Morris Vox AC-30TBR layout and its reverb issues

[Mark]

I was given a Rose Morris built Vox AC-30TBR to fix. This amp had faults in the vib/trem channel, the TB channel and the reverb/amp is noisy.

Of course the rail voltage and the bias are insane, now the amp works fine apart from the reverb. The reverb works but it’s noisy and even noisy when it’s switched out by the foot pedal. If I turn the reverb control to zero it’s a bit quieter but still very noisy.

I thought I could easily use a CRO to follow the noise but I found the probe attenuated the noise when on 1:1 and even on 10:1 and in the lowest voltage setting it wasn’t particularly useful.

If I bypass the reverb stage entirely the amp is much quieter though quiet enough for an AC-30.

I have included the layout for the amp as people will need to see the earthing and how it’s different from a vintage AC-30TB.

Any thoughts on fixing the reverb stage?

Corrected Rose Morris Vox AC 30TBR circuit diagram-12092026114549-0001.pdf

AC 30TBR power supply pcb-12092026114253-0001.pdf

1978 Rose Morris Vox AC 30TBR circuit diagram-12092026114903-0001.pdf

1978 Rose Morris built AC-30TBR trem panel brd.pdf

1978 Rose Morris built AC-30TBR preamp brd.pdf

AC 30TBR reverb tagstrip layout-12092026114413-0001.pdf

[pdf64]

My friend has that amp model, it's closer to schematic though, with a 22k trimmer acting as a variable load on the tank output. Where your's has a 10k resistor.
Yes the reverb was horribly noisy / hummy.
The simplest most effective fix I found was to disconnect the footswitch cable from the tagstrip, thereby disabling the footswitch.
Thankfully, the owner happy with losing that facility, so that's how it has stayed.
It's still a bit hissy when set full, but just about acceptable.

The tank had a lever to damp the springs during transport, but the foam in the damping mechanism had crumbled with age and it was causing intermittent drop outs, so I cleaned it out and removed the lever.

As I mentioned in your previous thread, a 100R 50W sag resistor had brought all the HT voltages down to a reasonable level, and it still puts out more power than my vintage AC30.

[chief mushroom cloud]

Mark, it has been my experience w/ these older amps (and even new ones!) is that they love using the chassis as a ground return. That is dead wrong with how we know currents travel and interfere in circuits these days.
If it was my customers amp....the FIRST thing I would do is replace the rectifier diodes with UF4007s, then take that green rectifier tail wire and solder it directly ONTO the capcan ground terminal (if that blue capcan is main HT filter).
The second thing I would is loosen the bolts/nuts (one at a time) and re-torque to break thru any corrosive buildup and electrically reconnect the two chassis. In fact, loosen all chassis ground fasteners and re-torque for the same reason.
THEN, I would start troubleshooting noise.
Ideally, all the reverb circuit grounds should end up at the cathodes of their respective capacitors before touching that sewer that is known as 'the chassis'.
Using cable shields as circuit returns is not a very bright idea unless you are transmitting RF or impedance controlled ultra high speed data ....something to think about w/ the footswitch.
LOL
Again, this is what I would do. It only takes about half hr. When I was doing repairs, I would just do it.
Good luck. Even a band-aid is ok.

[Mark]

Thanks so much for your help.

I did clean all of the earth connections previously and it didn’t help but as we know you have to try it.

I’ll give that a try and get back to you both. It’s now on the bench.