Vox Royal Guardsman 1131 problem

[R.G.]

The reverb return signal and the dry signal are both in phase, but turning the blend control from dry to wet gradually attenuates the signals. In fact, even the input to the reverb board attenuates too.

Where are you measuring the signals? Both ends of the blend pot?

[rf7]

OK, this thing is so weird. I have decided that it is some kind of grounding issue.

With the reverb on a channel, if you pan the Blend control from dry to wet most of that channels output signal is attenuated. In fact everything seems to be attenuated from the reverb board onwards, but the channel input stage is unaffected. If I lift caps between stages the signal is OK all the way up to the mixer input, no matter where the blend control is set.

Now, if I pull the RCA cable between the preamp and the power amp, the output of the Normal and Brilliant channels goes way down. The Bass channel output level is unaffected by the RCA cable connection.

If the RCA cable is connected and I just feed signal into the Normal or the Brilliant channel, I actually see input signal coming from all 3 channels into the mixer stage. The Bass channel does not bleed into the other 2 channels.

I think I need to remove that band-aid resistor from the power amp input and solve the pumping/ground problem.

[R.G.]

Could be a truly well farked rotary switch for assigning the reverb channel, perhaps.

The reverb blend pot is completely floating, driven on the dry end by an emitter follower from the front end of the reverb circuit, and on the wet end by the reverb recovery amplifier. From your description, I first thought that the wet end of the reverb blend pot was being held at either ground or a low AC impedance and was actually pulling down the signal. A loose ground wire touching the wrong end of a control or switch contact might do this too.

[rf7]

OK, but with the mixer input pulled, the switch behaves correctly. That makes me think that the switch is OK.

The craziness only occurs when I touch the signal end of the Preamp RCA to the inner sleeve of the Power Amp RCA connector. The ground does not need to be attached for it to get weird.

I wish I had a 1:1 iso-transformer right now.

Sorry about the double post this morning, I did not see the hidden replies below current depth stuff.

[rf7]

Yes.

[R.G.]

Hmmm. There is some weirdness I don't remember all that well about the ground on the RCA connector. It'll come to me. May even be here in the forum archives.

I would next check for DC levels on the switch contacts, and on the "inputs" to the mixer. A bum cap into the mixer input could hose things too. Shoot, I'd just remanufacture the mixer. There's three transistors, six or eight resistors and a few caps.

I'm forever dropping solder blobs and splashes onto PCBs. Could be something like that. DC volts on the controls and active devices that lead through the switch and into the mixer should tell the tale.

[rf7]

OK, so there is no appreciable DC on the switch contacts whatsoever.

Here are the Mixer transistor voltages:

Q207:
C: 3.36V
B: 0.6V
E: 0.05V

Q208:
C: 3.36V
B: 0.78V
E: 0.17V

Q209:
C: 3.36V
B: 0.65V
E: 0.09V

They don't match the voltages called out on the schematic, but they seem reasonable nonetheless.

[R.G.]

Here are the Mixer transistor voltages:
They don't match the voltages called out on the schematic, but they seem reasonable nonetheless.

Q208 is noticeably different,and possibly an indication in this low voltage setup. On top of that, it's where the reverb comes in. I would be very suspicious of that one.

Since all three mixer inputs are identical, try pulling the input caps and swapping a different "channel" into Q208 and the reverb into either of the other two inputs. If troubles follow Q208, that's an even stronger hint.

[rf7]

So, I just found an error in the Vox factory drawing of the main PCB layout. Capacitor C222 is drawn in the layout with it's polarity reversed according to the schematic. I replaced it and it did not make a difference.

I also tried swapping the channels into the mixer, and the amp behaves exactly the same.

It also behaves the same with the preamp disconnected from the power amp. So, it's just in the preamp section then.

I'm thinking of jumping the circuit paths together without having the reverb switch involved.

[R.G.]

So what are the pin voltages on the transistors in the reverb section?

[rf7]

Q201
C: 5.5v
B: 0.7v
E: 0.15v

Q202
C: 21.3v
B: 5.5v
E: 5.0v

Q203
C: 2.9v
B: 2.2v
E: 1.6v

Nothing too far from the schematic values.

[R.G.]

I dug out my schemos and reviewed the postings.

So, I just found an error in the Vox factory drawing of the main PCB layout. Capacitor C222 is drawn in the layout with it's polarity reversed according to the schematic. I replaced it and it did not make a difference.

This bothers me. Yes, the layout is incorrect then, because the schemo shows the correct way round. The (-) side should be at ground. Do a quick check of DC voltage with a meter. The layout should be what's faulty.

I also tried swapping the channels into the mixer, and the amp behaves exactly the same.
It also behaves the same with the preamp disconnected from the power amp. So, it's just in the preamp section then.

Yep, just the preamp.

The reverb return signal and the dry signal are both in phase, but turning the blend control from dry to wet gradually attenuates the signals. In fact, even the input to the reverb board attenuates too.

I thought about that for a while. There exists only three ways for the blend pot to attenuate signals. These are:
(1) the pot's bad, and turning it one direction runs the wiper into an area of the resistance layer that makes the wiper high resistance. Everything coming off the wiper is then reduced. It's worth doing a quick ohmmeter check of the pot, although I don't think this is the best candidate.
(2) one terminal of the pot is incorrectly connected somehow to a low impedance point, and turning the pot begins loading signals down. This would be the case if the wet side of the pot is somehow AC or DC shorted to somewhere else, perhaps even a crossed wire from the wet side.
(3) the pot is changing DC conditions and causing the amplifiers it's connected to to go to lower gains or output. Thinking about this one and looking at the schematic made me notice that two out of the three connections to the blend pot are DC blocked by capacitors: C206 and C208, 10uF each. The dry side is not blocked, connecting right to the emitter of the reverb input stage. If the blend pot were pulling down the emitter of the input amp, this could account for lower output of dry signal too by changing the DC conditions.

Have you watched what happens to the voltages on the reverb driver transistors and reverb recovery transistor as you turn the blend knob? If nothing, then it's not changing the DC conditions.

[rf7]

This amp kills me.

I spent a bunch of time on it and it was looking like the reverb return signal was just super low and that's why the blend pot seemed to be attenuating the output. And then when I went to look further into that, the amp started having all sorts of crazy oscillation problems, which I had not seen in quite a while and I just set it aside. You can chase something, but then the symptoms change and you've got to go back towards square one.

And earlier, the reverb return signal was the same amplitude as the dry signal.

It's just so inconsistent to work on. I think those 80 or so interconnect wires are probably the source of a lot of this. I've beeped them all out, but that doesn't mean that they're actually 100% OK.

[rf7]

The output was attenuated because some "genius" (in a previous repair) decided to swap the input and output wires on the reverb transformer.

[R.G.]

The fragile nature of the wiring is the single biggest issue with these amps. That's one reason I did this:

The circuit was split up functionally (normal preamp, brilliant, bass, reverb, etc.) and the whole circuit moved to mount on the back of the controls that run it. That dramatically reduces the wire bundles because 6-12 wires per circuit no longer make their way through that bundle. The wires that are left will be out where you can get at them.

I'm also solving the issue with breakage of the multi-stack power switch, as shown. Same function, same knob, done with mini-relays on one PCB under the switch.

I also did a PCB of the power amp, just for replacement. There's not much that can be done for the wires on that, as you still have to run three wires per power transistor. But I think that a setup with four LM3886 chips would replace the Beatle amp with no change in sound quality, due to the limiter before the power amp.

I'm still weeding out the bugs I put in the circuit boards just to test my debugging skills ... ... but it is all getting functional so far.