Two Peavey VTM-120, same issue…

[Wintercept]

Hi all, started my tube amp journey this year fixing amps for my friends. It’s been a great learning experience and I’m on a winning streak, but my latest two patients have me stumped.

Two Peavey VTM-120 of many my friend bought from someone hoarding them have the same issue. He said he was using each on separate occasions cranked to live/practice volume (pretty freaking loud), and suddenly the volume faded out and the indicator lights also shut off. He tried again later and he was able to play for another 30 minutes until the same thing happened again. I could never replicate this issue.

Now to me, volume fading out and lights off sounds like it lost power totally, as if the power switch was flicked. I also think not many power loss occurrences are recoverable without replacing something (like fuses).

I fired both amps up with the current limiter and all seemed good, switched it off for a play tested and they both worked, one sounds like crap tonally and I can see one of the output tubes isn’t lighting, will address that later. Got both out of their enclosure (what a pain that was!) and found the following:

Both amps: Original tubes and caps. Screen resistors measure good. B+ and negative bias measure perfect, all power transformer taps are working and the voltage is going where it needs to. I cleaned all pots and jacks, reseated the output tubes and all molex/ribbon connections for good measure, and tightened all the hardware.

Amp A (bum output tue) was definitely never opened or serviced. It is built in 1987 with an 1986 pre-revision preamp board (factory revised with ceramic cap tacked on. Filter cap board is beige material. The hot melt glue holding the filter caps dried up and released, and the caps wiggle a bit. Powering the amp on and poking the power board caps and miles connectors towards the big 10w 400ohm resistor makes a little sizzle sounds, makes the current limiter bulb flicker brighter. All tubes seem pretty microphonic.

Amp B has been opened (chewed up chassis screws ) and modified with parallel FX loop jacks inside the chassis. It is shoddy work and will need to be addressed but I have ruled it out as a suspect for the main issue. It is 1988 built, 1987 revised preamp board, and green filter cap PCB. No microphonics at all. Prodding the filter cap board the same as Amp A yielded different results, only faint sizzling by the bias circuit, but I wasn’t sure if that was just the PCB creaking.

I pulled both filter cap/power boards and inspected all solder joints. I see no cracks, looks like it was nicely wave soldered (not sure if that’s a thing back then, but definitely not done by hand). None of the wiggling cap leads are loose from the solder.

So here I am, a bit stumped. My prime suspects right now are the electrolytic caps, being as old as they are. Maybe the ones in the negative bias supply are faulty, which I think would cause the lights to go out along with sound. Sorry for the wall of text, but I would appreciate any ideas or advice on how to proceed.

[maxkracht]

Powering the amp on and poking the power board caps and miles connectors towards the big 10w 400ohm resistor makes a little sizzle sounds, makes the current limiter bulb flicker brighter.

Physically large components, especially hot ones, are always suspect for bad solder connections or cracks in the pcb trace. It's not always visually obvious. The led indicators come after that 10w resistor, so that is a pretty good suspect. This issue might only happen once the resistor gets hot and things expand. It is good practice to reflow solder joints that might have mechanical stress. Probably wise to remove that resistor and make sure there is no corrosion or charring before resoldering. Also a good idea to make sure all of the molex and spade connections are clean and tight, and replace all electrolytic caps regardless of how they test.

https://schematicheaven.net/newamps/peavy_vtm_120.pdf

Edit: looked again, just the status led would go off without that resistor. Could also be a bad standby switch (only affecting status led) or anything on the mains ac side of things.

[Wintercept]

Thanks for the reply @maxkracht. I reflowed all high-mass component solder joints on the power board on Amp A. I am hesitant to remove that big resistor since it’s in there pretty good. I reflowed its connections and it measures fine when hot. I narrowed the sizzle down to the molex connecter for the high voltage. I pulled all molex connections and this time cleaned them with contact cleaner and repeatedly plugging/unplugging. Then I tightened the female pins for each connection. Sizzle is gone. These 40 year old caps have absolutely got to go, and that is what I will prescribe.

Any thoughts on the 1N4003 diodes in the negative bias circuit? They look and measure find hot and cold but I have some 1N4004 I could throw in for good measure.

[maxkracht]

Any thoughts on the 1N4003 diodes in the negative bias circuit? They look and measure find hot and cold but I have some 1N4004 I could throw in for good measure.

The 1n4003 should have plenty of margin. I would probably leave it alone, but no harm in replacing if it makes you feel better.

[nuke]

This is not one of Peavey's better executions of build quality.

Lots of mediocrity in that box. Though, this amp does have its fans for its power and tone.

The build includes a bunch of "punch-down" plastic connectors. The wire in the connector is punched down between two fairly sharp teeth that pierce the harness wire insulation and make electrical contact. These are used in wiring closets for ethernet panels, and work great --- in a non-moving, stable environment. However, in a tube amp that gets hot and cold, sits on top of a big speaker cabinet making loud noises, gets hauled in and out of the gig and goes for a ride every night in the trunk of an '84 Buick that needed new shocks in 1994, not a great idea.

The loss of power, including LED indicator would suggest a problem on the primary side of the power transformer. The US domestic models have a thermistor that limits inrush current. Those do sometimes, fail. (TR1 in the schematic) Even the power cords fail, and I don't remember if this amp has an attached cord or an IEC connector. The wiring is poo, the fuse holder is even poo. Check it all very carefully. Also, the power outlet in the practice room where the failure happened, could be at fault.

The ICC branded filter capacitors (TTA series) are poo, especially the grey ones. In recent years, they've switched to blue, but same-same capacitor. Just replace all four 100uf 350v and the 22uf 450v on sight. Order a tube of "electronics grade" silicone from Amazon, ASI or some other brand, to glue them down with. Electronics grade means it doesn't have acetic acid in it, unlike the stuff at Home Depot. Leave a little bit of a kink in the leads for stress relief and rely on the silicone to hold them

The DIP switches on the front panel are frequently troublesome as well. Replacement is the only real fix if there's a problem with it. The pots are subject to damage and wear. Careful inspection of the boards, especially anything heavy or hot or both, will likely need attention.

If it's been messed with before, you might find absolutely any form of insanity in there. Look anywhere that a human has been messing with carefully.