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Hi guys, I have been looking for an answer to this question for a while now to no avail. I have an old d&a pdx-400 amplifier that has some issues. First off I am only driving it with 2 watts dead carrier, modulating to 12 pep, into a beam that every other amplifier I had worked fine into. Input swr is 1.4:1, output swr on channel 1 is 1.0:1 flat, then it rises into the red past channel 15.

Now the arcing, it will occur on the final loading capacitor inside the amp, In no specific location, it will vary where it arcs at. It will arc as soon as I key the microphone, then stop unless I talk into the mic, then it will actually produce a stong arc and start "sizzling". Almost like a welding arc. It only does it on the high power setting, not the low. The capacitor is clean and no plates are touching.

I found some good info here on this amp from a few users here but not on this topic.. I searched high and low for the answer to this with no solution. Any help would be majorly appreciated

3022 -

How does the unit sound on-the-air on low power? My first thought would be to check the contacts in the antenna relay. If all of the power in the high setting is not getting through the relay and into the load (antenna), it has to dissipate somewhere, and the loading cap would be a good place for that to happen. The antenna connector is also a probable suspect.

If the relay or the connector isn't the problem, I would check everything from the antenna connector back to the loading cap.

- 399

Things don't really work that way. If the relay was the weak point then the contacts would arc and burn and usually nothing would happen to the load capacitor.

One thing about an arc, if it happens once it WILL happen again unless everything is cleaned and polished. I know the OP said the load cap was clean and well poloshed but every time it arcs it will leave a small pitted spot that will arc again, and again and again unless it is repaired each time it arcs. There should not be that much voltage on the load cap to make it arc unless there is a piece of dirt stuck between the plates or the plate blocker capacitor is shorted allowing full plate voltage to appear across the tune and load capacitors. Since the tune cap has a larger spacing the load cap will arc first. Check the plate blocker cap and make sure the load cap is clean and no rough edges or traces of carbon left on it.

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Ok heres what I found, plate is clean, not even any carbon deposits on it from where it was arcing??? Input and output so-239 connectors are good with tight fitting connections. My coax is only a few months old and is rg-213 to the antenna, and rg8 to the meter from the amp. Coax all checks out fine.

Now I just read captain kilowatts post, and I flipped the amp over without the case and watched underneath as I keyed, and the middle relay does infact make a greenish/blue spark when I key the radio... What exactly does this mean?

Cap'n K.-

I agree with what you say. I just had trouble understanding why it worked in the low power mode, and why the cap never arced in the same place. Since the antenna switching relay in most of the D&A amps was a DPDT with the 2 contact sets paralleled, I thought if one contact set was bad that this could cause the problem. It was just a shot in the dark.
In any case, I would replace the final load cap, 'cause it can't be in too good a shape after that much arcing.

- 399

Thanks for the responses, heres what I found... The pdx-400, atleast this one uses 2, 3 pole relays.. On the antenna relay I tried advancing the tank circuit pole a few thousandths, so the antenna would connect before the input circuit, didnt work, still shoots sparks.. Now the 2nd relay is what controls the high and low I think? On the low setting sparks are nowhere to be seen in this amp. on the high setting the middle pole on the 2nd relay will spark along with the cap plates. Maybe this will provide some insight.

Here is a good article on amplifier arcing:
http://www.eham.net/articles/21381

Some mention there of capacitor plate spacing vs. voltage limit.
And among other things, the various causes of arcing.

The capacitor is fine. Believe me I read ever single article on arcing I could find, but thanks for that link, it was an interesting read. The capacitor is fine, I looked it over and none of the fins are touching, I tested it with a multi meter, no continuity between plates.. Cleaned it up and polished it to a mirror finish with 3000 grit sand paper, still arcs.

Could this be due to the tank coil....? I bought this and the tank coil looks out of whack, 6 turns, spaced far apart for less inductance I would think?, and soldered or tapped at the last turn to the 90 degree bend. Really want to get this thing on the air, Its just killing me to see it arc and self destruct with ALL brand new 6lq6 tubes and almost ever capacitor and component replaced.