Wonks wrote:

cyrano.mac wrote:I've had a look at the pics.

The first thing I'd suspect, are the three ribbon cables in picture number two. These are known to develop cracks that induce the kind of intermittent connection problems you're experiencing.

I'd certainly have a close look at them, but as those ribbons connect the drive channel controls to the main PCB, and it's the clean channel that's currently stopped working, that makes them far less likely to be the culprit here. There's no need to take the clean signal circuit output onto the drive circuit board.

I haven't seen the schematics, just the pics...

Schematics: https://drive.google.com/open?id=1vm-1a ... cyeYCwv7Ke

cyrano.mac wrote:

Wonks wrote:

cyrano.mac wrote:I've had a look at the pics.

The first thing I'd suspect, are the three ribbon cables in picture number two. These are known to develop cracks that induce the kind of intermittent connection problems you're experiencing.

I'd certainly have a close look at them, but as those ribbons connect the drive channel controls to the main PCB, and it's the clean channel that's currently stopped working, that makes them far less likely to be the culprit here. There's no need to take the clean signal circuit output onto the drive circuit board.

I haven't seen the schematics, just the pics...

Neither had I, but I had read the thread fully, so had seen photos of the front of the unit, so knew where the clean channel controls were situated.

DC-Choppah wrote:Schematics: https://drive.google.com/open?id=1vm-1a ... cyeYCwv7Ke

Well done for getting those. Not quite sure exactly how accurate those are, as the input socket wiring would never work as shown! The socket contacts have to be reversed per socket, otherwise the input signal gets sent to ground when a jack is inserted and no signal is sent to the rest of the preamp.

In fact, almost all the sockets are drawn incorrectly with the switched connection shown going to the circuit and the signal connection unconnected or led to ground. You normally have to mirror flip the diagonal arm with the arrow attached in each case to make it work.

The draughtsperson used the same template all the way through, when they needed to flip it over most of the time.

Undoubtedly the PCB has been created correctly, but this can't be the wiring diagram used, there must be a later version that's the corrected production version, despite this being a 1995 drawing and so of the mk 2 version of the unit

The schematics also fail to show the ribbon/plug connectors to and from the separate front panel circuit boards. Strange.

DC-Choppah wrote:This is new. I did something to it this time.

I'd suggest checking those connectors you opened and cleaned. I suspect you've either aligned them incorrectly, or have bent pins, or maybe the contact cleaner has pooled and is effectively shorting some pins together. It sounds to me like some contacts are either open or short...

As for the original fault... if both channels are affected the same way, that would point towards a power supply problem.

H

Of course, check PSU first. If there's 3V on a 5V rail, fi, look no further.

So what are the voltages?

When I listened to the recording it reminded me of two faults ive seen before.

[1] signal passing thru duff opamp/transistor or one without adequate power and then being amplified by the next stage

[2] output power-on-delay-thump-remover circuit duff mangling the signal > above a certain level.

in both these cases, low level signals were "ok" but louder stuff was distorted

The cause of these faults were components gone bad, so would neeed a tech to sort it out. It might be straight forward to do but time consuming tracing the signal through the circuit.

It would be helpfull if you could post a recording of the amp with just a pure sine wave ramping up in level, or even just a single guitar string pluck and let it fade away

N i g e l wrote:It would be helpfull if you could post a recording of the amp with just a pure sine wave ramping up in level, or even just a single guitar string pluck and let it fade away

Except that currently one channel works perfectly, whilst the other makes no sound at all. :D

DC-Choppah wrote:https://drive.google.com/open?id=1t1eCB ... b4skCGE0Xk

Shows the connector on the left of the PCB misaligned - it should go one pin closer to the front panel.

Though it looks right on

DC-Choppah wrote:https://drive.google.com/open?id=1hwrSo ... y977RsiocE

but what is all that liquid?

The liquid was De-Ox overspray.

This evaporated by next day and the problems remained.

I have taken your all’s advice and gave it to an audio repair shop that mostly restores old vintage stereo equipment and also pro audio. They will be following this thread too and I asked them to check out the features you guys described. I appreciate your advise.

This thing really sounds great and is pretty rare. I can’ find any others around. It does a sweet clean tone, drives pro-level line signals into stereo studio rack gear, and has a sweet marshal stack dirty channel that is outrageously good. I hope it can come back to life.

Currently, clean channel us broken up like the audio recording, but when the gain is increased the sound gets softer. At 25% up it is silent.

Green clean channel light never lights up.

Dirty channel is extra broken up, but red light comes on and it is loud and all controls work.

The tech is starting out with it like this and it is not an intermittent problem. It stays broke.

James Perrett wrote:

DC-Choppah wrote:https://drive.google.com/open?id=1t1eCB ... b4skCGE0Xk

Shows the connector on the left of the PCB misaligned - it should go one pin closer to the front panel.

Though it looks right on

DC-Choppah wrote:https://drive.google.com/open?id=1hwrSo ... y977RsiocE

but what is all that liquid?

Great catch man!

Ok the first picture with the misaligned connector was how it was when I first opened it.

After cleaning contacts and reassembling I took the second picture.

I wonder if it is supposed to go in the misaligned way?

DC-Choppah wrote:Ok the first picture with the misaligned connector was how it was when I first opened it. ... I wonder if it is supposed to go in the misaligned way?

It's rather unlikely, and since the unit was faulty before you opened it, I would suggest that could well be a significant part in the problems you've been having.

Without an accurate schematic that identifies the wiring through the connector it's impossible to say, but it would seem likely that the misplaced connector has resulted in something being damaged through the resulting circuit mis-connections.

H

Tech reports today that the issue is cracks in the circuit board near the back of the unit near where the jacks are. He also says that the some of the rear jacks lack the nuts that stabilize the connectors to the circuit board.

These guys who fix vintage audio are gold!

If it is OK with the mods, I would like to give a good word out to: https://www.just-audio.com/
for doing this repair for me!