Vox AD100VT = Fart Machine!

[drewl]

Odd,
when you turn the master output power level dual pot up slightly the output section starts oscillating some nasty low frequency square wave.

Power supplies all look good, nice clean signal getting to the output amp chip until the pot is turned up a bit.

Anyone replace the IC4 brick on one of these?
It's an SSTK404-130S, one like you'd find in a home stereo.

I swapped out the preceeding IC3 dual op amp with the same results.

[Jazz P Bass]

Check the master volume pot.

I had one that would go 'open' when turned all of the way down, which caused the output tubes to red plate.

[drewl]

Although they do go bad often, this one is okay.

Having it mounted on the back of the amp, the dual pot is always getting banged and goes bad.

[drewl]

Damn, I'm getting my ass kicked this week.

Replaced the brick power amp, problem remains.

Pulled all the power supply filter caps and they all tested good, I will replace them and see if it makes a difference.
The plus 15v regulator is hotter than the neg 15.

As soon as you get to about 50w out, it goes into this square wave pulse output.
Doesn't matter what the various gain, tone or volume pots are set, as soon as it gets to a certain power out it starts going nuts.

This model is actually a VT100, so if anyone has the correct schematic that would help.


I got too cocky, angered the gods of electronics and their wrath has been set upon me this week.

[mozwell]

Here you go
There are lots of circuits & power supplies around the power amp block, check all of these are ok (including the power level circuit)

[DrGonz78]

Any major differences between AD100VT and VT100 as far as design goes? I see online a VT50 and a VT80+ schematic. Perhaps those might help? Both of those can be found here...
Guitar Amplifier Schematics and more

[Jazz P Bass]

A CPU and a DSP.

Wheee!

http://bmamps.com/Schematics/vox/Vox_VT80+sch.pdf

[drewl]

Thanks, yrs there are quite a few differences.

[drewl]

Ooh, looks like I found it!


During my testing, I had blown Q11 on the AD100Vt schematic.
Designation and layout are pretty different on this model.

So, I had replaced Q11, but since it had blown I focused on that area and sure enough D23- the 1N4148 on it's collector was breaking down when pushed.

It measures fine but was crapping out.

[southdade60]

Hi Techs,
I struggled with this problem for awhile until I discovered the glue that holds the flex cables to the power board becomes conductive & causes all kinds of weird problems int volume ,int sound, oscillation after warm up etc. removed glue from both flex cable ends at power board. Problem solved, Hope this helps.

[drewl]

Wow, a thread of mine from days of old!
Sure it wasn't just bad or corroded connections from the connectors?
But if it is the glue, nice find!
Thanks.