Arcing Hot Rod Deluxe with no tubes installed

[Mr_bibbles]

Hey y'all,

I have a HRD on the bench that's giving me a headache, literally and metaphorically...

It's exhibiting a very loud crackling that sounds just like arcing with no power or preamp tubes installed when the standby is flipped. Other symptoms include filament bulb flashing in time with the crackling, limiter bulb as well. Relays will flip channels normally.

Things I have already checked;


1. The "lights off" test to visually inspect for any arcing, even flipped the board and powered it up with no luck.
2. Substituted two good switches for Power / Standby to check for internal arcing.
3. Checked the tube sockets for carbon tracing / arcing in the sockets.
4. Temporarily removed the flybacks on the power board
5. Re-soldered the cement relay resistors and surrounding area as a matter of course.
6. Removed the Transformer secondaries, got a good 300+ VDC out with a limiter installed
7. Checked Plate resistors in power section with MM
8. Removed Filament bulb to see if the arcing was related to the bulb-mount.

I'm beginning to suspect there is internal arcing in the Output Transformer?? That's a guess that I'm basing purely off process of elimination... Any old-heads ever experience anything like this before? Or see something I may be missing as far as testing goes?

Thanks!

http://www.prowessamplifiers.com/sch...rod_deluxe.pdf

[xtian]

I'm beginning to suspect there is internal arcing in the Output Transformer?

Disconnect OT primaries (or really, just the CT). Still trouble?

BTW, what wattage bulb is in your limiter? Over 40w will allow enough current to burn stuff...

[Mr_bibbles]

Disconnect OT primaries (or really, just the CT). Still trouble?

BTW, what wattage bulb is in your limiter? Over 40w will allow enough current to burn stuff...

<3 <3 <3

Disconnecting the center tap removed the crackling noise and the flashes from the limiter! I initially thought of removing the leads but figured it may present a false positive since the speaker would be disconnected... would you mind explaining exactly what we're testing here? I understand that a center tap is exactly what it sounds like, but why is it correcting our symptoms here?

Also Good tip about the bulb. I'm above 40W right now, ill look into a lower one.

[Leo_Gnardo]

I'm beginning to suspect there is internal arcing in the Output Transformer?? That's a guess that I'm basing purely off process of elimination... Any old-heads ever experience anything like this before?

Good guess, and yes do what xtian suggested. It's rare but does occasionally happen. I've seen OT internal arcing in 60's Ampegs mostly, but certainly not limited to those. If removing the B+ from the OT center tap eliminates the arcing, you could substitute, that is clip-lead in most any OT you have at hand to check if the amp behaves with another OT. And, if at that point arcing isn't happening any more, order up a replacement OT for that amp.

Curiously, those flyback rectifiers are supposed to help eliminate this kind of problem, if it does turn out to be arcing within the OT. So, if it does prove to be that, do replace those flyback rectifiers as well.

[xtian]

By disconnecting the CT, you're removing B+ (high DC voltage) from the OT.

Fortunately, these OTs are pretty inexpensive: https://www.tubesandmore.com/product...d-blues-deluxe

[Mr_bibbles]

By disconnecting the CT, you're removing B+ (high DC voltage) from the OT.

Fortunately, these OTs are pretty inexpensive: https://www.tubesandmore.com/product...d-blues-deluxe

I suppose I was expecting a complicated explanation but, it's more of a "DUH!" scenario!

I really appreciate your help on this one, and bonus points for the wattage / current revelations on the bulb too. Take care bud!

[Enzo]

Before we tear out the transformer, unsolder and lift one end of each of the "flyback" diodes, the diodes from the power tube plates to ground. One may be breaking down. That would be more common than a transformer failure.

[xtian]

I think OP already did that, Enzo:

4. Temporarily removed the flybacks on the power board

[Enzo]

I believe you are right, I overlooked that.

One other possible test, reconnect the OT CT, but pull the plate wires off the board. Now with no connection to the rest of the circuit, does the powered transformer still arc?

[Mr_bibbles]

I believe you are right, I overlooked that.

One other possible test, reconnect the OT CT, but pull the plate wires off the board. Now with no connection to the rest of the circuit, does the powered transformer still arc?

Thanks for your diligence, I'll try this test!

[Mr_bibbles]

I believe you are right, I overlooked that.

One other possible test, reconnect the OT CT, but pull the plate wires off the board. Now with no connection to the rest of the circuit, does the powered transformer still arc?

Kudos to Enzo, pulling the leads and leaving the center tap reproduced the arcing, so we know for sure that the OT, when supplied with B+ begins to arc internally, as there was no load or output attached. Great test method for OTs! I have rando-OT for testing but this saved me a ton of time.

Thanks again!