Marshall Valvestate 8080 has only 12V + and - on 15 volt rails and it has a loud hum

[rpape2601]

Xtrs 8, 9, & 10 are new. So is TR7. I'm going to try that tomorrow night.

[rpape2601]

I check the zeners out of the board and they check ok, but I'll replace them anyway.

[rpape2601]

Zeners are replaced, all the transistors TR8, TR9 & TR10 replaced. Did a "my bad". Had 8 & 10 reversed of course it would short...duhhh. Anyway they are in the correct position now and the offset is back. I have R101 and R102 lifted and I am getting -49.0vdc on R101 with the resistor lifted and 48.9 on R102 with the resistor lifted. I am getting 36.3 vac on both sides of the transformer now, just not sure the 49 vdc +/- is correct with the resistors lifted. Do the zeners act as a voltage drain to bring the voltage down to 15 volts?

[rpape2601]

Hooked the resistors R101 and R102 back in and I have -47vdc on R101 and +23.5 on R102. Don't know why I have this voltage now when it should be 15 +/-. Tube is out so it's nothing to do with that. This is all really annoying and I am just about done with this amp.

[rpape2601]

Looking at the drawing, the only thing I can figure is the 2 caps are shorted from that mishap and are causing the - & + to lose the separation and sum the voltages to give the 49vdc +/-. I get 49vdc +/- off the bridge too. Really frustrating.

[g1]

When you talk about measurements at R101 and R102, you need to say which side you are measuring.
Should be around +/-48V at the bridge ends, and around +/-15V at the zener ends of R101 & R102.
If the +23V you are measuring is at the bridge end of R102, switch your meter over to AC volts and see what you get coming off the + end of the bridge. (this will be the filter cap ripple voltage)

[rpape2601]

The funny thing is i'm getting 48vdc on the zener side of the resistor

[g1]

Are you sure the zeners are the right way around? (they go opposite of what you might expect)

[Enzo]

Without studying the whole thread, a common mistake I see is people assuming the negative lead of the filtar caps always go to ground. They don't. On the negative supplies the positive terminal is grounded. Make sure we did not install filter caps with all negative legs grounded.

[nevetslab]

Going back to 12/08/18 Post # 4, your secondary voltage of 36.3VAC....is that measured from GND to either of the ~ bridge terminals (AC input)? This would be loaded by the power amp circuit, and with R101 & R102 lifted. Measuring from both ~ terminals of your bridge (AC input), you'd get 72.6VAC or close to that, correct? Earlier, we had the DC supply readings of +/- 38.3VDC, and +12.1/-14.7V on the zeners.

If you are indeed now getting +/- 49VDC on the output of the bridge at the caps C62/C61, that sounds reasonable. But, if you're also getting +23.5V at C41/R102/ZD2 and -47VDC at C40/R101/ZD1, the preamp's IC's have now failed, ZD1 appears to now be open circuit for some reason, and ZD2 has also changed, as neither zeners are regulating for your +/- 15VDC supplies.

Enzo mentioned the capacitor polarity markings for the supply filters, where mistakes have been made by some who didn't understand that the (+) terminal would be connected to ground on the negative side of the bipolar supplies (such as C40, C61).

I fully appreciate the agony you're going thru, being on the learning end of these amplifier circuits, and contending with parts that failed to begin with, and not yet finding all else that failed. I just looked at the preamp schematic, finding 10 IC's in there...three of the M5201's which are not so common, TL072's, TL071's and a 1448...the last three being common parts. If these are socket-mounted, I'd remove them while trying to get the power amp circuit to work. If indeed you now have +23/-47V for the bipolar supplies to run these IC's.....they will all need to be replaced. The M5201 can be replaced with a New Japan Radio P/N NJM2120.

Not having one of these amps on the bench, I dug a bit on the internet to find the following photos.....some of these may be a different Valvestate model, but let us know. I'm looking to see if there's a simple way to isolate the preamp circuits power supply connections from that for the power amp. Sometimes those of us who have to isolate sections of the amp from a single-PCB construction like this, it requires cutting traces, which later would have to be jumpered back together by scraping off the green solder mask to expose the copper foil underneath, allowing jumpers to be soldered to re-connect the supplies.



Are these images what we're working on?

Earlier, when I was trying to find out why you had 4.7VDC on the output bus, I had suggested lifting R81 from the IC8 Pin 7 end, to see if that 4.7VDC shifted. That removes the DC feedback loop. I'd also lift the cap C48 from Pin 7 end as well, so the feedback is fully open. This allows working on the power amp circuit to re-establish DC stability, once all the faulty transistors, diodes and resistors have been replaced. And, IC8 also would have been damaged by that +23V/-47V supply potential.