Marshall VS65R powers on ..No Sound

[Begbie]

Hi,
I've got a Marshall Valvestate VS65R that I love. Unfortunately its stopped working. It powers up just fine and will output sound at very low volume if the Volume pots are turned right up, but nothing like it should.
Ive tried various things based on searches in this forum and elsewhere but have had no luck. So here I am cap in hand asking for help.
Some history;

I replaced the Valve a while back with a similar one 12AX7 and it was working fine. About two weeks ago I had the house to myself and was going to make some noise, but got nothing from the amp. I thumped it and still nothing!
I ran to google and found that there were reported problems with the large cement block resistors getting hot and causing bad solder joints and the dreaded switch fault with the jacks on the back. So I reflowed all the solder joints that looked even slightly dodgy (I tested continuity on these joints too) and tested all the jacks on the front and back. No change.

I had some spare electrolytic caps lying around and have replaced the ones in the amp no change there either.

I've tried the headphone jack and had no luck there.

So I took out the Muting gate (T16 J111) and tested without it and still no result.

Im not sure where to go next.

The best schematic I can find is this one which is hard to read (due to the original scan methinks) and not enetirely the same as my board (there are some discrepancies between my board and the schematic -probably just revisions)

marshall_vs65r_65w.pdf

I'm a shiftworker so my response may be delayed or erratic, but I am happy for any help getting my amp working again.
Also living where I do (Perth Western Australia), some parts can take months getting here, so it can take a long time for me to get results. So Id rather replace several components at once than one at a time.

[Pedro Vecino]

Two basic tests:
- Check the +15/-15 voltages on the zeners.
- Check what happens when you connect a signal on the return jack. To isolate the problem in the preamp or in the power stage.

[Begbie]

Thanks Pedro,
The return jack gives slightly louder signal than normal but is still very quiet and unaltered by adjusting volume pots.

I Measured the Zeners Number one (ZD1) was -15.7v
Number two (ZD2) however was 3.74v

Both were reading -7.5v on the shared rail which I believe should be the Earth. So something is wrong there.

Also after testing (only a few minutes) One of the large resistors was incredibly hot.

[Pedro Vecino]

It could be that C25 capacitor and/or zener associated is defective, or that some active element behind it generates a greater consumption. Try the first one. It´s relatively common in these amps.
If the problem comes from there, it´s advisable to replace them on both sides, and if the zener is 1/2 watt, better use 1.3 watts models.

[Begbie]

OK I will try and source a couple of 1.3 watt Zeners tomorrow (its getting late here now).
I have replaced the C25 Capacitor already as I had a supply here.

I wil let you know how I get on. Thanks

[g1]

Both of those big resistors should get really hot.
Is one cool, or are they both real hot and one even worse?

[Begbie]

It appears to be one much hotter than the other and it gets that way really quick! And yes g1 one is definitely a load hotter than its companion.

I have replaced Z1 and Z2 and the adjacent caps but no luck still! I did manage to lift one of the tracks on the board in doing so and have repaired and tested the continuity there.

[g1]

What I was getting at was that if one of them does not get very hot, then it is more likely the problem, not the hot one.
You measured both resistors and resoldered them?
Which one is hotter, R17 or R18 ?

[Begbie]

Its R17 that gets hot.
I rechecked them after I discovered the hot one. They are both OK and the continuity of the solder joints is good too.

[The Dude]

If you have +-15v rail issues, my bet is a shorted op amp loading down the supply. See if one of your op amps is getting hot. The best place to start is the first one in from the input jack. Lots of times these can short from someone plugging something into the amp that shouldn't be.

[Begbie]

OK I will check them out this morning.

[Enzo]

Would seem to me that the 15v supply that is measuring only 3v is likely to overheat the resistor. My first vote is a bad zener there.

[Begbie]

I've replaced both Zeners and still no luck. I tested the ones I removed too and they seem fine. I think The Dude might be onto something with the OpAmps and I think I might order some and systematically change them out.
Its crude and not the most graceful solution but given the time it takes to get parts here sometimes I'm inclined to replace them all at once, but curiosity is going to be the better of me here I think and I will end up working through them one a t a time until I fin the culprit.

[Enzo]

We call that an Easter egg hunt: just start replacing things until it works. Not very efficient. If the zeners are OK< then touch each op amp and see if any are getting hot. Same with the caps around the low zener. If push comes to shove, carefully remove the ICs one at a time to see if the supply recovers.

[Begbie]

I tried the touch test on the ICs this morning none were getting hot. I didn't try any of the passive components though. I will try that next thanks Enzo.