AD120VTH silent on one side

[KevinS1701]

Yes, I know Valvetronix amps, especially this original Blue series, are brilliant designs badly cheaped out in execution, but I really enjoy mine. Going on 9 happy years, I have my first problem: No sound from the Left channel. No hissing or popping or static, not even background hum turning up the volume. Dead silent. Right side still works and sounds perfect. Swapped around cables and cabinets to confirm it wasn't something outside the amp. There is just no signal out either of the Left channel speaker jacks.

With the great ideas offered here and at voxamps.com and valvetronix.net, I have rounded up the usual suspects. I understand the jacks corrode easily and create lots of problems so first thing, I vigorously cleaned all jacks, exercising the switched ones especially hard. No change (except for surprisingly better brightness and clarity, which shamed me for letting them get so bad!). The Power Select rotary switch is a common problem but this one seems to work and feel right.

In an attempt to narrow down where the problem might be, I connected up the Effects Loop with that of another amp, and I'm not sure what I learned. Signal in, from the "good" amp's FX Send to the AD120's FX Return, yields no sound, in either channel. But signal out, from the Vox's FX Send to the other amp's Return, does transfer a signal, in BOTH the Right and Left channels. So the Left channel is apparently working as far as the Effects Loop. I note that coming out from the Vox's Left channel its own speaker remains active but plugging into the Right channel mutes its own speaker output. There's some switching going on and I don't know if this is how it's meant to work. (I tried running jumpers between its own Send and Return, just to fill up all the sockets, but no help.)

So I'm about done, unless anyone (Enzo?!) can make further suggestions. And then I'll take a recommendation for a Los Angeles/Orange County area tech who has been inside Valvetronix amps and lived to tell of it. I've been to the nearest shop on the Vox/Korg website's list of "authorized" service centers and was not very encouraged.

Any ideas will be appreciated. Thanks. --k

[Chuck H]

If plugging into an effects loop return yielded nothing, and there is no sound whatever from that channel (ie: small hiss or hum) I would say there is no power from the power amp. Bad output transistors or a bad power supply for the output transistors. It would be helpful to have more info from inside the amp. Voltage reading and such. But if your asking for a tech right off I'll guess that you don't want to take it apart. I can't imagine a tech is hard to find in your area. I would probably call the music stores and ask who they think is good. Call any tech that gets mentioned more than once.

[g-one]

Here's a link to the service manual: http://valvetronix.net/docs/AD120VT_...l_Complete.pdf
Block diagram is on page 2.
Some of the statements you have made don't add up. There is a stereo FX loop, L&R sends, and L&R returns, is this correct?
The return jacks are before the master volume. When you send signal from the other amps send to the R return jack, with master volume up, you get nothing? Are you sure the other amp is sending ok?
"I note that coming out from the Vox's Left channel its own speaker remains active but plugging into the Right channel mutes its own speaker output." Please be more clear about this statement. Do you mean the right speaker in both cases? You said there is no sound from the left side so how would the speaker remain active?

[KevinS1701]

Chuck H: Thanks for your thoughts. I did pull the chassis to look for obvious signs of distress, a broken connection or blown fuse. Nothing apparent. And beyond that, I'm not up for going! I'm sure there is a good tech around here. Just wondering if a name surfaces, as you suggest.

g-one: Thanks for the manual link.Yes, the AD120's FX loop is stereo, L&R Send and L&R Return. The Fender's FX loop works fine on its own, so I'm sure there is signal there.

But let's try to clear up what's next. Your right, it DOESN'T make sense.

Yes, it's the Right speaker I'm referring to. (The Left remains silent through all of this.)

Taking signal out of EITHER the R or L Send does in fact get sound over to the Fender amp. So both channels are alive to that point.

But the AD120's own speaker (Right only) mutes when I plug into the Right Send jack, while it stays alive when I plug into the Left Send jack.

