Vox Royal Guardsman 1131 problem

[R.G.]

The culprit was a leaking coupling cap (electrolytic of course) in the power amp section. I believe it was the first one after the peak limiter. The bad cap was leaking DC onto the signal path I assume. Replaced the cap, worked perfect!!

My stock answer to people with Thomas Vox problems is to first replace all the electrolytic capacitors, then fix whatever else is wrong - which may be zero.

The caps on all these amps are at end of life, and if you replace one, the next one may fail a week later. This is one reason they're a tech's nightmare - owners don't want to pay for the time to replace all the caps, but they will sure be back to complain if only one cap gets fixed and then another lets go.

Adding a 3.3K resistor in series, in front of the input cap (50uf), makes the problem go away. It's a band-aid, but it works.

Now I can fix the rest of the amp's problems.

That cap is easy to get to. What happens if you replace it instead of patching in a 3.3K resistor?

I guess more importantly, what happens if you just gut it up and replace all the electros on the power amp section? They're easy to reach, and I promise that if they haven't failed yet, they will soon. Start fresh with all new electros and have some reliable life in the amp.

[Steve Conner]

RG: I was under the impression that he already replaced all of the electrolytics in the amp. If not, then it most certainly needs done!

rf7: That would kind of make sense, as the resistor would damp the instability. But it could also help if you put the input coupling cap in backwards. :-)

[R.G.]

RG: I was under the impression that he already replaced all of the electrolytics in the amp. If not, then it most certainly needs done!

rf7: That would kind of make sense, as the resistor would damp the instability. But it could also help if you put the input coupling cap in backwards. :-)

Doh! Sorry - yes, the post does say "re-capped". I have self-yo-yo'ed my self.
S'wat I get for not re reading.

In that case, yep, I'd then check the direction/polarity of the caps.

Note to rf7: even if you don't know the original polarity, it's easy enough to do with a voltmeter. Just measure the voltage across the cap. If it's + on the + side compared to the - side, you're OK. If the voltage is less than a volt or two but in the correct direction, chances are that an NP is needed there. If the polarity is reversed, swap the cap 'round.

[rf7]

I have a bipolar cap in there. When I ordered caps, I figured that one should be non-polarized.

[R.G.]

Hmm.

OK, back to checking transistor pin voltages.

Just for grins, check power supply voltages AND the difference between preamp chassis "ground voltage" and power amp "ground voltage" before and after plugging in the preamp cable.

Did you ascertain that the signal and ground were not switched on the cable?

[rf7]

I had a NP cap in there, and it did not make a difference.

[rf7]

Just for grins, check power supply voltages AND the difference between preamp chassis "ground voltage" and power amp "ground voltage" before and after plugging in the preamp cable.

1.5mV difference between preamp and power amp when connected.

4.5mV difference between preamp and power amp when not connected.

Did you ascertain that the signal and ground were not switched on the cable?

Yes.

I've kind of moved on to the other issues this amp has, such as:

1) The reverb control, when turned up brings down the output signal of the channel it is applied to.

2). The normal and brilliant channels oscillate when you turn them up.

Any ideas?

[R.G.]

1.5mV difference between preamp and power amp when connected.4.5mV difference between preamp and power amp when not connected.

OK. Not too bad.

I've kind of moved on

So does it still have the original problem, or has that gone away?

to the other issues this amp has, such as:
1) The reverb control, when turned up brings down the output signal of the channel it is applied to.
2). The normal and brilliant channels oscillate when you turn them up.
Any ideas?

Number 1 is a defect in the reverb send/recovery circuits.
Number 2 is either a ground wire problem or a control wiring problem most likely.

We'll still have to dig into what the actual transistor pin voltages are to make much headway.

[rf7]

So I'm digging back into this. I've checked every ground connection and beeped out every wire, and I still have the oscillation problem. Anything else I can check?

[R.G.]

So I'm digging back into this. I've checked every ground connection and beeped out every wire, and I still have the oscillation problem. Anything else I can check?

