Vox AD120VTH Amplifier Head Issue - Left Channel Silent / Dead / No Sound

[CamForrester]

Hi Everyone!

I've got two AD120VTH & AD412 head/cab sets (bought essentially as props for an authentic Velvet Underground tribute project I'm working on; as a substitute for original Vox Super Beatles, which are impossible to find in the UK), but one of the amp heads' left channel has stopped working.
Obviously they aren't period-correct, but they are fantastic sounding, versatile amps that really look the part...so before either taking this to a repair shop, or looking for alternative (and/or more reliable) heads to use with the 4x12 cabinets if a repair is uneconomical, I wanted to see if anybody who has experience with the early-2000's Vox Valvetronix 'Blue' series amps might be able to help me solve the issue myself.

I have some basic experience repairing Hammond organ amplifiers and other vintage instruments such as Vox Continental organs, but modern PCB's are a little out of my comfort zone.

The left channel IS working as far as the effects send/return loop stage, but the speaker outputs, headphone, and line outputs are all dead - so the issue is somewhere in the power amplifier.
I actually went as far as swapping the faulty power amp PCB into the working head, and the issue was still present - thus eliminating the other components in the faulty head as potential causes of the issue (the two other PCB's, mains wiring and voltage transformer).
I have checked all the fuses and swapped out the valves, I've also checked all the resistors, transistors, and both of the valve reactor output transformers for continuity with a multimeter, and all of them appear to give a consistent reading against those from the working PCB, or at least as far as I can tell - I've been as methodical and thorough as can, but it's entirely possible I may have missed something along the way.
I also took a chance and replaced the two pairs of power amp transistors (which amplify the signal from 1W to 60W per channel) with brand new OE ones, but still no change.

At this point I think it best to consult those who really understand what they are looking at - I have posted a link to the correct schematic below.

Any help you might be able to offer would be much appreciated!

Best Regards,
Cam.

(I'm aware of the previous thread from 2012 on this subject, but the OP ended up getting it repaired professionally - a faulty resistor was mentioned, but unfortunately not specified):
https://music-electronics-forum.com/...nt-on-one-side

Vox AD120VTH Service Manual link:
https://www.scribd.com/doc/235702467...-Manual

[Jon Snell]

"I have attached the correct schematic"
Sorry you haven't.
Please post it and we can take a look at what your issue may be.

[CamForrester]

Hi John,
Apologies; it's not attached (I tried to attach it first, but it seemed that only pictures can be attached), instead a link is posted at the bottom, I just forgot to amend the text to reflect that!
Thanks,
Cam.

"I have attached the correct schematic"
Sorry you haven't.
Please post it and we can take a look at what your issue may be.

[g1]

Is this one the same as the scribd link?

(edit: removed wrong schematic)

[CamForrester]

Hi G1,
No that's for the AD120VT (combo amp version). The PCB and arrangement of the output jacks are slightly different to the AD120VTH head version.
Thanks,
Cam.

Is this one the same as the scribd link?

[CamForrester]

Hi guys,

Sorry, I forgot that the link for the service manual requires you to sign up (I had to make an account and then delete it straight after to get the manual).

I've instead uploaded it to my DropBox and made a public link which should work for everyone, without any need to sign up:

https://www.dropbox.com/scl/fi/xhooa...=zz9pud62&dl=0

Thanks,
Cam.

[g1]

To download the file, scroll right to the bottom of the pop up box that is trying to make you sign in, choose 'or continue with download only'.

[g1]

I've also checked all the resistors, transistors, and both of the valve reactor output transformers for continuity with a multimeter, and all of them appear to give a consistent reading against those from the working PCB, or at least as far as I can tell

How did you check the transistors, and did you compare readings to working amp?

[J M Fahey]

Hi Everyone!

I've got two AD120VTH & AD412 head/cab sets (bought essentially as props for an authentic Velvet Underground tribute project I'm working on; as a substitute for original Vox Super Beatles, which are impossible to find in the UK), but one of the amp heads' left channel has stopped working.
Obviously they aren't period-correct, but they are fantastic sounding, versatile amps that really look the part...so before either taking this to a repair shop, or looking for alternative (and/or more reliable) heads to use with the 4x12 cabinets if a repair is uneconomical,

Honest answer:

You chose a VERY complex, bordering on nightmarish design amp to cut your teeth on.
Simply stated, not practical.
Quite beyond even regular Techs comfort zone, to put it mildly.
I would send it straight to an Authorized Tech and let him deal with it, anything needed from component level repair to full bard replacement.

I wanted to see if anybody who has experience with the early-2000's Vox Valvetronix 'Blue' series amps might be able to help me solve the issue myself.

I have some basic experience repairing Hammond organ amplifiers and other vintage instruments such as Vox Continental organs, but modern PCB's are a little out of my comfort zone.

See above.

