Vox Cambridge Reverb V1031 non-polar electro caps

[DrGonz78]

Okay one more important note to add. I just had the idea to disconnect the volume pot output wire from the board to test the preamp that way. It sounds fantastic so that for once and for all puts my idea that it is a bad pot to rest. The buzzing sound only occurs in circuit, so it is not the preamp. I am still scratching my head as to why this is occurring. The next part of the circuit is the reverb part so that might be what is the problem. C11 is new just to note.

[R.G.]

At this point I would scan and print a second copy of the schematic, then start measuring the idle voltages on all the transistors' pins one by one. I'd be willing to bet a certain amount of beer that one or more of them are way off from the "conducts signal well" voltages. If you're truly in a hurry, just do this for the two transistors in the preamp section.

I would write all the transistor pin voltages down on the second schematic and then start thinking about transistor biasing and output swing. Also measure the power supply voltages.

I'd be happy to help guess at causes if you want a second opinion on the voltages.

[DrGonz78]

Okay now this is getting interesting... All B+ 27v and 17v lines are present and healthy. I took measurements on all the transistor legs at idle.
Q1 E= 1.57v B=2.1v C= 17.6v
Q2 E= 900mv B= 1.57v C= 8.65v
Q3 E= 165mv B= 777mv C= 2v
Q4 E= 1.6v B= 2v C= 26.4v

So this makes more sense to me now that Q3 is not getting it's voltages on the collector, likewise Q4 on the base. I measure R15 one side is at 17.5v and on the other is that 2v where is should be 11.5v. Not to mention the 1.6v on the emitter of Q4 which should read 9v. So, what does this mean? I can only guess that is means bad R15 resistor, bad C12 or one of the Q3 or Q4 are bad. Also, I got to note that from the collector on Q3 there is a ceramic cap 680 written on it (perhaps .0680 or .0068 value) connected directly to the positive leg of C13. That cap is not on this schematic and I did replace C13. C13 is installed correctly in regards to the polarity of the cap.

Also, what are the jobs of Q3 and Q4? Can I just bypass the whole reverb circuit from C11 to the reverb pot output area? Not wanting to do that really, but just wondering.

[R.G.]

Okay now this is getting interesting... All B+ 27v and 17v lines are present and healthy. I took measurements on all the transistor legs at idle.
Q1 E= 1.57v B=2.1v C= 17.6v
Q2 E= 900mv B= 1.57v C= 8.65v
Q3 E= 165mv B= 777mv C= 2v
Q4 E= 1.6v B= 2v C= 26.4v

OK, I remember a bit more. The reverb driver and power amp driver are one of the places where the Vox service lit is either strange or wrong.

I popped the circuit into a simulator to see where this would land, and I get:

             E       B        C
Q3     1.3      2.0      7.3
Q4     6.6      7.3      26.2

So this makes more sense to me now that Q3 is not getting it's voltages on the collector, likewise Q4 on the base. I measure R15 one side is at 17.5v and on the other is that 2v where is should be 11.5v. Not to mention the 1.6v on the emitter of Q4 which should read 9v. So, what does this mean? I can only guess that is means bad R15 resistor, bad C12 or one of the Q3 or Q4 are bad. Also, I got to note that from the collector on Q3 there is a ceramic cap 680 written on it (perhaps .0680 or .0068 value) connected directly to the positive leg of C13. That cap is not on this schematic and I did replace C13. C13 is installed correctly in regards to the polarity of the cap.

There are a few possible interpretations. One is that Q4 is open. That could happen easily enough if the transformer had a flyback incident that popped the collector lead. I think R15 and the caps are OK, but Q4 is suspect. I would suspect the transformer or resistor in parallel to it if you didn't get 26+ volts on the Q4 collector.

I believe what's happening is that R15 is letting through all the current it can to the base-emitter of Q4, but no transistor action is happening and raising the Q4 emitter. Or very little transistor action. It's worth trying to replace Q4 if you have a suitable device.

And that's an issue. In my looking for replacements for Thomas Vox stuff, the type for Q4 is one of the few I didn't find replacements for. At a guess, you need a few watts of dissipation and at least 100V BVceo. I'll go look for a transistor.

Also, what are the jobs of Q3 and Q4? Can I just bypass the whole reverb circuit from C11 to the reverb pot output area? Not wanting to do that really, but just wondering.

Q3 does two things. First it drives Q4 so Q4 can drive the signal through the driver transformer to the tank. Q3 also produces a buffered "dry" signal at its emitter for the blend of dry and delayed that happens in the reverb control after the recovery amp.

[R.G.]

