanks for your great help. I was reading -39v with the old tube. The Gonset test point sheet said there should have been -90v at that point, and it is in fact right at -90v with the new tube. So I do not know how that squares with the data sheet.

73, Bill

I had the correct Gonsett VFO for the Gooney Bird II. It had a grey case and a very similar
dial that matched the receiver dial on the GB. As I recall, it even had a built in microphone
preamp. I used it for years on a homebrew two meter rig back in the early 60s.

Pete