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

[g1]

(Forget about diode testing semiconductors, that is not important).

If you are referring to my post, this is just rude bullshit Jon.
He has already isolated the defective amp by swapping channels, so I'm not sure why you wanted him chasing down whether it is getting signal or not.
He already had done the diode checks on the transistors. Whether they were good or bad, any discrepancies in the readings compared to the good amp would have helped narrow down and isolate the fault within the bad amp channel.

[Jon Snell]

If you are referring to my post, this is just rude bullshit Jon.
He has already isolated the defective amp by swapping channels, so I'm not sure why you wanted him chasing down whether it is getting signal or not.
He already had done the diode checks on the transistors. Whether they were good or bad, any discrepancies in the readings compared to the good amp would have helped narrow down and isolate the fault within the bad amp channel.

I didn't read that the amplifiers were swapped to determine the fault.

Odd comment indeed. No I am not being rude or personal.

If the amplifier, of this design, is not burning up or blowing fuses, it is obvious that one must insert a known good signal into its input, after checking there is the required DC voltages on the three critical points.
This is a time saving way of fault finding that most experienced engineers do.

By all means take a possibly perfectly good ampifier off the chassis and remove the PCB to test all the semiconductors. Be my guest but what is the point if you haven't checked the obious, it is likely to cause issues; DC not totally discharged and a slip with the probe, that is recipe for disaster.
Havinfg succesfully repaired a number of these, most of the issues were bad connections between boards.

[g1]

By all means take a possibly perfectly good ampifier off the chassis and remove the PCB to test all the semiconductors. Be my guest but what is the point if you haven't checked the obious, it is likely to cause issues; DC not totally discharged and a slip with the probe, that is recipe for disaster.

I was asking about diode checks he had already done, as I mentioned.
If he is going to do damage using diode check with the unit turned off, I'm not so sure it would be more safe for him to do voltage checks with the unit turned on.
As you said your comment was not directed at me, then I do apologize for being overly sensitive, which is one of my many shortcomings.
He did mention in the first post that he had tried the amp board in a different head and it was still faulty. But, as you say, the connectors on the amp board itself could still be an issue.

[DrGonz78]

And the whole while I am reading this thread I keep thinking… I would much rather have a Marshall Lead 100 Mosfet 3210

[CamForrester]

Hi guys,

Jon, you are clearly onto something!
I checked the voltages exactly as you instructed, and found 50v DC present in all locations:

+ between Q28-R162 and Q26-R112
- between Q29-R154 and Q27-R133
And 0v in the centre - all exactly as per your diagram, so the amplifier is fine.

Following that, I plugged the left channel signal input connector lead (CON2) into the CON1 (right channel input) socket on the amplifier PCB, plugged the amp into the cab and hooked up a guitar to the input, turned it on, and found that the left channel input signal is definitely working also.

Finally, I bridged PA1-IN and PA2-IN with a red test lead (initially I wasn't sure where to find PA1-IN and PA2-IN on the board, as the signals don't appear to be quoted anywhere...) at the last point I could trace it before it gets to the SW1 'Power Select' switch on page 9 (the page you drew on, Jon), which is on page 8 - after the output transformers, and R34/R36 (for the R & L channels respectively).
Picture link (file too large to upload):
https://www.dropbox.com/scl/fi/vjugq...=f60njzoi&dl=0

I turned on the amp again, and UREKA! There is now sound blasting out of the left channel...but interestingly, now there is nothing from the right channel, even though the left signal connector is plugged into the right channel amplifier, as explained above (the right channel is not plugged in at all, because the wire is too short to reach the board when it's out of the chassis).

I have now ran out of time to look any further until this evening, as I start work soon and wanted time to type up this post - but does this result make sense to you, Jon?
May the fault be with R34/R36, or something before or after them? I quickly checked and they both give a similar reading in ohms resistance on the multimeter (0.6ish)...as I say, I've ran out of time for today, but please do let me know what your next points of interest would be!

Thanks so much,
Cam.

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.

[CamForrester]

They do by all accounts appear to be great amps, but equally these Vox amps are too...when they work properly!

And the whole while I am reading this thread I keep thinking… I would much rather have a Marshall Lead 100 Mosfet 3210

[Jon Snell]

Pleased to hear the amplifiers are fine.
"Was working, now isn't on the previously good channel", usually means bad connectors or dry joints. Carefull examination will find it/them.
Knowing these amplifiers, there will be a few to repair!

PS, check the jack sockets for bad contacts.

[CamForrester]

Hi guys - just a little update:

Jon - after some further cross-checking, I'm not sure I was actually linking PA1-IN & PA2-IN from the right place previously (R34 + R36), as what this actually did was transfer the output from the right channel amplifier, to the left channel output jack - the sound is not actually coming from the left channel.
This has clarified that the fault is not in the power select switch, or any of the left channel output circuit...so in other words - anything after the valve reactor transformer is fine.

I've since been carefully tracing the signal with the amp live, using test leads going into an old practice amp (a trick learned from repairing Hammond organ wiring connections when an oscilloscope is not available - preceded by multimeter voltage tests first to confirm no high voltage is present at the test points of course, and being VERY cautious so as to not slip with the probes and short something else out!) - negative to ground, positive to test point - from the CON2 input which we already know is working (and is now correctly plugged back into the left channel; using the right channel to compare/verify my findings), and moving along the circuit as-per the schematic (picture link below).

