Vox Royal Guardsman 1131 problem

[rf7]

I've got a Royal Guardsman head that I re-capped and the output drops out about once a second, as if the tremelo is on, even though the tremelo is turned off. The dropout occurs at a rate independent of the tremelo setting; it's constant. And it is much slower than the tremelo will actually run. If I disconnect the preamp from the power amp, the problem goes away: the power amp output is fine and so is the preamp's output. Once I connect the two together, the problem comes back. It seems to affect the rails, but only when the 2 pieces are connected together.

Any ideas?

[Robert M. Martinelli]

I've got a Royal Guardsman head that I re-capped and the output drops out about once a second, as if the tremelo is on, even though the tremelo is turned off. The dropout occurs at a rate independent of the tremelo setting; it's constant. And it is much slower than the tremelo will actually run. If I disconnect the preamp from the power amp, the problem goes away: the power amp output is fine and so is the preamp's output. Once I connect the two together, the problem comes back. It seems to affect the rails, but only when the 2 pieces are connected together.

Any ideas?

Hi,
the V1131 had a "peak limiter" circuit right before the power amp, check that the voltages at the peak limiter are OK, that stage has a +17V supply, and uses +24V as a reference to set the peak limiter threshold, so, shouldn't the voltages be constant, the peak limiter could cut the signal down without a real need.

Just my "educated" guess, the problem, of course, could be elsewhere....

Hope this helps

Best regards

Bob

[Steve Conner]

Sounds like motorboating caused by some sort of unwanted feedback.

Did you use the same values of caps as before? If you made coupling capacitors bigger than they were, or filter/bypass caps smaller than they were, that could explain your problem.

Did you alter the ground wiring in any way when recapping? Signals can feed back through pieces of ground wire that are shared between circuits.

Last but not least, did you put any of the new caps in backwards :-)

[rf7]

Hi,
the V1131 had a "peak limiter" circuit right before the power amp, check that the voltages at the peak limiter are OK, that stage has a +17V supply, and uses +24V as a reference to set the peak limiter threshold, so, shouldn't the voltages be constant, the peak limiter could cut the signal down without a real need.

Just my "educated" guess, the problem, of course, could be elsewhere....

I disabled the limiter circuit, and I still have the problem.

Sounds like motorboating caused by some sort of unwanted feedback.

Did you use the same values of caps as before? If you made coupling capacitors bigger than they were, or filter/bypass caps smaller than they were, that could explain your problem.

I tripled the value of the initial filter caps, but there were a couple of 500uf's that I replaced with 470uf's. Also, for the 2uf coupling caps, I used 2.2uf caps instead. I've done this before without any issue. Should I up those 470uf's to 500uf?

Did you alter the ground wiring in any way when recapping? Signals can feed back through pieces of ground wire that are shared between circuits.

No, not at all.

Last but not least, did you put any of the new caps in backwards :-)

Found one of those yesterday, and replaced it. Still got the issue.

Thanks for the help guys. Any more suggestions?

[52 Bill]

I seem to recall a problem similar to this on a Viscount a few years back.

What I remember is that it would work fine as long as the two grounds from the power amp and the pre-amp were not connected at the RCA plug. What I mean is that if you didn't insert the RCA plug from the power amp chassis all the way into the jack on the pre-amp it would be fine.

I don't remember what the final cause was, but I remember investigating the grounds from the power supply plug to the pre-amp.

[R.G.]

I seem to recall a problem similar to this on a Viscount a few years back.

What I remember is that it would work fine as long as the two grounds from the power amp and the pre-amp were not connected at the RCA plug. What I mean is that if you didn't insert the RCA plug from the power amp chassis all the way into the jack on the pre-amp it would be fine.

I don't remember what the final cause was, but I remember investigating the grounds from the power supply plug to the pre-amp.

This is the right place to start. Ohm out power amp chassis to preamp chassis. In fact, as long as you have it pulled open, trace the ground wiring from the power transformer CT to the filter caps, then to chassis, then to the various places it goes. An open ground is a likely suspect.

Sometimes you'll get motorboating from open filter caps in the RC pi networks the Thomas Vox uses, but it's unusual.

[rf7]

What I remember is that it would work fine as long as the two grounds from the power amp and the pre-amp were not connected at the RCA plug. What I mean is that if you didn't insert the RCA plug from the power amp chassis all the way into the jack on the pre-amp it would be fine.

The ground does not make a difference, any time the signal is connected, with or without the ground, the output drops in and out.

This is the right place to start. Ohm out power amp chassis to preamp chassis. In fact, as long as you have it pulled open, trace the ground wiring from the power transformer CT to the filter caps, then to chassis, then to the various places it goes. An open ground is a likely suspect.

Sometimes you'll get motorboating from open filter caps in the RC pi networks the Thomas Vox uses, but it's unusual.

Found a filter cap missing a ground and corrected it. Ohmed out all the rest of the grounds and every one measured fine. Still got the problem.

