Vox AD60VT - I volunteered to help friend fix amp and am now stuck with just hum

[Vox Fan]

I volunteered to help a musician friend whos AD60VT was dead - and now I am stuck...

Seeing that the power amp pcb on his amp was cracked at the back (a lot of the retaining screws were missing - stress fracture) I ordered a new PCB, labelled all the connecting wires and then swapped in the new power amp pcb and plugged in the connecting wires - expecting this would fix it. On turn on, though, there was just a loud hum. So far I have tried:

* Changing the valve (I had some old ECC 82s from a hifi amp I once built) - result: no change
* Plugging headphones into the phone out jack - result: the same hum
* Unplugging the pre-amp from the power amp ('con2' on the power amp pcb) - result: buzzing just the same - so problem is in the new power amp PCB?
* Checking that I have plugged all the right wires into the right lugs/connectors (I took a photo at the outset) - result: everything is plugged in the right place as far as I can see

Now I'm a bit stuck - my friend needs his amp to work and I am not sure what to try next to hunt down the problem. Any suggestions please on next steps? (Inc the name of a good amp repairer in South London if you know one!)

Thanks very much


Chris

[Vox Fan]

Just a thought - old power caps?

In the post above, I called the replacement power/power amp pcb 'new' - but it was probably made the same time as the amp? - which would make the board more than 10 years old? Would that length of time with no voltage be long enough for the power caps to 'de-electrolyse' (sorry dont know what right word is)?

If so, can anyone tell me how best to check the caps when the caps are in situ? And could I fix just by leaving it on for a while so that the plate gets re-plated (I have a variac if that would help)? Or I could take the caps out of the old board and transfer to the 'new' board?

Any help/thoughts v welcome, and sorry for my ignorance! Thanks

[gearhead63]

My first question would be: has your friend been inside the amp? If so, has he disconnected/connected any of the wiring?
I ask because I have taken units in for repair and upon inspection all connections looked good. But after comparing to the schematic, I found that someone had connected wiring to the wrong board connectors.
Second, have you ran a cable from the preamp OUT into another amp and see if there is a proper signal there?
Looks like it will be the jacks labelled "LOOP". The send is from the preamp section, the return goes to the power amp section.
If you do have a proper signal at the preamp out, connect your cable from the preamp out to the power amp IN and see if it works right.
This way you narrow down the problem to either the preamp or power amp section.

[eschertron]

I had some old ECC 82s

Certainly you mean ECC83s? I'm not sure how the valve reactor engine likes valves other than a 12AX7. But still, probably not the problem.

My AD60VTX can produce some odd farts and buzzes, so I empathize. Have you cleaned all the switching jacks? That was a big problem for me. Also, have you tried cycling through the output power switch positions and impedance selector?

[DrGonz78]

Do you have a volt meter? First you need to determine if the hum is from a short on the new output transistor IC. The hum could be DC voltage on the speaker and that can kill your speakers. So disconnect them and measure with a voltmeter set to millivolts and then volts. Check the voltage reading across the speaker output terminals and post it here. If there is no problem with the DC Offset voltages then we can proceed with checking if the preamp is working on the amp. Also ECC81 ECC82 and ECC83 seem like similar enough tubes to at least pass the signal through the amp. The ECC82 I would think would just lessen in gain making the amp quieter in sound. I was under the impression that these amps would pass signal even with the tube volume turned all the way down. I remember working on an AD30VT and it worked like that exactly.

http://music-electronics-forum.com/t1046/
Check this link on the forum post #24 explains what part of the AD120VT schematic is shared with the AD60VT. Those schematics are a nightmare to follow for me.

[Vox Fan]

DC offset voltage and preamp test

Do you have a volt meter? First you need to determine if the hum is from a short on the new output transistor IC. The hum could be DC voltage on the speaker and that can kill your speakers. So disconnect them and measure with a voltmeter set to millivolts and then volts. Check the voltage reading across the speaker output terminals and post it here. If there is no problem with the DC Offset voltages then we can proceed with checking if the preamp is working on the amp. Also ECC81 ECC82 and ECC83 seem like similar enough tubes to at least pass the signal through the amp. The ECC82 I would think would just lessen in gain making the amp quieter in sound. I was under the impression that these amps would pass signal even with the tube volume turned all the way down. I remember working on an AD30VT and it worked like that exactly.

http://music-electronics-forum.com/t1046/
Check this link on the forum post #24 explains what part of the AD120VT schematic is shared with the AD60VT. Those schematics are a nightmare to follow for me.

Huge thanks. I just disconnected the speaker and measured the DC at the speaker output terminals (and AC, just in case)

DC = 28mv (varies c5%)

AC = .29v (I guess this is a pointless measurement - its just the hum)

On the valve, I wrote sloppily - apologies - I have actually tried 2 ECC83s and one ECC82 as well as the original 12AX7 - as things stand they all had the same 'loud hum' result.

