Marshall Superbass with mystery short in power supply. Exorcist required.

[christarak]

Marshall-JMP-Super-Bass-100W-1992-Schematic.pdf

I have what should be a very simple problem, but its driving me nuts. It came in with "Replace HT fuse holder" and "keeps blowing fuses". Easy, me thinks. A possible clue: He is running this 100w amp with 2 EL34's pulled, but I note that he's running the o/p transformer at 8ohms, I assume running a 16ohm speaker (I must check this with the customer) so that should balance the load out, right?

The short only happens when going from Standby to ON.

Here's what I've done so far:
1. Replaced the six original 1975 50+50 caps. (still shows short on light bulb current limiter). They were well overdue for this gigging musician.
2. Two of the diodes tested as short, so I replaced all 5, including, Bias diode. Also Replaced visibly leaking bias caps. (Still shorting).
2a. I used my newest toy ( a FLIR thermal imaging attachment to my iPhone.) Two of the new +HT diodes were putting out a fair bit of heat compared to the grounded anode diodes while I was ramping up the Variac with Standyby ON but before the light bulb was glowing much. I can't recall exactly, but I think I had the Variac at 50VAC (240VAC mains in Australia)
3. Removed HT fuse. Still shorting when switching Standby to ON. Oh Crap, me thinks...it must be the power transformer.
4. With switch Standby mode enabling me to monitor only the HT secondary winding, I variaced the voltage up to 240VAC mains. The secondary read 350VAC. Looks good. The filament was smack on 6.3VAC.

With the HT fuse out it seems that I have isolated everything except for rectifiers (replaced) and the first 2 50+50 caps in series (replaced). If the two 0.22uF caps or the Primary transformer had and issue they would have reared their ugly head with the Standby Switch open and 240v supplied.

It seems that there's nothing left for me to do other than get the amp exorcised or take a very large mallet to this historic beauty and tell the customer that its beyond repair. The fault that seemed so easy is now causing me anxiety. I'm sure your experienced eyes will say "Chris. You are are a fool. Its is clearly the hypergallactic interpolator at fault".

[Enzo]

Are you using slo blow fuses?


Oh, and make sure the fuses are upside down for Australia. (Yes, I never get tired of that joke)

[christarak]

Are you using slo blow fuses?


Oh, and make sure the fuses are upside down for Australia. (Yes, I never get tired of that joke)

HAHAHA! I reckon you've used that gag on me at least 10 times Enzo! Here's to the next 100!
Yep, slow blow fuse installed ...upside down. But my light bulb current limiter is glowing bright enough to guide the ships into Sydney Harbour.

[g1]

Nothing has been accidentally grounded at the center of the totem-pole filter caps? (cap shells insulated if required)

[christarak]

Nothing has been accidentally grounded at the center of the totem-pole filter caps? (cap shells insulated if required)

Thanks G1. I'll check that out when I get home. It's possible, as the symptoms are exactly the same as before I replaced the 50+50 caps. I'm not sure what you mean by "cap shells". Do you mean the blue plastic F&T wrapping around the 50+50 metal can?

I see that its how Marshall designed it, but I would have expected a couple of totem 220K resistors across those 2 series capacitors to keep the voltage evenly shared between the 2 totem caps. Do you think that's a possible issue, G1? (I suspect not, as the Marshall engineers would have already thought of it).

Thank you, G1.
Chris

[doombass]

Could it be the standby switch shorting when switched to on?

[HaroldBrooks]

Excuse my relative inexperience if these questions are too basic, or if you already covered these tests or types of tests already.

Did you try to measure the voltage of the transformer with the rectifier tubes out and out of standby ? there must be a baseline for that, and I would think it could expose a short in the transformer, if there was one and the light bulb limiter would come on without the rectifiers in place brigtly with the standby off.

Also, I've never done it myself, but can you measure the small resistance through the primary and secondaries of the power transformer (with unslodered connections) ?

Short in the power cord perhaps ? Shorted Standby switch (I would jump it out) ?

Keep at it, you'll get it with the help of all the great guys here !

[christarak]

Could it be the standby switch shorting when switched to on?

Thanks for that idea, doombass. It must be something weird like that. I'll check that when I get home in 5 hours and report back.

[christarak]

Excuse my relative inexperience if these questions are too basic or you already covered these tests.

Did you try to measure the voltage of the transformer with the rectifier tubes out ? there must be a baseline for that, and I would think it could expose a short in the transformer, if there was one and the light bulb limiter would come on without the rectifiers in place.

Also, I've never done it myself, but can you measure the small resisance through the power transformer ?

Short in the power cord perhaps ?

Keep at it !

Thanks for the suggestion, Harold. This amp uses solid state rectification, but still your suggestion is valid. If you look at the schematic, you will see that with the Standby switch open, I can measure the HT windings directly. The HT output with NO LOAD is in the right ballpark of AC. I did measure the DC resistance, but I'm not at my bench and can't recall the exact figure. Maybe 80ohms? But I'm thinking that if the were any inter-winding shorts, my HT AC voltage would be out. The lightbulb limiter does not come on with the diodes out of circuit by opening the Standby switch.
With the HT fuse out, the fault is limited to the Transformer, Standby switch, Rectifier Diodes (replaced) and capacitors (replaced).
A short in the power cord would appear continuously, so I expect its not there.
I really appreciate your input< Harold, and would welcome any other suggestions. I included the schematic at the top of my first post for your convenience.

[xtian]

[ATTACH]56788[/ATTACH]He is running this 100w amp with 2 EL34's pulled, but I note that he's running the o/p transformer at 8ohms, I assume running a 16ohm speaker (I must check this with the customer) so that should balance the load out, right?

If you pull two tubes, you need to match with half the load. So, on the 8-ohm tap, use a 4-ohm load. Isn't that right?

[Jazz P Bass]

I would disconnect the high voltage to the output transformer.
That would isolate the main B+ from the preamp voltages.
If everything holds up, I would tend to think the OT is shorting.

[christarak]

I would disconnect the high voltage to the output transformer.
That would isolate the main B+ from the preamp voltages.
If everything holds up, I would tend to think the OT is shorting.

Thanks Jazz. By removing the HT fuse, it isolates the HT voltage from reaching the OT, correct? With that fuse out, the short still appears when I close the Standby switch. Unless I've missed another path to the OT (EL34s are out of the amp).
Thanks mate

[christarak]

I think you've confused me, xtian. With 2 tubes out and the same speaker load do you use half or double the output tap? I thought it was half. . if I understand you correctly, if he had an 16 ohm speaker he would need to use a 32ohm (non-existant) tap, and not the 8ohm tap that I thought.

[glebert]

Half the tubes you should double the speaker ohms relative to the OT tap, so either keep the same speaker load and go to the next lower ohm tap, or double the speaker ohms.

Arc'ing tube socket?

[christarak]

Half the tubes you should double the speaker ohms relative to the OT tap, so either keep the same speaker load and go to the next lower ohm tap, or double the speaker ohms.

Phew! Thanks for clarifying that, glebert.

Arc'ing tube socket?

I think by removing the HT fuse, I have eliminated all the tube sockets. Everything from V1 to OT. The short is happening with no HT fuse AND with the Standby Switch closed. Standby switch open shows no short. Filament and bias supplies (replaced 2 leaky caps and its diode) are On with Power ON, and don't cause any issues.

Anything else you can think of Glebert, or can suggest I check? I have a couple of other suggestions to check on when I get back to my workshop.