Thomas Vox V1032 troubleshooting help?

[drewl]

Wish I had a replacement board for the original one I'm repairing this week.

[MWaldorf]

Guys,

I've made a high resolution PDF of the V1082 / V1032 schematic. Could someone please have a look and see what I might be doing wrong?

Given that my voltage problems are on Q5, Q6 and Q11, I wonder if the reverb transformer T1 might be bad. I've got proper voltage readings at Q4, and T1 is next in the signal path. Is there a way to test that?

Thanks again, Mel

vox-berkley-ii-v1082-1032.pdf

[R.G.]

Hi, guys. Just caught up enough to catch this thread. Where are we?

[R.G.]

I'm still trying to run down the email/posts. The first user on the Cambridge/Berkeley boards had issues with the reverb/trem, and had to stick an extra cap to the chassis. I'll find it as soon as I can.

[MWaldorf]

Interesting! I found a picture you had posted of a partially assembled Berkeley board, noticed an extra cap and had been wondering about that.

Also, I was following the schematic of the e-tuner and it looks like repair board wiring is different than the schematic - on the schematic the e-tuner signal is inserted in the main signal chain through R55 to R8/C4, but on the repair board it's connected through C36 to R16/C13. I'm not sure if this might be adding to the other issues, but I know that with the amp as I got it, the volume control worked on the e-tuner signal, and now it doesn't.

[drewl]

Dang, that's probably the cleanest version of the schematic I've ever seen!

We need to post the correct voltages of all the transistors in a good working unit for future reference.

[R.G.]

I've just about finished what amounts to a service/repair manual for each of the Thomas Vox "Suitcase" amps - the Pathfinder, Pacemaker, Cambridge, and Berkeley II. It's an analysis from the ground up on the original boards, and includes X-ray views of the board and components and wiring diagrams. The schemos are new-drawn and as they say "born digital" so they can always be clean.

The working voltages in the manual for the original boards (and the assembly manual with the new/replacement boards ) are from both measuring a working unit and simulating the circuits. The simulator of course produces unequivocal voltages for what it thinks is going on, but the real world has some fuzz on it, so the real world voltages will vary a bit.

The repair board part numbers will vary from the original Vox schemos somewhat, an artifact of making the one board fit all of the smaller amps. The parts in the schematic correspond.

... however, that being said, I recently received an E-tuner, the first one I've had in my hands since my Cambridge from the late 60s. They threw me a curve with the tuner. The tuner has the 2.7 ohm resistor on it, not on the main PCB. I believe that this causes the issue you're seeing. I'll know in a bit.

[MWaldorf]

R.G.,
Very interesting about the resistor being off the board. Also, what about how the e-tuner is connected to the rest of the circuit? The new PCB is different than the factory drawing (attached here with a fix to the e-tuner connection to the circuit - I had a joint where the two wires were just crossing over each other)
vox-berkley-ii-v1082-1032.pdf

As far as I can tell from photos of the original board, the wiring is pretty close if not exactly as the schematic (I don't remember and don't have the board with me).

Again, thanks so much for your help!

Mel

[R.G.]

The factory schematic for the Cambridge and Berk 2 do not show how the wires connect to the rest of the circuit. I'm a bit hamstrung by not having the e-tuner version of either one to refer to. The Cambridge and Berk I have are the pre-e-tuner version, so I got the exact way the e-tuner connects wrong for the actual e-tuner. It's fixable easily enough now that I know the issue.

A bit of background:
I started this whole tilting-at-windmills campaign with the objective of making a "Big Head" Thomas Vox PCB with wiring that could be done by mere humans. The wiring in those, much more than the Cambridge, is a bundled and laced rats' nest. So I adopted some personal rules. Every off board component - pot, jack, switch, etc. - had to have all of its wires to the circuit carried to it in a tidy PCB-to-part run. Each pot, for instance had three wires to/from the PCB if it was used as a voltage divider, two if it was a variable resistance. No pots had ground or other signals carried to/from other panel mount parts.

That meant that the PCB itself carries all of the lateral wiring, along the panel. So each panel-mounted part's wires are as easy to locate and fix as possible. Several wires run to that part from the PCB, and nowhere else. Further, the wires don't cross. Each part's wires come off the PCB on side-by-side pads, so there's not one wire from over there, another here, and a third from somewhere else to a pot. They're all parallel, short, and side by side. That made the PCB layout complicated, but PCBs can handle complexity, and once you get them right, they're always the same. And while I was at it, I made all the wires come off one side of the PCB, no wires off all four sides and none stretching out of posts stuck in the middle of the board. And it worked. The Big Head PCBs allow mere mortals to wire up a Beatle head. Several mortals have done Beatles, Guardsman, or Buckingham heads.

I carried the same wiring style into the smaller amps, which is why the Cambridge board is wired up that way. The Suitcase Amps are much simpler, and could have been rats' wired, but why go back? So again, all wires for a pot, switch, jack, etc. are in neat side-by-side wires from side-by-side pads on the PCB.

Then there's the E-tuner. The Vox schematic for the Cambridge does not show how the connectors work for the E-tuner. All the E-tuners (and G-tuners for bass amps) I've ever seen are a lump of circuit, one tunable inductor, some caps and resistors and one transistor, on a bit of phenolic, with a wire pigtail ending in a three-pin connector. Not having an actual e-tuner, I guessed wrong about where the 2.7 ohm resistor. So I put pads on the PCB for the e-tuner minus the 2.7 ohm resistor and put the 2.7 ohm resistor on the board. ACK.

