Mk3 sparks when txing on ssb

[vswr]

New member. Hi fellow radio lovers. About me. I have a tech background from the military. About txer. Browning G.E. mk3 ssb. I have sparks happening on the underside just near the final tube. I have not yet recapped this transmitter or replaced any of the weak or shorted tubes. I figured I would hook it up through my isolation transformer and see what happens. Could have been lucky but no go. It keys with over 300vdc on pin 2 mike socket to gnd. Dangerous design and could have been done using filament voltage through a bridge rectifier then to a 6 volt relay. Anyhow, arcing and sparking happens underneath on ssb mode and nothing out to my bird in any mode. There is a small signal getting through to my test rig. Enough to see that it keys up on all freqs with no drift (AM) and after seeing the light show, no more attempts on ssb. The 7558 tests bad and a few others too. 7558 comes up with a short on my old tube tester. Just wondering if anyone can comment or thoughts? Have the sams on it and all required test eq. Just need to find another RF sig genny with variable modulation capabilities on the cheap. I got one from Amazon and it has modulation but wont superimpose mod onto the rf carrier. (Too cheap). A few things to comment on. Yeah, maybe I should replace all the wax filled and electrolytics, I get that but
Thats whats throwing me off of putting in new fragile finals and drivers is that sparking which could be something else and may damage new tubes. Maybe I will go ahead and recap it, check out my voltages and resistances, pull the final and driver tube than key up and follow the signal first. Might have to sacrifice a few tubes while doing the repair. We'll see.

[2600]

Hi, I have a 1974 car that just won't start. It makes some clanking noises when I crank the starter motor. I hope it's just one simple cheap problem to solve so I can take it on a road trip.

The first question I usually ask about this kind of radio is how many folks have tried (and failed) to make it work since it first broke down?

Sometimes the sincere answer is a shrug. But for every different set of fingers poked into it you may find a new fault that was added, but nobody could tell. Didn't work before, doesn't work after the first guy accidentally caused one more tube to be damaged. How do you tell that it has two things broke in it now, not just one?

It's old enough to expect it to have more than one fault. If the sparking follows the voice audio, this may just mean that the final tube is not drawing normal current. But how many faults there are upstream from the final tube is worth knowing.

Maybe just one.

Audio voltages in the final tube's circuit get way out of hand if the final tube is on vacation, so this could be a red herring, especially if the sparks are across the air gap between the plates of the plate tune capacitor.

And maybe it has tweakeritis. A common response to this fault is to start twisting tuning adjustments. If nobody keeps track to put those adjustments back where they were before the failure, this becomes a new fault on top of the original one.

My routine advice for this symptom is to first see that all three internal frequencies are being generated. All three get mixed together, and if one of them fails, no output.

A receiver that can tune in 5.645 MHz, 5.05 MHz and 16.27 MHz can be used to eavesdrop on each of those three oscillator circuits. Making sure that all three of them are running is step number one. You can do this with the final and modulator tubes pulled out of the sockets, safe from surge risk.

And we haven't even discussed the age issue. That 1974 car will need every hose, belt and seal changed. A radio this old will need every electrolytic capacitor changed if you want to rely on it. Those capacitors might all check okay in a radio with no more than 500 original miles, but a radio that was used a lot should have more than one of them broken down. Those capacitors each have a rubber seal that holds in the chemistry that makes the capacitor work. Rubber ages, shrinks and gets brittle. Electrolytic capacitors just can't be trusted at this age.

Like a 49 year-old car, it's only money.

73

[vswr]

Thanks. I rifled through your response and got the meat I needed. Good luck with your 74 car!

73s!

[vswr]

It could very well be because of an adjustable air gap tuning cap. If its anything like the silver ones inside my zenith race track radios then the tuning cans have small pf caps inside that are silver and after 60 years they build up enough oxidization then arc across. A real headache to tear apart and fix. Havent tried anything after seeing sparks on sideband. I will pull those 2 tubes and check my IFs. Then check caps for shorts.(replacing each temporary to get running) this will get me at least a chance to follow the signal and see whats up. Like I said in my opening post. My test rig picks up minute signals when I key up on all freqs and to double check for adjacent activity, I key up on the next channel and dont get cross channel. I concluded that it is transmitting a small signal on all 23 am channels with no apparent drifting or splatter so that part could be working somewhat. I will order some tubes once I clear any dangerous vcc problems and ensure my plate and grid voltages are ok I will then test the txer with a fresh set of tubes after a recap. Thanks for your input. Will be looming for a pal vfo or a siltronix 90 or maybe they've standardized them now to just one easy to find Chinese made unit that actually works. We'll see. Thanks.

[2600]

Didn't think to suggest looking at the plate spacing on the Plate Tune control. Too easy for someone to bump the rearmost plate and cause the gap between the plates to become too narrow.

Just getting a look at where the arc is jumping would serve to narrow down the fault.

73

[vswr]

That's true. I will have a look during the week. How's you're 74 car going? I hope its a mopar. Lol.

73 back