Vox AD120VT Test Mode

[dmeek]

Does anyone understand the AD120VT Test Mode? The manual is hard to
understand.
I have one that passes signal but has a cyclical white noise/crackle
coming from the DSP, even with no input. Power supplies are clean.
I don't know that Test Mode will help me but it's my last hope.
The noise sounds like it's amplitude modulated by an LFO, to put it
in analog synth terms.

Doug


http://music-electronics-forum.com/a...1&d=1393786642

[teemuk]

Mu quick guess would be that you need to push those buttons either sequentally or simultaneously to toggle the various test modes...

[soundguruman]

a lot of them had
master volume (power control in rear of amp) was worn out. Dual 100K PC mount pot.
send / return jacks- change them to Cliff Jacks. The switch contacts fail. I would change the foot switch jack too.
Power amp board- re-solder the whole board.

That usually fixed it. Unless Rob spilled beer in it.

[Fragger]

I have a AD30VT would play for 10-20 minutes and would hear noise/static/hum come up in the background before the signal was modulated by a ramp wave.

Wouldn`t do it with all the different amp model selections. DSP chip effects would change with the wrong knobs.

When it was messing up I got in there with freeze spray and found it was the 7915? voltage regulator.

[Mick Bailey]

I know it's from the service manual, but the test procedure doesn't give you the table to look up the fault code that you get from the LED combinations.

For the 150 there's a comprehensive procedure that tells you the item being tested and what is displayed at each stage of the test. You can then see the NG code and refer back to the schematic. But that procedure doesn't apply to the 120 and I've never found the lookup table for that model

It's power-on test for all the models, but it wouldn't be any use anyhow.

[soundguruman]

In these amps, the grounding is quite poor.

The ground depends on plastic jack nuts, holding ground contacts against the chassis. They fail.
The metal oxidizes and the nuts come loose...

You are better off soldering a wire to the ground points.
Then connect those ground wires down to the chassis, with a ring terminal, screw and a nut.

After bypassing the pathetic ground points, that depend on plastic jack nuts...
Or bypassing the screw post grounds, that contact the circuit board...

You can usually get it to work correctly again. And it will keep working.

I can't believe what a poor job they did grounding the circuits.
I can't believe that they would design the grounding that way....

But after bypassing these, and hard wiring the grounds to the chassis, you can have success.

Typical of Chinese engineering. Designers who don't understand how a guitar amp should be built...

[dmeek]

I'm making some progress on this. I freeze sprayed the DSP board and noticed a change around the memory chip IC14. Looking more closely I noticed a blob of waxy substance covering some of the resistors beside the chip: R12,R123,R98,R132. I scraped it off and there is a big improvement. For a while the noise was gone but it's come back slightly. Maybe there is more gunk under the chip.
I also removed the power amp board to look for bad connections. One of the ext speaker jacks fell off, all 4 solder connections were broken. My first thought was bad grounds but I didn't find any.
Thanks for that info Mick, I guess there are some pages missing from the PDF copy.

[Mick Bailey]

I've never seen a copy of the service manual with the full test procedure for that particular amp.

[dmeek]

Success!!!
The "resistors" I mentioned earlier are actually 0 ohm jumpers. They route the address bus to the memory chip address pins. One of them, the one that was covered in goop, was open. Replaced with wire and everything is good.
Thanks SGM and Mick and all for your help.
Doug