Lots. ...eek... 8-)

I had my trial by fire with a Beatle head that had pernicious hiss and oscillation in the reverb that simply appeared everywhere. The hiss was a "fix" resistor someone had added in front of the tremolo, and the oscillation was the emitter bypass of a transistor in the reverb that wasn't bypassing. I darned near rebuilt the thing before I found those.

I have some possibilities for you.

Look at the controls on the schematic. The bass/treble/volume controls are DC blocked from the circuits, so there should be 0.0000V on every lug of all of these controls. If there is not, there is a problem with either the capacitors feeding them or the wires connecting to them. The tremolo speed and depth pots do NOT sit at ground. The speed pots are up at about 5-6V, all terminals, and the depth control is about 4V, the emitter voltage of the first transistor of the tremo section. The reverb depth control sits above ground by a few tenths of a volt.

DOH! Does this thing have a "repeat percussion" setup in it? If so, disable it. That could be the issue with the dropouts.

IF none of that helps, make up a BFC* on a pair of clipleads, or just tack-solder the BFC where you need it. About 1000uF/35V to 4700uF/35V will do nicely. Attach it to the power supply and local ground near the preamp transistors. This will swamp any issues with bypassing that may be getting you. If oscillation stops, it's a power supply bypassing problem. Maybe put a 47K across that BFC so it'll run down between uses. 8-)

Break oscillation paths. Get out your signal generator and scope, and temporarily remove the wire from the wiper of the volume pot on the normal channel. Scope the wiper and look for oscillation as you turn it up. If no oscillation, hook up the signal generator into the input and see if you can get good signal through. By the way, these amps have remarkably low gain in the preamps. You're looking at maybe half a volt of signal max. All the voltage gain in the preamp is pretty much in the "mixer".

*Big Freakin' Capacitor

[rf7]

Thanks RG!!

Lots. ...eek... 8-)


Look at the controls on the schematic. The bass/treble/volume controls are DC blocked from the circuits, so there should be 0.0000V on every lug of all of these controls. If there is not, there is a problem with either the capacitors feeding them or the wires connecting to them. The tremolo speed and depth pots do NOT sit at ground. The speed pots are up at about 5-6V, all terminals, and the depth control is about 4V, the emitter voltage of the first transistor of the tremo section. The reverb depth control sits above ground by a few tenths of a volt.

I'm seeing about 1.4mV on the Bass/Treble/Volume pots. It's similar to what I see on the Normal channel, which is not oscillating. The Trem and Reverb pot voltages seem fine.

DOH! Does this thing have a "repeat percussion" setup in it? If so, disable it. That could be the issue with the dropouts.

No.

IF none of that helps, make up a BFC* on a pair of clipleads, or just tack-solder the BFC where you need it. About 1000uF/35V to 4700uF/35V will do nicely. Attach it to the power supply and local ground near the preamp transistors. This will swamp any issues with bypassing that may be getting you. If oscillation stops, it's a power supply bypassing problem. Maybe put a 47K across that BFC so it'll run down between uses. 8-)

I tried it, but it did not help.

Break oscillation paths. Get out your signal generator and scope, and temporarily remove the wire from the wiper of the volume pot on the normal channel. Scope the wiper and look for oscillation as you turn it up. If no oscillation, hook up the signal generator into the input and see if you can get good signal through. By the way, these amps have remarkably low gain in the preamps. You're looking at maybe half a volt of signal max. All the voltage gain in the preamp is pretty much in the "mixer".

*Big Freakin' Capacitor

No oscillation from the Brilliant channel preamp itself. And I do get signal through it.

Should I lift the coupling caps from the Normal and Bass channels into the mixer, and see if that makes a difference?

Any other ideas?

[R.G.]

At this point, I'd quit easter-egging and do some orderly detective work. I personally would power it, check the power supplies, and then measure and record the pin voltages on every transistor. Base, collector and emitter on every one, as well as the power supplies.

[rf7]

The oscillation was caused by a bad ground on the reverb return line.

Now, onto the reverb attenuation problem.

[R.G.]

That'll do it.

I just found out that replacing the driver transistor in the power amp with too good a modern transistor ruins the cut-to-fit compensation on some power amps in the TV series.

[rf7]

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.