It's fixable. A wire jumper goes on the PCB where the 2.7 ohm resistor was, and the actual tuner now works. It's just that it has to be hard wired from PCB to tuner, or else you have to find and/or re-use the PCB-side connectors for the module. I'll draw it up better and get you a revised manual for the PCB.

I've uploaded the E-tuner page from the Vox Owner's Safety Net. The schemo shows the connector-ization like I thought it was. I'll go get that fixed.

[R.G.]

And here's the wiring fix.
http://www.geofex.com/FX_images/Camb...ring%20fix.gif

Since there's no PCB-edge pad for this, the wire needs to go to the position that the 2.7 ohm resistor used to occupy. That place on the PCB is now a jumper, and the wire carries ground. A cable tie holds the bundle of wires for the off-board e-tuner together.

[MWaldorf]

Excellent. I'll make the mods and let you know how it goes!

[MWaldorf]

I made the mod and sadly still have the same problems. I even tried swapping the green and black wires from the e-tuner and it made no difference.

I pulled out the original board and reviewed photos I took before I replaced it and I've made mods to the TV board layout image to include the e-tuner connections as they existed in my amp when I got it. It didn't match the schematic, and who knows, maybe that's why the amp was having problems to begin with. Note that the 0.47uF cap tied to the green wire from the e-tuner is not original, but the 100 ohm resistor appears to be.

For the moment, for me to continue troubleshooting, is there anything I need to do besides pull the green pink and black wires from the e-tuner module to remove it from the circuit? Do I need to put jumpers in place of R56, R57 and C36, or remove them from the circuit?

"revised" V1032 board layout diagram:
V1082_board.pdf

[MWaldorf]

R.G., another question - on page 13 of your repair supplement, you write that for the 18V circuit, to use either the original R103/C35 or regulated U1/D101/R103A. I chose to go with the regulated supply. However, in this photo of a partially build Berkeley, C35 is on the board in addition to the regulated circuit parts. I know this isn't a finished build, but I just want to confirm I don't need to include C35.

Also, on this build there's a capacitor that appears to be going from the reverb return ground R19 to the chassis. Is this something I need to add to my amp?

[R.G.]

Still digging on the e-tuner stuff. What you drew helps. None of the Vox literature shows the E-tuner connected that way that I know of. Maybe it was an end-of-the-manufacturing-line change, or maybe a custom mod. Don't know. But I'll go figure it out.

Meanwhile:

R.G., another question - on page 13 of your repair supplement, you write that for the 18V circuit, to use either the original R103/C35 or regulated U1/D101/R103A. I chose to go with the regulated supply. However, in this photo of a partially build Berkeley, C35 is on the board in addition to the regulated circuit parts. I know this isn't a finished build, but I just want to confirm I don't need to include C35.

You do not have to include C35. The original circuit used R103 between the C34's and C35 to drop the voltage down to 17-18V, and C35 to filter that. The regulator provides all the filtering of C35 and more. But having C35 in there doesn't hurt things either with the suggested 7818 regulator.

With the regulator, it's optional. With R103 and no regulator, it's mandatory.

Also, on this build there's a capacitor that appears to be going from the reverb return ground R19 to the chassis. Is this something I need to add to my amp?

That is a picture of the first builder's board. He put that in to correct an issue with his reverb. I'll go dig out my notes on that. I didn't need it on the proto I built here, but then I'm finding that there are quirks in all of the Thomas Vox stuff. I would say, leave it off for now. The board works without it, but it may be needed when you apply the final gloss to it.

[R.G.]

OK, dug some more on the E-tuner.

The schemo you attached for the 1032/1082 is one I don't have. I used the schemo for the 1022 for how the e-tuner connects. It is the same circuit as the connection for the 1032/1082 shown in your schemo. The 1032/1082 schemo does not match the board layout you posted, as you noted. The board diagram is a second and different way to get the e-tuner signal into the preamp. It's different from any of the Thomas Vox amps I've ever seen, or worked on, but it can be made to work.

But it did show me the way to wire up the board you have. Take a look at this:
Cambridge 1032 E-tuner version 1.gif

Remove R55 and the jumper in its position on the PCB. Connect the missing "third wire" to the upper hole/pad in the R55/jumper position on the PCB, and take it off-board with the other two wires from W15 and W16. This wire carries ground to the E-tuner module. It is *possible* that you could connect this to W14 as well. That signal is also ground, but it's ground to the tone stack. Might work OK. I was very fussy about which ground wire went where on the PCB.

W16, shown as red/pink on the diagram, carries the voltage to the E-tuner module, and goes through the panel e-tuner switch to do it. The switch interrupts the power to the module to turn it off.

W15 carries the e-tuner signal back to the PCB. This signal is injected into the ground side of Q2. It amounts to common mode ground noise to Q2, and is injected whenever the e-tuner is on. This is because the 100R emitter resistor for Q2 is disconnected from power ground and "raised" above ground by the 2.7R resistor in the e-tuner.

That's a potential problem, and may be why Vox changed the e-tuner wiring - if it was Vox that did it. In the original and schemo setup, an open connection to either ground or signal on the e-tuner module disables the preamp entirely and the amp doesn't work. Bad contact, or missing e-tuner = busted amp.

The board diagram you showed indicates a second and different way of injecting the signal. In this version, the original Q2 emitter resistor R8/100R is left connected to ground, so the preamp works if the E-tuner is missing, and the E-tuner signal is coupled in in parallel through the 100R/0.47uF shown on your board diagram. Now the amp will work in all cases.

The additional 100R will change the preamp gain a bit as well.

I think that in your case, you should first try the wiring shown in the link above.