Marshall 2203 JCM800 Lead series (100W) reissue

[the3secondrule]

Got a 2203 reissue on the bench here - initially was waaay down on gain / volume. Plenty of volume when putting signal into FX return.
Opened up and discovered R11 (V2 cathode resistor 820r) had burnt/failed open.

Replaced it, and powered up and its now oscillating. Shortly after this, discovered that the Hum balance Resistors R31,32 and VR2 have also failed open.

Suspected coupling cap C4 was failed/leaking but this has been pulled, tested OK, but was replaced for good measure.

Have replaced V1 & V2 and the fault persists.

C24 & 25 have been discharged, disconnected and are measuring OK.

Am running out of ideas, any other suggestions?

Cheers

J

[Jon Snell]

R11 open circuit can be just a failed resistor unless burnt. If burnt, I don't think even an anode cathode short can cause that as there is a 100k fanode feed resistor limiting the current but an incorrect valve in V2 position could do that. Pin 1 to pin 8 short ... highly unlikely.
The heater trim resistor chain can fail when an EL34 fails. I have known the chinesium version go heater cathode short circuit on a regular basis. It may be the resutlt of a V2 failure but then again, quite unlikely.
C4 failing will not cause R11 to fail for similar reasons already described.

[Mars Amp Repair]

Hello the3SecondRule,
Need a bit more information regarding the oscillation. In an effort to localize the source of the oscillation; is the oscillation affected by tone or volume controls? Is it present in both channel 1 & 2 operation? whether or not reverb is engaged or reverb level up or down.
f you pull V9 or V10 or any other tubes down the line does the feedback persist? Typically pulling the driver tube V-5 would kill any output, but pull it & see if the oscillation persists if pulling other tubes doesn't remedy the issue.
You can reinstall the tubes as you pull them, or sometimes it helps to leave the tubes out as you pull them.
Does it look like anyone has been in there dicking around? Any obvious replaced parts?
That kind of stuff....thanx, Glen

[loudthud]

Did you fix the blown resistors on the Hum Balance circuit ? Floating heaters can cause oscillation because of the feedback path from output tubes back to the input triodes.

[drewl]

I've seen those resistors fried on a couple of these.
And there's no reverb on these.

Aside from oscillation, are you getting full or more power output?
You have an oscilloscope or voltmeter?

[the3secondrule]

R11 open circuit can be just a failed resistor unless burnt. If burnt, I don't think even an anode cathode short can cause that as there is a 100k fanode feed resistor limiting the current but an incorrect valve in V2 position could do that. Pin 1 to pin 8 short ... highly unlikely.
The heater trim resistor chain can fail when an EL34 fails. I have known the chinesium version go heater cathode short circuit on a regular basis. It may be the resutlt of a V2 failure but then again, quite unlikely.
C4 failing will not cause R11 to fail for similar reasons already described.

Thanks Jon - that gives me a few places to start looking - R11 was burnt - the first thing I spotted when I opened the amp was the burnt resistor.
The valve in V2 was the stock ecc83, but I have since subbed in a new EHX 12AX7

I have a few old but good EL34s here I can sub in for testing too

[the3secondrule]

Hello the3SecondRule,
Need a bit more information regarding the oscillation. In an effort to localize the source of the oscillation; is the oscillation affected by tone or volume controls? Is it present in both channel 1 & 2 operation? whether or not reverb is engaged or reverb level up or down.
f you pull V9 or V10 or any other tubes down the line does the feedback persist? Typically pulling the driver tube V-5 would kill any output, but pull it & see if the oscillation persists if pulling other tubes doesn't remedy the issue.
You can reinstall the tubes as you pull them, or sometimes it helps to leave the tubes out as you pull them.
Does it look like anyone has been in there dicking around? Any obvious replaced parts?
That kind of stuff....thanx, Glen

Thanks Glen, no reverb and only the one channel on this amp (hi/lo inputs).

V1 & 2 are the preamp tubes, V3 is the Phase splitter, and V4-7 are EL34s.

Prior to replacing the burnt out resistor I had fed signal into the power amp which was coming out clean, loud and clear with leads me to believe the fault was in the preamp section (v1,2).