The left channel IS working as far as the effects send/return loop stage, but the speaker outputs, headphone, and line outputs are all dead - so the issue is somewhere in the power amplifier.
I actually went as far as swapping the faulty power amp PCB into the working head, and the issue was still present -

That does not look good.

I have checked all the fuses and swapped out the valves, I've also checked all the resistors, transistors, and both of the valve reactor output transformers for continuity with a multimeter, and all of them appear to give a consistent reading against those from the working PCB, or at least as far as I can tell - I've been as methodical and thorough as can, but it's entirely possible I may have missed something along the way.
I also took a chance and replaced the two pairs of power amp transistors (which amplify the signal from 1W to 60W per channel) with brand new OE ones, but still no change

Same thing.

(I'm aware of the previous thread from 2012 on this subject, but the OP ended up getting it repaired professionally -

Excellent decision.

[Jon Snell]

A particularily horrible digital side, luckily from your description so far. the fault is on the feed to the output stage.
Check the leads and connectors if the output stage and fuse is not taking a photograph of you.
Check for +-47v on the power transistors, 0volts on the emiters etc.
Forget about the bits that work; pre amps, digi bit and concentrate upon the signal coming from the many connectors.
The amplifier is a simple amplifier, the switching is confusing at first but straight forward.

If you are UK based I would take it in as a repair job as long as it hasn't been butchered.

[CamForrester]

Using a multimeter set to diode mode; I took readings of all transistors in the PCB, both ways round, and wrote the results down - the results are consistent (the circuit is copied) on both the L & R channels. I have not checked the transistors against the working amp (I had reassembled it by that time), but when I had both PCB's out on the table I did check all the resistors from the faulty PCB against the working one, and again all readings were consistent across both.

How did you check the transistors, and did you compare readings to working amp?

[Jon Snell]

What are your DC voltage readings on the amplifier as described in my last post?

"Check for +-47v on the power transistors, 0volts on the emiters etc."

Scope the input and check for signal. If none, swop inputs over and check for output.
(Forget about diode testing semiconductors, that is not important).

Is power where it should be? That is the first thing to check!

[CamForrester]

Well the first one I have had for almost four years, used as my main home/practice/recording amplifier. It's so easy to use, sounds great (the amp models are fantastic), and appeared to be completely reliable - so it seemed a no brainer to buy another for the tribute project, especially as there's no way I could find (or afford, even if I could) a pair of original Super Beatles, and that part of the goal with the project is to replicate the acoustics of the live sound arrangement the Velvets (and many other bands) used back in the day, which was 120W solid state guitar amplifiers on stage, with only vocals and drums going through the PA...However, shortly after the second amp arrived, the left channel went dead in the first - damn!

So now it's decision time; whether to either put my mind to fixing it myself, put money into getting it professionally repaired (with the risk that one or the other may break again in the future, and be equally complicated and expensive to fix), or instead buy two more reliable 120W SS amp heads for live use, and place them on the floor behind the Vox cabinets with the trapezoidal heads on purely for aesthetics.

I'm considering the Marshall Lead 100 Mosfet 3210 (old, but well-regarded for sound and easy to work on), or the Orange Super Crush 100 (new, and very well regarded for sound).

But before abandoning them completely I would at least like to try and attempt repairing them first. I just need some guidance on how to trace the signal, as I'm new to reading schematics...but I am keen to learn!

Honest answer:

You chose a VERY complex, bordering on nightmarish design amp to cut your teeth on.
Simply stated, not practical.
Quite beyond even regular Techs comfort zone, to put it mildly.
I would send it straight to an Authorized Tech and let him deal with it, anything needed from component level repair to full bard replacement.

See above.

That does not look good.

Same thing.

Excellent decision.

[CamForrester]

Hi John,

Thanks so much for your messages - I really appreciate your assistance!
Unfortunately I am working today so can't get to the amp to carry out the checks you have suggested, but I shall do tonight/tomorrow morning and get back to you asap with what I find.
In the meantime, could you tell me where to find the power transistors you've mentioned on the schematic? Or which "Q" number I'd be looking for on the PCB?
I'm assuming by power transistors you mean the pair attached to the heat sink, but I wanted to confirm first!

Also I don't have an oscilloscope unfortunately, but I will try switching the input connectors over. I'm not sure if the wire is long enough to reach, but if it isn't I will extend it.

Best,
Cam.

What are your DC voltage readings on the amplifier as described in my last post?

"Check for +-47v on the power transistors, 0volts on the emiters etc."

Scope the input and check for signal. If none, swop inputs over and check for output.
(Forget about diode testing semiconductors, that is not important).

Is power where it should be? That is the first thing to check!

[Jon Snell]

If you get as per the drawing, voltage wise, the amlifier is probably OK.
If you link with a test wire between points shown in green to test.
One channel works so both channels should work with a temporary link.