Thanks to your question I poked around further and I find plugging ANYTHING into EITHER of the Right channel Loop jacks mutes the right speaker. Plugging into the Left Send or Return leaves it working. What's going on? Of course, all the Loop jacks are switched, so as to pass signal when the Loop is not in use, and so I could imagine a bad jack (switch stuck "open") in the Left Loop killing sound on that side. But what's with the Right Send and Return muting the speaker? (That would account, though, for no sound from the AD when taking signal in the Loop from the Fender: the Left is silent and the Right mutes.)

I hope I'm describing this more clearly. But I admit, I'm not yet grasping the logic of the symptoms. --k

NEW EDIT: Okay, okay, don't all laugh at once.

OF COURSE the speaker "mutes" when plugging in to one end of its FX Loop and not both. I'm using the AD120's Send and Return Loop jacks as, effectively, Preamp Out and Power Amp In jacks, for the purposes of trying to isolate where the problem is in my dead Left Channel. All these jacks are switched, so as to pass the signal when the Loop is not in use, and when I plug into the Send jack to run into another amp and check for signal, I have broken that circuit and not re-closed it by bringing the signal back with a plug in the return jack. So it works in the slave amp but the AD's own speaker (on the side that is working!) goes quiet.

Got it. Sorry.

At least I have learned, I think, that the Left channel problem is somewhere downstream of the FX Loop. --k

[Chuck H]

I think, that the Left channel problem is somewhere downstream of the FX Loop. --k

Almost certainly. Your observation of no sound whatever (small white noise. Hiss or humm) is very telling. No amp is utterly silent unless it's not working. So this tells me that the final, ie: the power amplifiers, are not working for one reason or another.

[g-one]

Well that makes more sense, the sends and returns are both switched. So plugging into the right send disconnects the signal. If you use a patch cord and then plug the other end into the right return, the sound should come back. Try it. If so, it implies that a signal plugged into the return jack should produce sound. Now try your guitar straight in to the right return. What do you get?

[KevinS1701]

Correct. Guitar straight into Right Return produces a (low level) sound. Plugging into Left Return is still nothing.

So there's some progress. I now understand how the FX Loop jacks are wired, and the Left channel issue is clearly somewhere after the Loop. Likely in or around the power amp as Chuck H surmises. Voxman over at voxamps.com says after corroded/sticking jacks, a dry solder joint in a critical connection is the next most common fault on Valvetronix. It's headed to a tech as soon as I find one I trust.

Thanks, you guys. --k

[g-one]

That's good, it does indicate that the send from the other amp did not have signal present.
I had thought there may be a problem with the muting circuit, but that mutes the signal at the send jacks, not the returns. The only other thing you could try is the L line out or headphone out. If you get signal there, then the left power amp may be blown. If no signal at left line or headphone out, the left power amp may be ok, and the fault may be between left return and left power amp (see block diag. page 4, power amp is called "current" amp).
Good luck getting it fixed, hopefully not major power amp failure.

[KevinS1701]

Just wanted to close this up, and include a recommendation for a Valvetronix-savvy shop, if anyone else needs one.

I finally decided to try Advanced Musical Electronics in Los Angeles. They are on the Vox/Korg list of authorized service centers, but I had already discovered that being on that list did not in itself mean much. However, they seemed to know the Valvetronix when I contacted them, so I took it in. They charged nothing for an initial diagnosis, then came back with a quote of $140 to fix the dead left channel and do a general service and cleaning. I said yes and gave them half up front (credit card number over the phone).

When I picked up the amp a week later, I paid the balance of an actual $137 total bill. They had found and replaced one bad resistor on the pcb, resoldered several connections, and cleaned all the pots, jacks, switches and plug-in connectors. Not only are both channels now working normally, but the amp sounds rich and brilliant in a way I'm not sure it ever did. I am again smitten by this thing!

So for anyone else looking to get a Valvetronix serviced competently, at least in the LA area, I can recommend Advanced Musical Electronics. The place doesn't look like much when you pull up, but they are comfortable rooting around inside these complicated amps, and that's all I wanted. They are at 8665 Venice Bl (just west of La Cienega) and can be reached at 310-559-3157 or emailed via their website at Advanced Musical Electronics - 29 Years of Service in Los Angeles. --k