Vox AD60VT Valvetronix

[Mark Black]

Got this one in for blowing fuses and was pleasantly surprised when I quickly found a shorted bridge rectifier - quick replace and no more fuse-blowing. At this time I also did my customary check for shorts at transistors, diodes and such as well as inspecting and retouching a few solders.

Aah - but the mystery started when I hooked it up to the scope for an initial test. To my horror I saw full negative rail voltage at the output, but with low current draw (no-load). So I cut out all my freshly-replaced cable ties (a lot of them in these) and pulled the power section again only to find the problem gone.

To shorten things up a bit - I have found the problem will happen only when the power attenuator switch is set to the lowest power setting, and THAT is intermittent. The switch seems to be functioning just fine with pretty close to zero ohms where it should be even when the problem is happening.

Could anybody share even a partial schematic showing the circuitry associated with the power switching please? Apparently VOX/Korg is not fond of distributing schems except to authorized service centers (which of course I am not).

Thanks -
Mark

[Enzo]

The file is 9MB and I am on a dial up. I'll see if I can slice a page out of it, but the Adobe reader doesn't like to let me do that - no editing.

The impedance switch is the feedback switch, it selects one of two resistors or open. Remember the solid state amp has no impedance-ness. I suspect the open is the problem, not sure why I do. They are adjusting damping or something maybe

Make sure R66 and RV3 are not open. SHould be close together, and the wiper of the impedance switch connects to them. They are at the input of the power amp. Resistor is 62 ohm, and the twiddle pot is 220 ohm. Just a hunch.

[Mark Black]

Thanks Enzo for the reply. I am quite amazed at all the schematic requests you have answered here and indeed your level of general participation, especially now I know you are on dial-up (shudder - I remember well - I'm now on DSL and everybody tells me to go cable because DSL is so slow).

R66 measures about 37 ohms in-circuit (it's late here now so I will desolder and check in the morning) regardless of switch position although it is indeed marked as 62 ohms. RV3 however eludes me - could it be on another PCB? R66 is physically close to the output transistor section and RV1, RV2 (both marked as 10K) which I took to be bias controls based on location and circuit layout although the values seem a bit high.

Thanks again for your help - hope some day I can reciprocate (need any old ETA or NSI lighting schematics?).
Mark

[Enzo]

One end of R66 connects to the RV3, the other end of RV3 goes to ground. So the two parts in series are from input to ground. For all I know RV3 is replaced by a resistor on your board. Follow the traces. However since teh 62 ohm resistor measures half that, the parallel paths must be intact which means nothing open at either end of the resistor.

OK, I fished the page out. Send me an email with Valvetronix power amp as the subject line.

tmenzo at msn dot com

Wow, I haven't seen an ETA in a long time. I have a pretty fat file on them, though nothing newer than 1994. Used to work on some model controller a lot with various color push buttons and sliders. And of course dimmers. In my drawer is a little dimmer tester I made - just a box with a 9v battery and four push switches and a cable to plug into the analog inputs of the dimmer. Exercised th four channels of the dimmer on and off.

I have done a lot of NSI work too, much more recent. Some fancy boards, but mostly controllers in the 500 series - 508, etc. - and plenty of dim packs. Got documents too. Thanks though.

[Mark Black]

Thanks for looking into the connections, and thanks for the schematic offer (email sent).

The problem, whatever it ends up being, goes at least as far upstream as the pin-1 output of the 4556 chip which swings to full positive and stays there sometimes after switching to low-power setting.

Funny about the ETA test box. I've still got one hanging around too but went more elaborate with a power supply and copy of a channel circuit sending to all channel pins of the 6-pin DIN and 10-pin connectors they used on their dimmers. Haven't used it much in years but still every now and then see one of the ETA dimmers of that era so can't quite part it out...

[Mark Black]

Problem solved?

Looks like I won’t need that schematic after all (at least not for this one).

I am guilty perhaps of being overly cautious - I have done all this testing with no load or high resistance loads as I usually do with a suspect modern transistor output stage. Tonight just on a hunch I hooked it to my 8 ohm load and fired it up through a light-bulb limiter (just in case it got hairy).

The result was – nothing. No more glitches during power level switching and no more swinging to the negative rail at the lowest power setting.

Could it be these are unstable without a load? Not the kind of thing I’m used to seeing on transistor amps but then most things I work on don’t have power level switching of this sort either.

Crap I’ve wasted a lot of time (and wasted Enzo's as well – sorry) on this thing. Looking forward to moving on...

Thanks for all your help.

Mark

[Enzo]

This still bothers me, the amp ought to be OK without a load. I worry about an open ground reference for the input. If the speaker is always there, it might never matter. And if there is no speaker, the DC offset is not drawing excess current, so again it probably doesn't matter. But my spidey sense still tingles...

[picturevalve]

Hi guys, I'm new here and a friend has a advt60 head with a few problems - it suddenly drops off in volume. Could you possibly email me that section of the pdf with the circuit/schematic on it?
Thanks in advance for any help.

[picturevalve]

Thanks to Enzo for the pdf, much appreciated - especially the time taken for the send attempts.

[Enzo]

And good luck, I absolutely hate those things. I hate the convoluted circutry, I have the way they are put together.

[RussU]

I am troubleshooting a fuse blowing issue with my AD60VT. I see that some members have access to PDF copy of schematic. Would it be possible for me to get a copy of the schematic? Would greatly appreciate any help. I have been unable to find one on the web. I have high speed internet. My email is rustyund@verizon.net

Thanks- Russ

[picturevalve]

On it's way to you Russ, check your inbox. Edit it's 75% there atm - it's a 10MB file.

[Mr_Ken]

Picturevalve, What ended up being your problem? Mine is started doing the same thing. Thanks!

[picturevalve]

Hi Ken,
It's hard to remember now, there was another intermittent problem later on. I ended up changing the output power switch and resoldering just about every joint on the main board. It's a bit of a pig to work on with all the various connectors - make sure you label them. It's a great sounding amp but not really robust enough for life on the road imho.

[Mr_Ken]

I agree, it's not really something to haul around. But it has only gone out 3 times total. Once for a gig, twice to the shop.

They have issues, but it's a great band room amp for modeling. It actually helped me to decide between an AC30 and a Fender HR Deville. I ended up getting the Fender. But I still like the Vox to get other sounds to play with when the guys come over for a jam.

Just gotta figure out how to keep the kids off it and use the cheap 100w Peavey Transtube.

Oh wait! I did! The last one just moved out!

Thanks!