Marshall 50 JMP Hum - Heater Filament Pot Candidate?

[solderstain]

Thanks, solderstain.

I've pulled V1 and the hum is still there. I've replaced nearly everything that touches V2 - since I think that's the spot - but with no luck.

BTW, the 4 input jacks are metal as I changed them years ago because I like that look better than the plastic ones.

This will be short, as I don't have time to really look close at the pictures....

Regarding the plastic versus metal input jacks... I'm a bit confused about the 'metal input jack' comment - those look like plastic Cliff jacks in the picture to me...???? What am I missing? Since pulling V1 leaves the buzz, it's clearly not in Stage 1 of either channel of the amp, so it's basically a moot point...

Since you say it 'just happened', how about telling us exactly WHAT happened? It was all fine and wonderful and one day you fired it up and BINGO - 60 hz hum? Out of nowhere? Or was there an event - tube change, or a tube shorted or failed and you replaced it and the buzz was there afterward? SOMETHING happened.

[hazy]

Jana: It's hard to tell from the pic, but, the green wires are up off of the amp chassis. I did that the other day putting them as high in the air as possible - no noticeable change in the hum.

The input jacks are plastic with metal nuts on the outside of the chassis. They used to have the plastic black plastic nuts on the outside but I liked the metal look better. The rest of the jack is plastic - I was referring to the external face being metal.

You know, there was no real event. I originally changed the filter caps 6 - 9 months ago because of hum. I remember that after the filter cap change, it was very quiet and gigged with it several times. A while back ~ 2 months ago, I gigged with it and the next time that I fired it up at home to practice, the hum was back as it is now. I didn't make a hard mental note of the time it arrived because I thought that I had it fixed after the 1st filter cap change.

Perplexed. Hazy

[Structo]

Yes, those jacks are not metal, you just used metal nuts on them but that does not ground the jacks at all. You have to wire them to the ground.

Have you substituted known good tubes for what is in this amp?

I can't think of a thing that would cause the symptom you are describing unless something has physically happened to the amp like, was it dropped down a flight of stairs?

Did somebody pour a cocktail in the top vents?

Did a power tube short out and fry something?

Is there a bad solder joint from the cap job that is just now becoming noticeable?

First thing, get some decent tubes in it then check all grounds by touching them up with a bit of solder and the iron.

In fact I would probably walk through the whole amp and touch up every solder joint.

Make sure heater wiring is routed away from signal wires, pay attention to lead dress around the sockets and to the board.

Do you have access to an oscilloscope?
Even without a scope you should be able to eliminate which stage is the culprit by process of elimination.

[Rick]

Have a qualified tech check out the filter section to see if any of the caps have gone bad.

[hazy]

Have a qualified tech check out the filter section to see if any of the caps have gone bad.

I replaced all of the filter caps - twice - with F&T 50/50 uf caps. I checked the ones I put in 2 months ago after I replaced them last week and they were fine.

I pulled the caps to and from V2 and checked them with a meter. I realize that's only 9v instead of amp voltages, but they checked out fine.

The tubes are NOS Siemens. They were brand new when I put them in - they replaced Mullard EL34s.

I think I'll just set up one day (in the not so near future) and re-solder the whole board, sockets, caps, and jacks. For right now, I put my '76 50 watt JMP in the 1x12 combo that housed the problem child. This is it with the '78:

[IMG:700:493]http://i123.photobucket.com/albums/o285 ... ont1-1.jpg[/img]

Thanks for the help. I know it's frustrating when I can't ID the event causing the issue. If I could do that, I'd have this done. I bet I'll be laughing when I get the answer.

Hazy

[Dai H.]

(Maybe it's there, but) I don't see a ground connection from the input jacks to the rest of the ground line. By any chance did it come off, or perhaps did you forget to put it back on when the jacks were changed to the type w/a metal ferrule?

[Structo]

Yes that may be the issue.

The first thing I would do is straighten out the lead dress.
There is simply too much wire in there.

The grid wires, the OT wires in and out,etc.
There are many avenues for crosstalk and ac induction problems in that amp.
Personally, I don't see how that amp was ever very quiet due to what I see there.

[solderstain]

Yes that may be the issue.

The first thing I would do is straighten out the lead dress.
There is simply too much wire in there.

The grid wires, the OT wires in and out,etc.
There are many avenues for crosstalk and ac induction problems in that amp.
Personally, I don't see how that amp was ever very quiet due to what I see there.

I suspect not, regarding the 'apparent' lack of input ground - he says the noise is still there with V1 pulled, so that kinda lets the input jack off the hook.

I'll be a voice from the other side of the fence regarding peoples' comments about the lead dress, etc. My old metal panel SL lead dress looks pretty much like that, and it doesn't hum. That lead dress looks pretty much like EVERY Marshall I've opened. They don't all hum.

Given that Hazy says the buzz is still there with V1 pulled, but it's gone when V2 is pulled, I have strong suspicions about the dual 50uf can under the circuit board or the one immediately after the choke (or with wiring associated with it), myself...

[UR12]

Like Structo aluded too, make sure those cliff jacks used for the speaker jacks are grounded. Because of the negative feedback coming from the secondary of the OT, if the speaker jacks aren't grounded or loose their ground you will get hum.

[Firestorm]

It might be worth taking a look at The Trainwreck Pages 36 and 37 (in the Files section) and attempting the lead dress Ken described. Couldn't hurt.

[hazy]

Like Structo aluded too, make sure those cliff jacks used for the speaker jacks are grounded. Because of the negative feedback coming from the secondary of the OT, if the speaker jacks aren't grounded or loose their ground you will get hum.

I'm going to check the input jacks' ground again. I replaced those a long time ago and might not have grounded them properly.

I agree on the lead dress. I have another Marshall (1976 JMP 50 watter) and this one (1978) is light years better as far as the wiring and wire position. Also, the board is a Metro board that I installed in 2005.

Hazy

[UR12]

Like Structo aluded too, make sure those cliff jacks used for the speaker jacks are grounded. Because of the negative feedback coming from the secondary of the OT, if the speaker jacks aren't grounded or loose their ground you will get hum.

I'm going to check the input jacks' ground again. I replaced those a long time ago and might not have grounded them properly.

Hazy

I wasn't refering to the input Jacks I was talking about the jacks where the speakers plug in on the back of the amp.

[hazy]

I wasn't refering to the input Jacks I was talking about the jacks where the speakers plug in on the back of the amp.

I never touched the speaker jacks. I did replace the input jacks. Could the speaker jacks have come un-grounded?

I was off-line for a few days without a modem.

Thanks,

Hazy

[M Fowler]

It looks like there is a ground wire on the jacks going off to the left side check to see if that is going to the chassis grounding. The other wire goes to the imp. switch.

Mark

[hazy]

It looks like there is a ground wire on the jacks going off to the left side check to see if that is going to the chassis grounding. The other wire goes to the imp. switch.

Mark

I'll check it. Thanks.

Hazy