JCM 800 2203 Hum

[Enzo]

Gtr - I believe I mentioned pulling the wires off the socket, so the battery in my idea was only clipped to the one socket that seemed to have the problem. The remaining tubes would all still be powered as before by the amplifier.

[rf7]

Unfortunately, replacing the Filter caps and the Bias caps did not really do the trick.

The hum seems to be induced through the R7 to C7 path into V2a. If I short either end of R10 to ground, then the hum is negligible. As noted before, with tube V1 pulled, it still hums. The B+ side of R7 looks clean, but I see a small 120Hz dip at the R7 and C7 junction, which matches up with dips in the hum's waveform.

The hum now has a 60Hz period, but looks like 120Hz with 2 different peaks.

Any ideas on how to proceed?

[Chuck H]

OK... Since grounding either end of R10 stops the problem, that rules out V2 entirely. Since pulling V1 doesn't stop the problem, that rules out V1 entirely. So the hum must be getting into the amp in the coupling circuit between V1b and V2a. Check grounds for that circuit (reflow questionable joints, etc.). Check circuit values in this area. Check to be sure it's actually wired correctly. Check to see that no leads in the amp have been re routed incorrectly so they are near this circuit. Check to see that the filters are grounded IN THE RIGHT LOCATION. Try removing C8 to see if it's picking up noise.

[big_teee]

On my 2204 Amp build I had ground hum issues.
The only way I got it to stop was to ground it like the Ceritone Layout.
The preamp ground chain is tied together and only grounded at one point.
I chose to ground it to chassis under the board at the 50/50 Preamp Cap, near the input.
If I tied the other end to ground it had the hum.
http://www.ceriatone.com/images/layo...3Ceriatone.jpg
Good luck,
Terry

[Chuck H]

it's possible... something is grounded in the wrong place.

Check to see that the filters are grounded IN THE RIGHT LOCATION.

The only way I got it to stop was to ground it like the Ceritone Layout.
http://www.ceriatone.com/images/layo...3Ceriatone.jpg

Is there a theme forming???

I wasn't clear about what exactly to ground differently to stop the hum (because I couldn't be sure). But BT was. The Ceriatone scheme can't be that different from the stock scheme. Probably just a couple of leads. Change a couple of ground leads now that you have a working guide and see if it helps. Just because the amp was made by Marshall doesn't mean it's perfect the way it was built. And as I said earlier, it could be that someone inadvertently misplaced one or more grounds at some other time.

[big_teee]

I would concentrate on the Preamp ground buss, everything else seemed less critical, if properly grounded.
The important thing is to not have the Preamp ground buss grounded on each end or additional chassis grounds in the middle of the Buss.
B_T

[rf7]

Thanks for the ideas everyone!

Alright, I checked everything out between V1b and V2a, and nothing was bad. I removed C8 and it made no difference. I moved the Preamp ground to the ground for the preamp filter caps and the hum decreased by one third, so it's definitely better. Went from 140mV pk-pk down to 80mV pk-pk with the Master Volume cranked. But it is still a bit much, really. Moving the preamp PCB ground from one end of the PCB to the other did not make a difference.

[big_teee]

I also cycled through all the preamp tubes and put the quietest one in V1.
Make sure you have good quality Shielded wire on the Grids of V1.
I used a double shield wire for that and it helped.

Terry

[Chuck H]

I moved the Preamp ground to the ground for the preamp filter caps and the hum decreased by one third,

Try putting the preamp ground back and move the preamp filter ground to that location also. And check if any grounds are doubled up. As mentioned, the buss should only be grounded at one end. The end nearest the input jack is probably best.

[rf7]

Try putting the preamp ground back and move the preamp filter ground to that location also.

Oops, I thought that did it, but the impedance switch was set wrong. So that did not change a thing, unfortunately.

[big_teee]

Bingo! That did it. Made a big difference, enough to make the amp useable now.

Thanks alot Chuck!!!

Glad you fixed it.
So is the grounding now stock, or more like the Ceritone Layout?
Terry

[rf7]

Sorry, I messed up and that did not fix it.

[drewl]

For the heck of it, check the filter cap ground connections.

I had a JCM900 last night with a filter cap ground wire sitting on the cap looking like it was connected, but the wire had broken.

Trying to fix a similiar hum on a Roccaforte marshall clone was a pain....finally found a couple 12AX7's that would work without too much hum/noise.

[rf7]

All the grounds check out OK.