As to driver transistors;

the only one I find at Mouser that might be the same package is the 2N5682 in a TO-39 package, for $4.50. It ... might work.

Most of the good candidates are in TO-126 or TO-220 packages. You can get similar specs in a KSC2690AYS for $0.58, about 1/9th of the metal package cost and probably better at getting heat out of the chip.

[DrGonz78]

Okay I replaced Q4 with 2n5682 and no change.

Q4 E= 1.6v B= 2v C= 26.4v

Replaced Q3 with a 2n3392 and no change. I was extremely careful to get the right pinout on both transistors, so that can't be the issue.

Q3 E= 175mv B= 800mv C= 2v

The 2n3392 is sourced as a good replacement on RG's Vox transistor substitution documents.

Let's say the owner of the amp never wants to use a reverb unit on the amp. Do you think it is possible to just bypass the reverb circuit completely? Do I need that Q3 as a buffer to the final pre-driver output stage? At this point I do want to figure out the reverb circuit problem, but lot's of labor on something that might not be necessary.

Also, any idea where/why my voltage is weening away after R15? Thanks as usual.

[R.G.]

Okay I replaced Q4 with 2n5682 and no change.
Q4 E= 1.6v B= 2v C= 26.4v
Replaced Q3 with a 2n3392 and no change. I was extremely careful to get the right pinout on both transistors, so that can't be the issue.
Q3 E= 175mv B= 800mv C= 2v
The 2n3392 is sourced as a good replacement on RG's Vox transistor substitution documents.
...
Also, any idea where/why my voltage is weening away after R15?

OK, I wrote a whole reply before thinking to myself, something is just not right here. Got the simulator back out and reentered the circuit. It acts different. I think I hosed up my first sim run.

I've been back to it three times now, and I think I have it right now. Bottom line - your voltages are not all that bad. The schemo is wronger than I thought. I also checked the voltages and such on the other models of the amplifiers, and it's in line with my revised thinking here.

First, Thomas uses an abysmally low internal signal voltage in these amps. It's only 25-50mV, actually smaller than guitar level. This lets them get away with certain tricks in the effects circuits, I think. So the emitter follower output really can be OK with a DC bias of 100mV on the emitter of Q3. I ....think.... that your base voltage on Q3 is really higher than 800mV, and that your meter is dragging down the base. Try measuring from base to emitter on Q3 instead - this is a lower impedance measurement and easier to get closer on circuits like this.

In any case, if you wanted to make this thing more like the others in the line, you could change the 330K to 470K, which raises Q3 emitter, and change the collector resistor from 27K to 12K-15K, which gives more base current to Q4 and more emitter current to Q3. But it may not be needed.

If you can see the input signal on Q3 emitter at about 50mV with volume up full, It's probably working OK. The transformer output to the tank connector should be loaded with about a 1K-2K resistor, or just short the primary of the transformer if you don't have a tank to plug in.

Let's say the owner of the amp never wants to use a reverb unit on the amp. Do you think it is possible to just bypass the reverb circuit completely? Do I need that Q3 as a buffer to the final pre-driver output stage? At this point I do want to figure out the reverb circuit problem, but lot's of labor on something that might not be necessary.

Sure. Just lift the cap after the volume control, lift the cap at the power amp input, and patch the power amp input cap to the wiper of the volume pot with a wire. Q3 buffer isn't needed for the power amp... I think. It would be good to parallel C14 with a 1K or lower to keep the recovery amp all neutered.

But if the transformer, cables and tank are not hosed, you may be very close to a working unit.

[DrGonz78]

Good news!! I had only looked at the amp as far as voltages were concerned last night and being late I did not get to plug it in to hear it. Well, it's working great now! So one of those transistors were buzzy/leaky or something of that nature. I had also read something where a guy was working on reverb for a similar circuit and had low readings on the reverb drivers transistors. So now to impress the customer I am entertaining the idea of finding a reverb replacement tank. I have a nice Accutronics on hand 8BB2C1B 190ohm input 2575ohm output. I don't think that tank is gonna work in this situation... I am thinking something like a 4AB3A1B as I am wondering if the input is 8ohms? Also, makes sense to have 2250ohm at the output, I think.

Anyway thanks RG your advice has helped me and many here to come, very much appreciated.

Edit: One minute later I read that the recommended replacement is 4FB2A1C...

[R.G.]

Congratulations! Good work!

Glad you found the recommended replacement. I would have guessed something like 1K-2K for the input side of the tank. Lower than that is really intended for either line level drivers (200-600 ohm stuff) or speaker level drive (8-16 ohms).

