Vox Valvetronix AD120VT lights but no sound

[Yazman]

Hey everyone,

I purchased an AD120VT (with the "blue cloth") that was not in working condition, but had supposedly worked recently. The amp turns on, and all the lights come on, but there is no sound. I don't get noise from turning any knobs, except I get a click through the speakers from moving the "power select" knob. If I run a cable from the either the R or L "send" jack into another amp, it sounds perfect and works great (on the other working amp). I have tried patching a cable from the R send to R return, and L send to L return, but I still get nothing. I have tried running a cable from all the other jacks into another amp, but do not get anything except for when I use the send jacks. I have sprayed every jack 6 times with electric contact cleaner and shoved the cable in the jack about 100 times. I also replaced a T1aL glass fuse that the previous owner had wrapped foil around when it burnt out. I looked the board over as best as I could, and I can't see anything obviously wrong. I've also tried the 3 button reset to no avail. I've tried different power cords, different guitars, and different instrument cables. I've spent the last week reading every thread I could find on this topic, but I am still at a loss as how to proceed. I do have a multimeter if that helps. Any ideas would be greatly appreciated.

Thank you

P.S. I have posted a link to the schematics http://www.valvetronix.net/docs/AD12...l_Complete.pdf

[The Dude]

The secondary fuses are T1.6A, T100mA, & T630mA. So, I'm presuming you replaced the primary fuse? Fuses blow for a reason, and it's not a good idea to wrap a fuse with foil. I'd start by measuring your supply voltages and see if the supplies are up and working as they should be. Also, make sure none of your secondary fuses are blown.

[Yazman]

The secondary fuses are T1.6A, T100mA, & T630mA. So, I'm presuming you replaced the primary fuse? Fuses blow for a reason, and it's not a good idea to wrap a fuse with foil. I'd start by measuring your supply voltages and see if the supplies are up and working as they should be. Also, make sure none of your secondary fuses are blown.

Thanks for your reply. I checked all the secondary fuses, and they all appear to be in good shape. Oddly, the manual states the amperages as you have listed, but the PCB board itself has the fuse amperages stamped on the board at slightly different values than above. And yes, I was disappointed to see that someone had treated the amp so poorly as to use foil to bridge a blown fuse. This amps had a rough life and I don't know what other surprises I might find...

When you say measuring the supply voltages, I'm assuming you mean measuring the voltage coming in from past the on/off switch?

Thanks again,

Rich

[eschertron]

A fuse blown suggests that there is an actual problem that needs to be fixed. having said that...

I have the 60W (mono) version of that amp, and one issue I did run into with it was after using the headphone out - which mutes the amp's speakers - I couldn't get the sound to work until I'd exercised the headphone jack a few times. You mentioned send and return jacks, I thought I'd add the headphone jack to the pile. It's not in the audio path, but has a switch that affects power amp muting.

[Enzo]

Power supply does not refer to the mains from the wall outlet. The internal circuits run on voltage s like +5vDC, +15vDC, -15vDC, likely 3.3vDC, maybe 12v or 24v. Whatever. Those are your power supply voltages.

[The Dude]

Ok, so the circuit board fuse values are different. Which fuse was replaced, then? That may be a clue. On page 4 of the schematic, you can see all of the separate supply voltages labeled on the secondary. Measure each of those and report what you find.

[Yazman]

Thanks again everyone for your replies.

The board has T125mAL for the F1 spot (orange), T1.6AL for F2 (blue), and T1AL for F3 (red). This doesn't match up with the schematic at all, as the schematic doesn't even have a fuse listed for blue, and I don't see a green set of wires with a fuse. In any case, the voltage on the red connectors is 9.6VAC on one side and 10VAC on the other. For the blue, 34.8VAC on both sides, and 196 & 192VAC on the orange connectors.

I got the amp to work after replacing the 1.6A fuse. It looked fine initially, but when I first put the voltmeter on it, one side read 0.2VAC and the other side 191. I then removed the fuse and saw a teeny, tiny brown spot. I replaced the fuse and everything fired up and sounded great! I guess this is a lesson to me to always check fuses by pulling them or by checking with a voltmeter.

Now I'm wondering why the fuses (1.6A and 1A) blew in the first place. I have no idea what the history of this amp is, but I have a feeling it's been abused. Could the fuse have blown from someone starting the amp with the volume cranked? I'm hoping this was just a fluke.

Thanks again to all of you, I wouldn't have figured this out without all of your help.