Ampeg V4 arcing

[melvin]

Anyone know what might cause this? It's a picture of a V4 where wires from the plates meet the board. It looks like there was a rather serious arc from one side to the other.

[Chuck H]

I don't have a V4 board layout in front of me to see exactly how those two contacts relate. I do see a lot of splattered flux and grime on that board though. I have seen old flux and grime create unwanted semiconductors before. Those two points seem far enough apart that no voltage created in an amp could spontaneously arch it without the benefit of some residual carbon. Not 100% but I would say clean the board (very well) and replace any burned wire/components and see how it goes.

[Prairie Dawg]

I've got a V4B on the bench right now and looking at it those wires run through a conduit to the primary side of the output transformer. The amp I've got has surge suppressor diodes in this area but they may have been bodged in by a previous person. It's possible your amp may not have them? The one thing that comes to mind could be flyback voltage? Perhaps operating the amp without a speaker load attached could have caused this lightning bolt?

[melvin]

It is possible that the ohm selection switch for the secondaries of the OT vibrated into a position that disconnects the speaker. I can certainly do it manually.

Also, I wonder if that Ampeg symbol that got torched was made of copper trace.

[Diablo]

It is possible that the ohm selection switch for the secondaries of the OT vibrated into a position that disconnects the speaker. I can certainly do it manually.

Also, I wonder if that Ampeg symbol that got torched was made of copper trace.

I doubt you can move that switch by vibration. My switch on the V4 is very difficult to move, and the chassis uses rubber Lord mounts to isolate any vibration.

[melvin]

With this switch it's hard to know if it's even in one of the three positions.

I didn't know these were engine mounts. That's interesting, it's a V4 with engine mounts...

[jrfrond]

Whatever the cause, you will need to excise the burnt portion of the PCB. My weapon of choice for the chores is a Dremel with a small HSS straight bit or diamond burr. My thought here is surface contamination, and Ampeg tended to leave a lot of flux on the PCB's in that era.

[Prairie Dawg]

Once you get an arc like that you've got a carbon track. I would do like John says, and then finish it off with Glyptal 1201 or a suitable insulating lacquer of some kind.

[Diablo]

With this switch it's hard to know if it's even in one of the three positions.

I didn't know these were engine mounts. That's interesting, it's a V4 with engine mounts...

In its day, it was the Hemi of amplifiers.

[rf7]

I've seen that exact same burn before on those amps. No fun.

[Prairie Dawg]

The one I'm working on has no subtlety at all. It's a sledge hammer.

[jrfrond]

Once you get an arc like that you've got a carbon track. I would do like John says, and then finish it off with Glyptal 1201 or a suitable insulating lacquer of some kind.

I usually use thin Super Glue on the edges cut area. Those boards are phenolic, which is kraft paper heated and compressed with phenol resin, so they burn pretty easily. Leave that carbon in, and you're gonna have another light show!

V4's are great amps, and I'll take one of those anyday over some of the crap that's being churned-out currently.

FYI- I've repaired holes that I cut out in PC boards with the old PlastiPair acrylic plastic repair system, using a lubricated piece of aluminum foil as a backing to hold the syrupy liquid until it cures.

[olddawg]

I usually use thin Super Glue on the edges cut area. Those boards are phenolic, which is kraft paper heated and compressed with phenol resin, so they burn pretty easily. Leave that carbon in, and you're gonna have another light show!

V4's are great amps, and I'll take one of those anyday over some of the crap that's being churned-out currently.

FYI- I've repaired holes that I cut out in PC boards with the old PlastiPair acrylic plastic repair system, using a lubricated piece of aluminum foil as a backing to hold the syrupy liquid until it cures.

+1 or 5 minute epoxy. Even bondo. Depends on where the hole is and how big. I've even sandwiched a couple of scrap pieces of board over the hole. That looks the best IMHO. At the end of the day though, as long as the amp works right, nobody really looks inside. I usually just leave the hole after the carbon is excised and jumber any traces with the appropriate insulated solid copper wire.

[melvin]

Thanks for helping me out. A hole that big should be shored up somehow. I'll see what I can come up with. I think I could find a lot of uses for something like Plas-T-Pair.

Would it be a bad idea to run the OT primaries directly to the pins on the plates and bypass the 10ohm resistors? What is their purpose?

[g1]

Those plate resistors are basically acting as fuses for the power tubes. Should one tube short, the resistor opens and the amp can continue operating with the remaining power tubes. The values changed from year to year depending on model (some use 5 ohm).
Bypassing them would be a matter of personal preference I suppose. The downside of having them there is
a)possibly not knowing you have a tube failure
b) having to open the amp and replace a resistor rather than just blowing the mains fuse.