Fairly Ancient Carvin Amp Problem

[Runewalker]

My good buddy, has the 100W VTX-212, JBL version of this amp:
www.carvinmuseum.com/decade/80-guitaramps.html

An unusual amp that gets a pristine clean sound, and a bit ridiculous distortion sound (leave it off). It is a hybrid: SS preamp with 7band EQs, 2 separate and mixable channels, and a 4 tube power side output. This 'combo' amp is HEAVY, and I mean in mass. The JBLs are pretty incredible, almost high fidelity, with a tight defined and clear bass, balanced midtones, and a very high end, articulated treble.

At some point a local tech replaced the transformer. It has hummed ever since. That termonlogy is me not trying to be street cool (hopeless). I mean that obnoxious 60 cycle AC hum, that has steadily gotten louder over the years.

Also the right channel does not work. Dead. Nada.

Any ideas on rehabbing or testing this sucker in a way that will still leave us with both hands, as it is difficult to bar chord without them. We know practically nothing about a lot (as though it does not show), but especially amps.

RW

[UnklMickey]

runewalker said:

...At some point a local tech replaced the transformer. ...

which one? output or power transformer.

is the humming in the speaker, or is it "mechanical"? it happens sometimes when replacing the PT, that there can be a mechanical hum. usually due to a small gap between the chassis and the transformer. sometimes the transformer itself will hum due to loose laminations or ill-fitting covers.

if its in the speaker, power supply caps are the first suspect.

carvin has always been pretty good about sending out diagrams, when requested. so send away for a schematic before you even think about doing any work under the hood.

and of course, you will be working with about 400v so proper caution will be required to maintain functionality of all your body parts.



unk

[Runewalker]

which one? output or power transformer.

Ok, this is where the buttshow starts, again. Not sure, but will check with the owner.

is the humming in the speaker, or is it "mechanical"? it happens sometimes when replacing the PT, that there can be a mechanical hum. usually due to a small gap between the chassis and the transformer. sometimes the transformer itself will hum due to loose laminations or ill-fitting covers.

Happens regardless of extension speakers used. It does not sound like mechanical vibration. When it boots, you can hear the hum increase as the tubes come to warmth. It gets louder if you turn up the volume.

I think the owner already has the schematic.

If we could fix it inexpensively it would be a good aux. to have around, just not necessarily worth a big investment.

[UnklMickey]

runewalker said:

...When it boots, you can hear the hum increase as the tubes come to warmth. It gets louder if you turn up the volume....

the first sentence is not entirely conclusive. if it were a power transformer mechanical hum, it would increase as the tubes warm up and draw plate current.

the second statement points directly to "electronic hum"


if the power supply caps are an issue, that could still be getting back to earlier stages (before the volume control).

it does seem curious that this problem first appeared after a tranformer was replaced, then progressively worsened over time.

trying to do detective work by "remote control" is always a tricky business.

runewalker said:

.....If we could fix it inexpensively it would be a good aux. to have around, just not necessarily worth a big investment.

if you're lucky enough to find a 15 year old kid, that has some background with tubes, this sounds like a good project for him.

good luck finding one of those guys these days.



unk

[sumgai]

RW,

Also the right channel does not work. Dead. Nada.

One thing at a time, please. Let's get the humming issue resolved first, that could lead to "accidentally" fixing the dead channel problem.

..... It gets louder if you turn up the volume.

If the volume control affects the loudness of the hum, then the first thing we suspect is the voltage regulator circuitry that supplies the preamp stages. There should be at least one or two voltage regulators, probably not right next to the rectifier diodes, but several inches away, and they're probably mounted on heat sinks. Look around them for the associated electrolytic caps, which are at least 1000 mfd, and probably greater.

Visually inspect them for bubbling, cracks, loosened mounting(s), discoloration, anything that looks out of the ordinary. If they look OK, then set your meter to AC volts (auto range), and hook it across the regulator output. Better yet would be to use an o-scope, we're looking for massive ripple here, but a meter should also be able to tell you if you're seeing AC riding on top of the DC.

Either bad cap(s) or bad regulator(s), that's my bet.

HTH


sumgai