As you will see; the last point at which there is sound on the left channel, is C39 and R93 (the latter on the side closest to the transistor, after the voltage drop). Everything after that point is dead silent.
On the right channel, the next point at which there is sound after the transistors is R81 (the equivalent of R91 on the left side), so the fault is somewhere around here - it could be one of the several resistors in the area, or the Q3/4 transistors.
From the voltage tests I carried out, there was an inconsistency at R110 and R111 compared to the right channel; as I recall - R111 should drop 240v down to roughly 55-60v, and then give a similar figure across R110, but one side of R111 is reading in mv, and R110 the same, so I think the power to the transistors (or the circuit in general) is being cut by one of these failed resistors.

I have ordered an assortment set with all the required values as well as several more, so fingers crossed I'll be able to give some good news after they arrive! If not, next to order will be those transistors.

Wish me luck!
Cam.

https://www.dropbox.com/scl/fi/jr3u4...=bk98l2n1&dl=0

Pleased to hear the amplifiers are fine.
"Was working, now isn't on the previously good channel", usually means bad connectors or dry joints. Carefull examination will find it/them.
Knowing these amplifiers, there will be a few to repair!

PS, check the jack sockets for bad contacts.

[Jon Snell]

I would look at R111 for being open circuit or C70 short circuit.
The transistor pair are just a phase splitter. R111/110 sets a bias point for the fets. High impedance input on their gates so not likely to be them.

[CamForrester]

Fantastic, thanks for the info Jon!
I'll report back as soon as possible.

Also I feel a need to mention that for anyone who - like myself - is not a trained in electronics, and happens to come across this thread after encountering the same or similar issues with this model amplifier - please be aware that whenever probing/testing around high voltages, from the outset I have been - and you must also be - wearing rubber safety gloves and rubber-soled shoes, and also be turning the MAINS POWER OFF whenever moving test leads or multimeter probes from one location to another, to avoid electric shock or further damage to the amplifier. Also be mindful of residual currents around capacitors etc. even after mains power has been isolated - in some cases I have even been doing this with my partner present, who isolated the power for me when needed while testing multiple locations, to speed up the process.
Time consuming? Yes, but safety first!

I would look at R111 for being open circuit or C70 short circuit.
The transistor pair are just a phase splitter. R111/110 sets a bias point for the fets. High impedance input on their gates so not likely to be them.

[CamForrester]

Hi everyone, apologies for the dead air - I been trying to find the time to report back with an update:

After much scratching of the head, and several attempts to locate and rectify this fault, I've had to accept my limitations, and admit defeat unfortunately.

I replaced C70, R111 and R110, and the Q3/4 transistors...the left channel finally came back and was fine for a few seconds, but then started to break up, and after another 5 seconds went completely dead again.
On the next power up, the signal faintly came back immediately after switching on, but after a second or so, as the circuit current charged up to normal operation level, it disappeared again.

It seems there is a voltage issue somewhere in the vicinity of Q3/4, as I took readings of every component in the left channel all the way up to the transformer, then did the same with the working channel, matched them up on a spreadsheet and compared the readings against one another - and all of the inconsistencies surrounded this area.
Not wanting to waste any more money on buying replacement components that I'm not certain would fix the problem, I decided as a last-ditch effort to cut the power to the right channel by removing R98 and installing it into its equivalent (R107) position on the left channel (one of the suspects), as well as some others to see if I could correct the voltage problem, to no success.

There's clearly a chain reaction going on which I'm unable to make sense of due to my lack of knowledge, meaning I'm just blind-chasing components that are only one element of the issue, not the sole source.
I gave it a good go, but there's only so much time one can spend on something before considering more practical options.

I'm certain a professional would have been able to fix it - thanks for the offer Jon - but the cost of labour and postage etc. combined would have probably ended up being at least close to, if not more than the cost of buying a more reliable alternative.
Even a local amp tech I found who has experience with these told me: "Our labour is charged at £75ph...and knowing those amps, there's every chance the cost of the repair would outweigh its value."

In the end, I decided to go for a Marshall Mosfet 3310 (100W @ 4 ohms - a great match for the AD412 cabinets unlike most amp heads these days, which are 8/16 ohm), which I was able to grab dirt cheap at £150 - it's over 30 years old, still works perfectly and both channels sound great on their own (very similar to the earlier 3210 Lead 100 Mosfet, but with warmer low/mid-range which I prefer), but the clean channel also sounds great as a pedal platform too.
I've currently got an old Zoom MS50G I had lying around in front of it, with its Vox/JMI amp model engaged to give it that "chime"...I've A/B-ed it against the AC30 model on the working AD120VTH head side-by-side, and I actually think it sounds better...and that's before pairing it with a more sophisticated analogue modelling pedal, such as a Tech21 Liverpool or similar. I also prefer the mono 100W 4 ohm sound compared to the stereo 2x 60W 8 ohm.
After a couple of weeks living with the 3310, I decided I liked it so much that I grabbed another one (this time for £200), so now I shall just keep the AD120VTH as a backup, and use the 3310's primarily.

So to conclude; it's not the outcome I was hoping for, but equally I'll feel a lot safer gigging with these Marshalls, especially having seen how simple their circuitry is in comparison to the Vox!
I hope this account/experience is of some use to other owners of these Valvetronix amps - don't open them up unless you really know what you are doing...and consider all your options before forking-out for a repair!
Thanks to everyone for your contributions!

[DrGonz78]

I think you’ll end up very happy with the Marshall mosfet amp.