Vox AC30 problems

[MarkusBass]

I'm looking for the schematic and layout of Vox AC30 amp. It seems that there were several different versions of the amp. This one is with solid state rectifier. I've got it with a request to fix some minor noise problems. But when I opened it, I found out that the original boards were replaced with some home-made boards. What is funny is that the author of this brilliant modification put a signature on the boards .
The noise problems shouldn't be to difficult (most probably power supply issues) but I'm wondering whether it would make sense to restore the amp to its original state. I know that the tag boards can be purchased. To complete the task I need the layout of the amp. Please check the attachment and tell me whether you know which version it is.

EDIT: after looking at the board I'm not sure any more if this is Vox made board, or some replacement. Could it be some late version of the amp?

Mark

[52 Bill]

What does the amp look like? Does it have a standby switch? Reverb? Top Boost Channel?

I've only seen a couple of schematics that are for the solid state rectifier versions.

[Jazz P Bass]

Here is a 1978 version with SS rectifiers.

Vox_ac301978.pdf

That board looks stock to me.

[MarkusBass]

Thanks. The amp is slightly different: no reverb, different HV power supply (two-section capacitor and a choke in between), no standby switch. I'm especially interested in solving power supply issues. Currently there are two 2x50uF 500V capacitors. Also I need to know what was the burnt component.

Mark



EDIT: I'm closer to the amp identification. It appears to be Rose Morris version: http://www.voxshowroom.com/uk/amp/rm...1979_hood.html although the amp I have has a choke which was removed in Rose Morris version. The boards are slightly different but it seems that they were factory made. The burnt component was 100, or 120 Ohms 10W resistor. I need to clarify problems with capacitors in the power supply of the amp (and in the power amp). They are not original and icorrect values may be installed.

[J M Fahey]

That PCB by itself, even if home made, definitely is not the problem

Just for curiosity, if you have the chance look at the foil side: do tracks look screen printed (sharp definite edges, properly drawn) or clumsily hand drawn with a Sharpie pen?
Although the proud signature points to a homemade one (again, not a problem if well made, which it seems to).

Trace what the burnt part (presumably a resistor) does and connects to, it might even be an improperly rated discharge/bleeder resistor.

[MarkusBass]

The traces look good. It seems that this is Rose Morris version but with some alterations. This is the closest schematic I could find: http://www.korguk.com/voxcircuits/circuits/ac301978.jpg . But in this amp there is a choke (which was removed during Rose Morris era), the burnt resistor is a cathode resistor and it was most probably 68 Ohms (compared to 47 Ohms in earlier versions), reverb circuit is not installed. The PC boards look almost like in RM version but still different. They may be original but from some experimental series.
I solved most of the problems by replacing 2x50uF capacitor in the power supply with 2x100uF (although there is nothing wrong with the old one - capacity is exactly 50uF and ESR = 0.13). It's difficult to say which one was originally installed.
The amp has 220-240V mains switch. With 220V the voltages are to low (including heater voltage), with 240V they are to high. The amp works better with 220V setting (we have here 230V mains power supply). I'll install the cathode resistor as it was originally (68 Ohms 10W) and try a new set of matched tubes.

Mark