1979 JMP - No Sound After Turning Up Master?

[bluesriffdev]

Hi all,

I recently got my dads old 79' JMP repaired as his birthday gift, after it sat for 12 years gathering dust. Today he was playing it and after turning the master from 4 to 6, the signal suddenly cut. It didn't blow up or make a massive fizzle sound before dying; it just died, but the pilot lights all stayed on, and the tubes were all lit. I'm somewhat joyous at this because it didn't look / sound like an expensive fault.

Has this happened to anyone before? Got any ideas? My guess is the output transformer. I would have thought if it were a fuse, all the lights and valve lighting would dim. Similarly, if the speakers died I would have thought it'd be a bit more dramatic. It was a new speaker cable too and I tried moving it around to see if I had it in a short-out spot.

Let me know if you've got any ideas. It's a Sunday arvo here and it'll be a couple days before I'm at the techs store next, so if there's a few simple things I can try myself or some ideas of what has done it, that'd be great. Cheers!

 

[redcoats1976]

maybe a bad master volume pot?it has sat long enough for dust/carbon to build up.try some electrical contact cleaner on that pot,turn it back and forth.even a static sound while you are turning it could indicate a bad pot(wait for the contact cleaner to dry before plugging it in and turning it on)

 

[bluesriffdev]

maybe a bad master volume pot?it has sat long enough for dust/carbon to build up.try some electrical contact cleaner on that pot,turn it back and forth.even a static sound while you are turning it could indicate a bad pot(wait for the contact cleaner to dry before plugging it in and turning it on)

I'll give it a go. Is that likely though? There's literally no sound, even when I twist it. It's a bit like having the standby on.

 

[redjunior]

Check the High Voltage....

 

[tazzboy]

Check the fuses first you might have blown one.

 

[Pappy58]

Be careful putting your hands in there......check DC voltages before proceeding...

 

[Cjsinla]

Check the fuses first you might have blown one.

How many fuses are in that amp?

 

[bluesriffdev]

How many fuses are in that amp?

I believe there are 2. Surely if it were a power fuse, the whole thing would have shut off, I'd have thought.

 

[Cjsinla]

I believe there are 2. Surely if it were a power fuse, the whole thing would have shut off, I'd have thought.

Just asking. Lots of amps only have one. I know my JCM’s have 2.

 

[Big John]

Did you try all the input jacks? Might just be a dirty/bad jack. Or, perhaps a cold solder joint if the repair tech changed anything out.

 

[Mike_LA]

The Master on my JMP did the same thing.
Dirty pot, though you didn't say if it came back or not.
If it stayed off, I would check the glass, preamp tubes and fuses.
best of luck

 

[bluesriffdev]

Just asking. Lots of amps only have one. I know my JCM’s have 2.

View attachment 310947

That's all good. I might have a look. I'm thinking of replacing the fuses and pot before I send it back to the tech. I'm confident that I can at least do that without blowing myself up. Yeah the early JCMs and late JMP 2203s are almost identical.

Did you try all the input jacks? Might just be a dirty/bad jack. Or, perhaps a cold solder joint if the repair tech changed anything out.

Tried both jacks, unfortunately.

The Master on my JMP did the same thing.
Dirty pot, though you didn't say if it came back or not.
If it stayed off, I would check the glass, preamp tubes and fuses.
best of luck

I'll check that they're all seated properly. All power tubes were lit up but the pre-amp tubes have the covers on them so I'd have to check that. Sound didn't come back, even after I brought it back to a lower volume and tried turning it off and on. I thought it was a dirty pot too but there's no static at all when I twist it. The amp is silent with no hum, but lights are on and valves are lit.

I'll try turning it on soon and seeing if it emits volume again. That might narrow down the problem.

 

[Big John]

...It was a new speaker cable too...

Just a shot in the dark, but maybe try an old one that you know is OK? Also, try all the speaker jacks on the back of the amp?

 

[bluesriffdev]

Can confirm that it's a blown HT fuse. I'll replace it and see if it goes again. Does that immediately point to a faulty power tube or could it be the output transformer? I really hope its the tubes but they were all replaced when the amp got serviced; is it likely a brand new JJ EL34 would die like that?

And the replacement fuse would be a 1A 3AG Slow Blow Glass Fuse or a Fast blow one?

 

[tazzboy]

If the power tubes were still lit than it wasn't them because they wouldn't be on if that were the case.

 

[Cjsinla]

Can confirm that it's a blown HT fuse. I'll replace it and see if it goes again. Does that immediately point to a faulty power tube or could it be the output transformer? I really hope its the tubes but they were all replaced when the amp got serviced; is it likely a brand new JJ EL34 would die like that?

And the replacement fuse would be a 1A 3AG Slow Blow Glass Fuse or a Fast blow one?

The mains fuse is 3A, the HT is 500mA. The wankers on the Marshall forum claim that the HT fuse is slow blow. Are the cap cans original? They also suggest that might be the problem if they are going bad.

 

[bluesriffdev]

The mains fuse is 3A, the HT is 500mA. The wankers on the Marshall forum claim that the HT fuse is slow blow. Are the cap cans original? They also suggest that might be the problem if they are going bad.

Probably original. Labelled as DALY and Made in UK. I think if the fuse blows again (which it inevitably will) I'm going to take it back to the technician because there's too many things it could be. He seemed to think the caps were fine after he'd turned it on and let it sit for a while. I mean, there's no hissing or farting, which is a sign of bad caps (or so I'm told). I'd have thought that testing an amp after you've repaired it would involve turning the master past 4, but old amps that haven't been serviced for years can be unpredictable I guess.

Thanks for letting me know that they're fast-blow. They're labelled on the back as 2A Mains and 1A HT; the values vary depending on voltage. I'm in Australia where appliances run at 220-240V.

 

[Cjsinla]

Probably original. Labelled as DALY and Made in UK. I think if the fuse blows again (which it inevitably will) I'm going to take it back to the technician because there's too many things it could be. He seemed to think the caps were fine after he'd turned it on and let it sit for a while. I mean, there's no hissing or farting, which is a sign of bad caps (or so I'm told). I'd have thought that testing an amp after you've repaired it would involve turning the master past 4, but old amps that haven't been serviced for years can be unpredictable I guess.

Thanks for letting me know that they're fast-blow. They're labelled on the back as 2A Mains and 1A HT; the values vary depending on voltage. I'm in Australia where appliances run at 220-240V.

Ok, but they said slow blow. The guy replaced the caps and the HT fuse blew right away. He then used slow blow at their suggestion and it held. The HT fuse seems to be located right after the power supply and between the two cap cans, schematics show that’s the b+ voltage at 465v. Let’s see if some other people will weigh in on this. @Leña_Costoso @Soul Tramp @Splattle101

 

[D'tar]

Pull the tubes, replace the fuse, power it up. Fuse blow, y/n. Shut down power.

Y= tech time
N= install tubes and power on again

Fuse blow?

You can do a generic quick test with your multimeter check for short circuit between pins on the tube, only the heater circuit should have continuity.

A lightbulb limiter would save a couple $ worth of fuses and possibly more expensive components.

 

[redjunior]

Do not put a slow blow in your hi-tension fuse block, that's asking for an expensive repair.
Went to slow blow on my 2204, next thing I know I'm buying a PT....