Marshall valvestate 8080 repaired?

[zoblitz]

Hi!

Some time ("15 years") I was playing the last song of a concert (At that time I had a rock band) and suddenly the amplifier stared to sound really bad and then went out. As I approached, the smell told me that something very bad had happened and that the band would have to finish the song without guitar.

The truth is that I did not open the amplifier until two weeks ago, have changed: TR4, TR5, TR6, TR7, TR8. TR9, TR10, TR11, TR12, I had to rebuild many tracks because the failure vaporized many of them in the power stage.

After that I plugged my limiter circuit, the multimeter to measure the DC voltage at the output connection without speaker and I turned on the amplifier.

The lamp shines a bit, the DC output voltage is 70_mV (Channel Two) and 160_mV Channel One (clean channel), nothing connected to the input and all potentiometers are on zero.

I'm not sure why the dc voltage difference between channel one and two, is that normal? there may be something wrong with the pre-amp?

Thanks in advance.

[J M Fahey]

The lamp shines a bit, the DC output voltage is 70_mV (Channel Two) and 160_mV Channel One (clean channel), nothing connected to the input and all potentiometers are on zero.

I'm not sure why the dc voltage difference between channel one and two, is that normal? there may be something wrong with the pre-amp?

Never saw that before, maybe it wasthere but didn't notice, wouldn't lose dream because of it, 70mV is perfect and 160mV not bad by itself.
Are you sure you are measuring DC and not AC?
For one you don't state polarity and on the other side you might be measuring residual noise/hiss.

Anyway, the main point here is whether you fully solved the main trouble or not, the burnt power amp.

All this with the limiter still in line:
1) just as a reference, measure +/-V rails, you should have +/- 30 something V .
2) connect a speaker, play a few chords, at low power , say 1W or so .... any problems?
3) if fine, play a little louder, say 5/10W: does the limiter bulb shine in rhythm with the music?
Is sound still fine?
4) plug the amp straight in the wall, play first at low power, then mid power, then loud, always watchingbthe board, stopping a few seconds to touch the heatsink, etc.
5) if no problems visible, you have repaired your amp
As a final test, play loud for 10/15 minutes, check (a finger is fine) that heat sinks warm up but you still can touch them (no sizzle )
Enjoy.

[zoblitz]

Never saw that before, maybe it wasthere but didn't notice, wouldn't lose dream because of it, 70mV is perfect and 160mV not bad by itself.
Are you sure you are measuring DC and not AC?
For one you don't state polarity and on the other side you might be measuring residual noise/hiss.

Anyway, the main point here is whether you fully solved the main trouble or not, the burnt power amp.

All this with the limiter still in line:
1) just as a reference, measure +/-V rails, you should have +/- 30 something V .
2) connect a speaker, play a few chords, at low power , say 1W or so .... any problems?
3) if fine, play a little louder, say 5/10W: does the limiter bulb shine in rhythm with the music?
Is sound still fine?
4) plug the amp straight in the wall, play first at low power, then mid power, then loud, always watchingbthe board, stopping a few seconds to touch the heatsink, etc.
5) if no problems visible, you have repaired your amp
As a final test, play loud for 10/15 minutes, check (a finger is fine) that heat sinks warm up but you still can touch them (no sizzle )
Enjoy.

Hi JM, I'm very grateful for your response,

I'm sure those 70 and 160_mv are DC, in fact I'm reading -70_mVDC and -160_mVDC.
Before turning on the amp I read 47_VDC and -47_VDC on the rails.

About the clean channel, I remember that the sound wasn't completely clean even I remember it sounded a little distorted when played a little hard, I'm not sure if this has any relation to the actual 160_mVDC.

Tonight I'm going to do the tests with the guitar.

Thanks

[Mick Bailey]

Check your +/- 15v too, and see if this holds up with the channel switching.

[zoblitz]

Check your +/- 15v too, and see if this holds up with the channel switching.

Hi Mick,

Great idea, I took measurements of the +/- 15_VDC on the ZD1, ZD2, IC8, R83, R78 but only with channel 2.

I'm going to do that.

Thanks.