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.
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Ampeg V4 arcing
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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."Take two placebos, works twice as well." Enzo
"Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas
"If you're not interested in opinions and the experience of others, why even start a thread?
You can't just expect consent." Helmholtz
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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?
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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.
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Originally posted by melvin View PostIt 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.
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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.
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Originally posted by Prairie Dawg View PostOnce 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.
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.
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Originally posted by jrfrond View PostI 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.
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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?
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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.Originally posted by EnzoI have a sign in my shop that says, "Never think up reasons not to check something."
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