There were two versions of reverb drivers in the "big head" section of the Thomas Vox line. One was the transformer style you see here and the other was what I'd call a "line level driver" circuit, a kind of mini power amp. In my first Beatle head, the 200uF cap on the equivalent to the Q4 emitter was bad, and it made the driver oscillate. The squealing went everywhere. It took me a week of evenings to run it down to that cap, and it was only due to easter-egging that I found it, not clever, insightful circuit analysis. This was the start of my "replace all the electro caps" crusade.

So now you're part of the very rarified few people who have successfully repaired a Thomas Vox amp. I get panicked emails all the time looking for a tech who will do that. You want a career? Advertise that you both can and will repair them.

[DrGonz78]

Well unfortunately the customer wants to put in a reverb tank but is saying he'll do that later. So onto terminating the input/output leads of the transformer...

Just clip one 1K across each of the plugs going to/from the missing tank. The output cable is then terminated in 1K, which is an OK-ish match to most reverb tanks and won't drive your driver transformer or its transistor crazy, and the 1K across the input keeps hum and noise down. You can short the cable from the output of the missing tank, but then you'd have to know which cable was which; with two 1K resistors, you can treat them equally.

So if I understand this correctly... We want to take two 1k resistors and put them from ground to the input/output terminals, one resistor on each side. I know the output from tank has a direct connection to ground so that is fine. The reverb transformer has the two wires 26v on it's primary and a red & black(ground) terminal on it's secondary. So do I just stick a 1k across secondary side of the transformer red(input to tank) to black(ground)?

[R.G.]

Well unfortunately the customer wants to put in a reverb tank but is saying he'll do that later. So onto terminating the input/output leads of the transformer...

I tell you - it's always the humans that are the problem!

So if I understand this correctly... We want to take two 1k resistors and put them from ground to the input/output terminals, one resistor on each side. I know the output from tank has a direct connection to ground so that is fine. The reverb transformer has the two wires 26v on it's primary and a red & black(ground) terminal on it's secondary. So do I just stick a 1k across secondary side of the transformer red(input to tank) to black(ground)?

Part of what to do depends on how much you like the customer.

This thing has cables that end in RCA plugs, right? The neat thing to do would be to grab some old panel-mount RCA jacks and solder a 1K across signal and ground, push them onto the RCA-end cables, and wrap the ends up in heat shrink or tape. This makes putting a tank in super-easy when the customer gets around to it later. And it might save you from pulling it open again if you already have it buttoned up. You could also use a surplus RCA extension cable that has a couple of females on the cable. Cut the cable, solder a resistor from center to shield, wrap in heatshrink or taps.

Otherwise, if it's open or simply easier, yes, just solder a 1K across the secondary of the driver transformer at any convenient place, and also across that little cap on the PCB. Or where the RCA cables connect to the PCB.

Your customer *does* understand that with this done, the reverb knob is now an auxillary volume control too? When reverb is turned up, the signal is panned to the output of the reverb recovery, which is recovering the thermal noise of that 1K resistor.

[DrGonz78]

Great info RG and again thanks mucho. I ended bringing the amp back with the resistors set in place and it will be a quick fix to install the tank. Actually just found out that we might be installing the tank after all since the customer had more time to think about doing so. I highly recommended installing one for such a great amp. Either way I will be happy to finish the job.

[DrGonz78]

So I got the amp back again to put the reverb tank in and I got it all hooked up. The problem now is that it is feeding back just turning up the reverb. It gets so bad I have to shut off the amp so it won't kill my ears or the amp. Actually it will feedback a bit without turning up the reverb too in some ways. Anyhow seems likely a problem that might have been encountered before. All the cables seem to be wired up correctly signal flows out of the reverb transformer secondary into the IN on the tank. The other connection on the reverb transformer secondary was already connected to the chassis. So my cable is only shielded on Input side of the tank(not directly connected to the chassis ground). The Out side of the tank is connected to ground on the board and signal flows to the board to appropriate lead. So I hear signal and some reverb, but the feedback take over. Any ideas how troubleshoot this one? Perhaps it is a grounding problem?

BTW I am using a 9FB2A1C instead of a 4FB2A1C tank, but I don't think that should matter much.

[R.G.]

What is the nature of the feedback? Single frequency screech? High tone? Low growl? Does it sound like guitar-speaker feedback?

[52 Bill]

The problem now is that it is feeding back just turning up the reverb. It gets so bad I have to shut off the amp so it won't kill my ears or the amp. Actually it will feedback a bit without turning up the reverb too in some ways.

Are you talking about feedback or oscillation?

When the term feedback is used regarding reverb, I think acoustic feedback, where the springs are being vibrated by the speaker and feeding back, just like a mic.