[redneckgeek]

Why stop now??

I have a Royal Guardsman with the same problem. I was delighted when I found this thread but then it stopped. Did you find the problem? I have checked the PS and found it to be fine. The problem shows up in the DC voltages at the output transistors as a voltage swing plus and minus relative to 0 volts on an analog meter. It is also present as a DC change on the oscilloscope. I have not isolated it yet to a specific section. Just got the schematic yesterday. Any ideas??

[R.G.]

The ground does not make a difference, any time the signal is connected, with or without the ground, the output drops in and out.
...
Found a filter cap missing a ground and corrected it. Ohmed out all the rest of the grounds and every one measured fine. Still got the problem.

Has this been worked on previously? I got this flash that a previous repair may have reversed signal and ground on the preamp end of the shielded cable from preamp to power amp.

But that was one of those flashes which fit the symptoms with no other reasoning behind it.

It's time to do the hard stuff. Get out the voltmeter and the 'scope. Plug power into the AC input and connect the DC power plug to the preamp unit. Check the voltages on the power supply - yep, all of them - with the preamp signal connected and disconnected. Do they change? How does this match what the schemo says they should be? Is there DC on the end of the preamp cable?

If that doesn't turn it up, start tracing the signal through the amp with the o'scope, or an audio probe. Thomas Vox is relatively hard to do this on because they keep the signal level down in the under-1/2V range throughout most of the preamp, and only build it back up in the "mixer" before the power amp.

I have a Royal Guardsman with the same problem. I was delighted when I found this thread but then it stopped. Did you find the problem? I have checked the PS and found it to be fine. The problem shows up in the DC voltages at the output transistors as a voltage swing plus and minus relative to 0 volts on an analog meter. It is also present as a DC change on the oscilloscope. I have not isolated it yet to a specific section. Just got the schematic yesterday. Any ideas??

Yep - do what you started to. Trace it out, report back what voltages change to when it's connected.

[rf7]

The problem shows up in the DC voltages at the output transistors as a voltage swing plus and minus relative to 0 volts on an analog meter. It is also present as a DC change on the oscilloscope.

Just got back on this. I noticed the above too, the DC rails are pumping exactly like the output is pumping.

This seems weird, but I noticed that the 2 rails, just off the first filter cap and rectifier diodes, have different sized ripple. The positive rail's ripple is 3 times the size of the negative rail ripple. When the preamp is connected, the positive rail ripple tries to collapse down to the size of the negative rail ripple, whereas the negative rail ripple tries to expand up to the size of the positive rail ripple. This is all in time with the pumping of the DC rail itself.

I have not done RG's latest suggestion yet, but throughput is not a problem from the input to the output.

Has this been worked on previously?

Yeah, the guy gave up on it.

[Steve Conner]

Definitely sounds like a touch of motorboating.

Now, I assume the preamp is powered off one of the power amp's rails through a dropper resistor. Can you run the preamp off a separate supply (bunch of 9v batteries? lab power supply?) to see if this fixes it?

This is "cutting the problem in half". If it fixes it, we know the problem is feedback through the supply rail. If it makes no difference, we know the problem is feedback through the ground system.

There is a third possibility: maybe the power amp's NFB loop is unstable (any NFB loop with a transformer in it is an accident waiting to happen!) but only when its input is "loaded" by the preamp's output impedance. This is possible, because connecting the input to something adds the input coupling capacitor's time constant to the NFB loop.

You can test this one by cutting the NFB to see if the problem goes away. Or try shorting the power amp input to ground, instead of connecting it to the preamp. If this were the problem, it would make the oscillations start even worse. The cure would be to leave the NFB off, or reduce it, or experiment with the value of the input coupling capacitor. The original design may well have been on the verge of oscillating, and you just gave it that extra push over the cliff.

So try these two tests and get back to us.

[redneckgeek]

seperating the two halves

I have unplugged the RCA cable that feeds the preamp output to the power amp. I have connected an audio signal to the p/a input and it runs fine. I have also connected the preamp output to another amp and it works fine that way also. I have tested every component in the P/S and P/A. Replaced several out of tolerance parts including two electrolytics and resistors. Tested both transformers.

While I was following the signal path in the preamp I found the voltage swing to be at about the same percentage. For example the 25V fluctuates between 25V and 18V. Changing the Reverb channel control switch does affect these voltage readings. The input is clean through to the volume control on the normal channel. Somewhere in the reverb section is where it shows up and the ruins everything I tested all the way to the output transistors.

The manipulating of ground connections does make a difference.

[Steve Conner]

Ah. Is the reverb tank touching anything that it shouldn't? Or are the send and return connectors swapped? Problem still there with the tank disconnected altogether?

[Leftydave]

Wow, I had a similiar problem with my Buckingham in that the amp was low in power and distorted. I ran the output of the preamp to a separate amp and it sounded fine. Also, a signal sent into the power amp sounded fine. 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!!

[rf7]

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.