Will have a play with tone and volume and see if that has any effect

[the3secondrule]

Did you fix the blown resistors on the Hum Balance circuit ? Floating heaters can cause oscillation because of the feedback path from output tubes back to the input triodes.

That's useful info thanks!
Waiting on a new trimmer so I can restore the Hum Balance, so that could be the cause of the oscillation

[the3secondrule]

I've seen those resistors fried on a couple of these.
And there's no reverb on these.

Aside from oscillation, are you getting full or more power output?
You have an oscilloscope or voltmeter?

Have a scope and signal generator, and DMM.
Got a few hot tips to check out, sounds like I need to track down why the heater balance cooked itself, and repair that - floating heaters might be the cause of the oscillation.
If not then I'll get the amp on the scope and see what's happening.

[Greg_L]

Have a scope and signal generator, and DMM.
Got a few hot tips to check out, sounds like I need to track down why the heater balance cooked itself, and repair that - floating heaters might be the cause of the oscillation.
If not then I'll get the amp on the scope and see what's happening.

Does the 2203x even have a filament center tap wire? It shows "NC" on the schematic. I wonder if it's actually there. I have a 2203x right here but I'm not cracking it open right now lol.

Marshalls implementation of a resistor-balanced filament center-tap and a hum balance pot is sketchy at best. Thousands of Marshalls existed without this "technology" with great results for decades. Cost cutting parts and design aren't helping matters even on otherwise great amps like the 2203 reissue.

Bypass the board mounted hum balance pot and resistors and redo it elsewhere, more solidly. Floating the heaters on an elevated DC reference isn't a bad idea - many amps do something similar. No need for a balance pot. Make your own 50-ish VDC reference off the PI node and center-tap the filaments there.

[the3secondrule]

Does the 2203x even have a filament center tap wire? It shows "NC" on the schematic. I wonder if it's actually there. I have a 2203x right here but I'm not cracking it open right now lol.

Marshalls implementation of a resistor-balanced filament center-tap and a hum balance pot is sketchy at best. Thousands of Marshalls existed without this "technology" with great results for decades. Cost cutting parts and design aren't helping matters even on otherwise great amps like the 2203 reissue.

Bypass the board mounted hum balance pot and resistors and redo it elsewhere, more solidly. Floating the heaters on an elevated DC reference isn't a bad idea - many amps do something similar. No need for a balance pot. Make your own 50-ish VDC reference off the PI node and center-tap the filaments there.

Doesn't appear to have the center tap no. Will look at implementing the floating heaters, I assume by the PI node you mean the R16,17,19,20 intersection? I may just revert to the original setup as this is slightly above my paygrade.
Need to figure out why the heater circuit cooked in the first place though, doesn't seem to be any shorts between heaters / cathodes.

Investigations continue..

[drewl]

They use a cheap little pc mount Bourns pot for the hum balance, maybe too much current for it or random failure?

[Greg_L]

Doesn't appear to have the center tap no. Will look at implementing the floating heaters, I assume by the PI node you mean the R16,17,19,20 intersection? I may just revert to the original setup as this is slightly above my paygrade.
Need to figure out why the heater circuit cooked in the first place though, doesn't seem to be any shorts between heaters / cathodes.

Investigations continue..

No that's the cathode side of the phase inverter....the long tail pair. Don't mess with that. You would make a filtered voltage divider node at the can cap filtering the phase inverter B+ supply. It's two resistors and a capacitor. Measure the voltage at that can cap and do some math to figure out what resistors you'd need to make a 50-ish volt spot between them.

Or if that's too much...just do the old tried-and-true 100 ohm resistors to ground off each half of the filament winding.

Bottom line...get rid of the hum pot and get that stuff off the board.

[Greg_L]

Looking more at the schematic....I suppose it's possible that you could elevate the heaters at the fx loop W1 connection. It shows a 30VDC reference. I think that would do it. Connect the center tap of the heater winding there, and add a small 10uf electrolytic cap to ground to filter that node and off you go.

Maybe it's a bad idea....someone chime in please.

[mozz]

Why would you elevate the heaters if there was no hum problem? Solution looking for a problem? I have to add, now if you did have elevated heaters, then had a shorted tube, would you be looking at more problems than if they weren't elevated? Just curious on all these points.