Vox AD120VT Problem

[imf]

Hey all... I should say, I'm not a tech and don't really know amp circuits or electronics all that well. But I have a DMM, soldering iron and determination to try and learn if that counts for anything. I can somewhat follow the schematics.

I have a vox AD120VT valvetronix from circa 2001. I liked it a lot actually, but after a few years it crapped out on me. I took it to a tech here on Long Island a while back, and he told me I shorted it out by dropping a pin onto the board (through the vents on the top of the unit). He wanted nothing to do with this thing is what I got from it... he told me go pick up a simple tube amp.

I have done some research (searched this board, which has a few threads on it) and some other forums, and I get why he didn't want to work on it... nobody does, it's basically a computer lol. I'll try to keep this short, I know you guys like it to the point here.

I took it out of the closet a week ago and turned it on. Everything lights up perfect as it did when I last powered it on. So for shits and giggles, I plugged in and took the effects loop send out of the amp into a small Roland cube 30 I play on for practice. It sounds great - so obviously the preamp section is fine.

I plugged headphones into the phone out - nothing just light humming.

The speakers and phone out have a low cycle hum which gets a bit louder as I move the power select switch, but overall it's not THAT loud.

The speakers are fine and there is no DC current on the speaker out wires. (I've read to check that from someone on this forum).

I have read that the headphone and other jacks can cut the signal it if the contacts are dirty. I have deoxit d5 on the way to try an clean all the jacks, but since the tech guy told me I shorted something.. I'm not counting on that being the problem. I never gigged the amp so it has no road abuse either.

I don't have a scope, but I can build a small signal tracer probe to see where the signal cuts out if that's worth it. Maybe I could check continuity between all the switching. Not sure where to start.

I opened this thing up and there is no burnt anything anywhere, looks clean as far as components, but I've also read the solder joints are garbage on this as well.

Just looking for advice on what I could/should check out first, as maybe it's worth investigating since it seems to be in the power section.

here's the schematics link that I'm looking at:
http://www.valvetronix.net/docs/AD12...l_Complete.pdf

[nickb]

Plug your guitar into the return jack and see if you get anything out of the phone jack and speaker.

If it works the return jack is the issue - simply plugging into it a few times is often enough for it spring back into life.

[imf]

The line/phone out jack sits right is fed from the return jack after a single chip buffer. Either the return jack contact is bad or the buffer is. Plug your guitar into the return jack and see if you get anything out of the phone jack and speaker.

If it works the return jack is the issue - simply plugging into it a few times is often enough for it spring back into life.

Hey nickb, thanks for checking it out.

I just tried plugging into the return jack, nothing. I plugged the jack in/out many times... still nothing. before I do anything I'll be getting the cleaner to try with also.

It seems the signal is gone maybe well before the line/phone jack, although I'm not great at reading this schematic. It seems the problem may start from Master Volume onward. Power Section is fed from the Master Volume W09 into CON1(R-IN) + CON2(L-IN) to the power PCB where it goes to the VN2410L's and on... looks like signal to the line/phone outs are after the valves right?

[nickb]

Standby... I looked at the AD100 schematic....

[nickb]

OK.. Ahem.. Looking at the correct schematic now

Yes the return jack goes thru the master volume VR15 and on to the Q3/Q4 and Q1/A2 differential pair amps. Now since this is stereo and both outputs are dead it suggests something common like a power supply. I suggest you check the voltage on the center tap of the transformers for about 218V.

[imf]

OK.. Ahem.. Looking at the correct schematic now

Yes the return jack goes thru the master volume VR15 and on to the Q3/Q4 and Q1/A2 buffers. Now since this is stereo and both outputs are dead it suggests something common like a power supply. I suggest you check the voltage on the center tap of the transformers for about 218V.

Yep, I'm thinking check the output transformers as well, will do. I was just looking them up, seems hard to find new ones (vox part# V904050013). Don't want to jump the gun though.

Thanks for looking it over, much appreciated. I'll update once I go digging later this week.

[imf]

Ok I tried cleaning the jacks and as expected, that wasn't the problem. I propped the PA board up and started probing around underneath to look for some voltages and I'm not getting much on the output transformers, not even a volt. These are PCB mounted OT's with 6 pins on each side so maybe I'm doing it wrong, I just checked each of the 12 solder joints on each one. The Tubes are glowing though if that means anything.

[The Dude]

If you don't have voltage to the output transformer center taps, check that 100mA fuse just off the orange wires of the power transformer. It should be near "TAG2".

FYI: I don't think Nick was saying your output transformers were bad. He was just wanting you to check voltage there. That voltage comes from the rectifier circuit off the orange wires of the PT.

[imf]

If you don't have voltage to the Output transformer center taps, check that 100mA fuse just off the orange wires of the power transformer. It should be near "TAG2".

FYI: I don't think Nick was saying your output transformers were bad. He was just wanting you to check voltage there. That voltage comes from the rectifier circuit off the orange wires of the PT.

Just checked it, those were the first things I looked at when I took the chassis out. They seemed fine but I just checked continuity on the F1 and got the beep.

[The Dude]

If the fuse is good, check the voltage on the cathode of D11 or D12 (same point). It should read about 216V.

[imf]

If the fuse is good, check the voltage on the cathode of D11 or D12 (same point). It should read about 216V.

Reading barely anything on D11 or D12, R84 gives me 195V right before them.

[The Dude]

Your meter is set for DCV and not AC?

[imf]

AC on R84, DC on rectifiers right?

[The Dude]

Yep.

[imf]

Yep.

ok yeah, 13.1mV on the cathode side (side with the stripe).