Overcooked amp?

Frank Freniere · Post by **Frank Freniere** » 14 Apr 2022 9:47 am

I’ve had a Vox Pathfinder 15R for about 15 years now. Solid state, 1x8”, 15 watts. It’s got reverb, tremolo, line out, headphones out, overdrive, lightweight: everything I could ever want in a club gig amp.

Lately, after about an hour or so, the amp starts to break up, gets dirty and distorted. What gives? Can it be repaired? They don’t make these anymore and I’d hate to say goodbye.

Thoughts?

Dave Grafe · Post by **Dave Grafe** » 14 Apr 2022 10:41 am

I wouldn't use it again until a good tech checks it out. It may be as simple as a failing tube or two, but it could also be power supply caps or other critical components which will cause a lot of damage if they fail under power.

Which part of the coast do you inhabit? I would be happy to look at it if you're in
the Hudson River Valley. Othetwise someone here will know of a good tube amp tech near you.

Jeff Highland · Post by **Jeff Highland** » 14 Apr 2022 12:55 pm

Solid state so no tubes- outside my expertise (LOL) but could be as simpleas failing electrolytic caps at that age, should be an easy fix for a tech.

David Nugent · Post by **David Nugent** » 14 Apr 2022 7:00 pm

Frank..You might try cleaning all of the input/ output jacks on the amp (especially any that do not get regular use). Dirty jacks will occasionally cause the problem that you describe. I recommend using 'Deoxit' that contains both LUBE and cleaner. Take a normal guitar cord and spray a generous amount of Deoxit on one of the 1/4" jacks, then insert and reinsert it four or five times for each jack.
Also, check your amp's controls for any scratchy pots. Sometime turning them full on to full off position and back a few times will clear up any static. This should be done periodically to avoid flat spots on the wipers caused by remaining at one setting for long periods.
Hope that this proves useful.

Frank Freniere · Post by **Frank Freniere** » 15 Apr 2022 3:38 am

Good advice - I'll try it. Thanks, David.

Robert B Murphy · Post by **Robert B Murphy** » 15 Apr 2022 5:45 am

Frank, I agree that the power supply filter electrolytic capacitors are the likely culprit. They can dry out over time and yours are certainly old enough. The supply voltage can sag and you can get signal clipping distortion and/or nasty 60 cycle hum. I doubt it will damage anything else like transistors in your amp and it's a straightforward job to replace them. Replacement caps of the same value and voltage rating have gotten a lot smaller so it's possible to get a larger (more micofarads) cap in the same space. Find some old fart like me with a scarred workbench, an o-scope and a weller soldering station and you'll be good to go for another 15 years.

I just looked at your schematic and the power supply puts out +/- 20vdc which is regulated down to +/- 15vdc to the transistors and op amps. There are two filter caps of 2200uF at 25vdc rating. Jameco lists a radial can at 79 cents apiece. It might be a little different for the axial can. You can go up on the capacitance and rated voltage with no problem but not down.

George Biner · Post by **George Biner** » 17 Apr 2022 8:06 am

I don't think degraded (but not shorted) electrolytic caps can cause distortion. Voltage sag would bring distortion, but degraded electrolytics will not cause a voltage sag -- their voltage is regulated by the power supply circuit upstream.

Can't hurt to swap them though -- just don't go too much higher on capacitance because the inrush current at turn-on may get too high.

Bad caps *can* cause buzzing because they are no longer filtering like they should.

Frank Freniere · Post by **Frank Freniere** » 22 Apr 2022 2:06 am

I used the Deoxit on the amp last night but I’m wondering now if the problem is the pot in my Goodrich volume pedal which I bought used a long time ago. For the second time now, about 2 1/2 hours in, my volume suddenly weakened. I had the VP flat and it wasn’t loud enough. And no, the other musicians hadn’t gotten louder. Next step is replacing the pot I guess.

Michael Brebes · Post by **Michael Brebes** » 22 Apr 2022 6:27 am

With solid state amps, the biggest culprit for intermittent problems after a warm up period is bad solder joints on the circuit board.

David Nugent · Post by **David Nugent** » 22 Apr 2022 3:20 pm

Frank..A bad pot will normally sound scratchy when going bad. If the volume is suddenly dropping, you might check to determine if the string is slipping on the pulley. Removing the rear panel should give you a clear view.