Marshall '77 JMP 2004 power supply issues

[simonallaway]

Hello everyone! It's been a while since I posted, I hope all are well.


Sadly today my Marshall 2004 was either producing low volume, or farting a lot. One output tube was giving me blue glow when played, the other not.  It did exhibit the occasional crackling sound, but no sign of arcing. Then it blew an HT fuse (500mA). I've measured voltages on all tube pins.


I think it boils down to this: Voltage at the HT fuse is only 370V (and output tube plate voltage is the same). Voltage prior to the PT is definitely 120V A/c. Preamp plate voltages are anywhere from 40-50% of what one would expect. Can I conclude the power transformer is toast?

I referred to this for voltages: http://www.mhuss.com/MyJCM/vchart.html
But then this is somewhat closer to what I'm seeing: http://el34world.com/charts/Schematics/files/marshall/Marshall_ampchart.gif

What else can I do to pin it down? Should I isolate the PT and measure again to discount the filter caps?


Simon

[shooter]

it blew an HT fuse

is it still blowing?

this is somewhat closer to what I'm seeing

Your idle volts are close to good volts on the chart?

[DummyLoad]

HT fuse blowing and lower than spec voltages are the likely result of bad cap(s). if it is a '77 model that's had a lot of play time, then i would suspect power supply caps, not the power transformer.


--pete

[simonallaway]

is it still blowing?

Your idle volts are close to good volts on the chart?

I've attached a chart of my voltages, and also the 50w Marshall MkII from the EL34 library for comparison. I think the earlier schematic I used for voltages was for a later JCM800 2204 which appears to have crazy-high B+.


The filter caps are 'only' 5 years old. BUt naturally that doesn't mean they're not defective now.


One thing I did do is measure the mV across the 1 Ohm resistors on the power tube cathodes. One tube has the expected 40mV, but the other had 80V...not mV. I ran out of time so I haven't swapped out the tube to discount a bad one, but I will try that this evening.

[sluckey]

but the other had 80V...not mV

Really? That would mean 80 amps is flowing through that tube. We know that ain't possible. I suspect that 1Ω resistor is either open or has greatly increased in value. Check it. That tube may also be bad and caused the 1Ω resistor to fail.

[simonallaway]

but the other had 80V...not mV

Really? That would mean 80 amps is flowing through that tube. We know that ain't possible. I suspect that 1Ω resistor is either open or has greatly increased in value. Check it. That tube may also be bad and caused the 1Ω resistor to fail.

Will do. Thank you.

[simonallaway]

You nailed it. The 1 Ohm resistor was toast. I replaced it, rebiased and I no longer have 80V across it. I now have somewhat uneven cathode current between the two EL34s. One is at 35mV and the other 27mV.


Does that indicate they're simply mismatched or that one is on the way out?

[sluckey]

The resistor probably failed because the tube was drawing too much current. Your tubes are mismatched now but do we really need a perfectly matched set for a guitar amp? I don't think so. If the amp sounds OK, then play it. But keep an eye on it. And maybe get another set of tubes just in case.

[simonallaway]

You're right, it does sound just as it did now, so I'm happy. I just wondered if it was evidence for impending failure.

I've ordered more tubes so I don't get caught out, and I'll keep an eye on it. Thanks again for your help.

[PRR]

> One is at 35mV and the other 27mV.

Sounds fine to me.