I then checked the preamp using gearhead63's suggestion of plugging the AD60Vt's send output into my (working!) amp's power section - and it works fine, no hum (thanks gearhead63 - this was a really reassuring test).

I also have the schematic - but need to own up (no surprise) that I am no electrical engineer - I can manage a valve amplifier circuit, but anything with silicon in it might as well be magic for all I know.


Assuming you think 28mv is ok for DC offset (?), where would you suggest checking next, please? Thanks again

[Vox Fan]

Thanks very much gearhead63. No I havent checked wiring v schematic and I will do that - but have to admit to pessimism there as I am pretty certain my friend thinks that electrical stuff is far too scary to open.

I have just taken your idea of wiring the broken amp's 'send' to my own amp's 'in' and it works beautifully - no hum just a nice guitar tone. So I guess that means the hum is definitely coming from the 'new' power/power amp pcb (ie the pcb combines rectification and smoothing with audio power amplification)?

Huge thanks for the ideas - any more tests you could think of would be gratefully received. Thanks

[DrGonz78]

28mv is very good indeed and measuring AC is a good idea too, which shows not much AC ripple on output. So the preamp is working good and that means the problem is in the power amp, figured it was headed that way. Where did you get your replacement board and power IC? Is it used and bought through a Fleabay source? In other words do we feel confident the replace board and IC are in perfect working condition?

[Vox Fan]

Hi eschertron - thanks very much for the ideas - I am sorry I miswrote about the valve - I have actually tried 2 ECC83s, 1 ECC82 - as well as the original 12AX7. I have also cleaned the jacks - which was helpful as they were quite dusty, but the hum is still all the power amp is doing. Thanks again and thanks in advance for any other suggestions you may have.

[Vox Fan]

Thanks again. I should have said that the board comes direct from Vox, so I guess it is broadly sound (albeit not working...). The thing that occurs to me (wrongly?) is that Vox would have manufactured it whilst the Ad60VT was in production, which was more than 10 years ago? So, maybe 10 years sitting in a cupboard is long enough for the smoothing electrolytics to 'de-electrolyse'?

[DrGonz78]

Sure anything is possible in the long run, but I would still look into the idea that one of the connectors is not making good contact. Perhaps check continuity through all wires that lead to the power amp board. All it takes is one wire not making good contact or bad solder joint for the signal to get lost.

[Vox Fan]

Sure anything is possible in the long run, but I would still look into the idea that one of the connectors is not making good contact. Perhaps check continuity through all wires that lead to the power amp board. All it takes is one wire not making good contact or bad solder joint for the signal to get lost.

Thanks - sounds like good advice - i will get onto it. Thanks again

[Zipslack]

Thanks very much gearhead63. No I havent checked wiring v schematic and I will do that - but have to admit to pessimism there as I am pretty certain my friend thinks that electrical stuff is far too scary to open.

I have just taken your idea of wiring the broken amp's 'send' to my own amp's 'in' and it works beautifully - no hum just a nice guitar tone. So I guess that means the hum is definitely coming from the 'new' power/power amp pcb (ie the pcb combines rectification and smoothing with audio power amplification)?

Huge thanks for the ideas - any more tests you could think of would be gratefully received. Thanks

Have you tried the other way? Use a known-good amp as the preamp "Send" and route it into the Vox "Return". Send/Return loop jacks can get futzed and cause odd problems. Sometimes just removing/inserting a cord can free-up the gunk - if so, use some cleaner.

Also, I would say check all your Ground connections and check for cracked solder joints (great source of annoying static/pops/hum).

[Vox Fan]

Thanks all - case closed

Have you tried the other way? Use a known-good amp as the preamp "Send" and route it into the Vox "Return". Send/Return loop jacks can get futzed and cause odd problems. Sometimes just removing/inserting a cord can free-up the gunk - if so, use some cleaner.

Also, I would say check all your Ground connections and check for cracked solder joints (great source of annoying static/pops/hum).

Zipslack, Eschertron, gearhead and DrGonz78 - many thanks, you saved the day and my mate's prized amp. I think I would have panicked and tried something mad (like changing out the power caps). Your advice narrowed the problem to the poweramp - I then spent today going through the schematic and checking continuity as DrGonz78, Gearhead and Zipslack had said - and Con1 wasnt connected properly. Just reflowing the solder made it work, no hum.

Thanks

One last Q - there are a couple of trimmer pots on the pcb. Even after staring at the circuit diagram I cant see what they do (not valve biasing?)? Im not too worried as it sounds fine but is there a test point or something to put these to the right value, do you know?

Huge thanks again

[Vox Fan]

You saved the day and my mate's amp - thanks all. Your advice stopped me from panicking and doing something stupid. Instead you advised that I go through the schematic, checking continuity. I spent today doing that and found a dry joint - I reflowed it and the amp magically works. Huge thanks for calm advice!