MarshallJCM800 2210 hum

[jvm]

Thanks for your input, guys! I unhooked the filament winding, including the CT, and still get 6.3 Vac

[Chuck H]

What about the voltage from either end of the winding to the CT?

[jvm]

Hi, Chuck H. I measure 2.9 volts per side to the center tap

[g1]

So the heater winding will not hold up under it's rated load (5.5V with tubes in is too low). A separate transformer to run the heaters would probably be the most reasonably priced option.

[Chuck H]

Does the amp have a replacement PT??? That the filament winding is symmetrical inclines me to think that it isn't shorted. So why shouldn't it handle the load? Is there a lot of rewiring in the amp? Do the filament leads look especially skinny?

Then there's the initial problem of hum.

I want to second g-ones inquiry about the primary voltage. But I'll elaborate. Do we know the PT's primary spec so that it may be compared with the AC voltage source?

If it is the stock transformer but the amp has been worked on there is the possibility that someone hooked up the primary incorrectly. A couple of Marshall's I've seen have several, not just four leads from the primary wind to facilitate three selectable primary voltages. I'm not sure what the extra leads do, but I'll bet it's possible to wire the primary wrong in more ways than two.

Also see post #9 and check for any over current conditions.

[jvm]

Ok. I have installed a new PT as per owners request. ( He said he wanted to upgrade anyway). No change at all. The primary is wired correctly according to the schematic. I still get 5.5 vac at the filament string. So, to update, ALL electrolytics replaced, PT replaced, ch switching IC replaced.

[big_teee]

Have you tried testing with a different Volt meter?
You may have said, I didn't read every post.
T

[gtrplayr1976]

I'm not an expert by any means ,but have you worked your way back from D2 ,and D3 (filament supply) to see where the voltage changes ?

[Chuck H]

When you removed the IC from the filament circuit, did you lift the 6.3V wind from the diodes or did you disconnect the IC from the diode junction?

[g1]

Definitely try disconnecting D2 and D3 from the heater supply as Chuck is suggesting. Does the heater voltage improve?
Also, while the heater measures 5.5VAC, triple check that there is 120V at the primary connections of the PT (not at switches or fuse or anything but at the actual PT wires).
And much earlier, Enzo had asked about the other secondary windings. When the heater is at 5.5V, what is the high voltage AC at the standby switch?

[jvm]

New ball game! I noticed a Mercury choke on this amp and asked the owner about it. Someone installed it for him and the hum started afterwards. So he brings it to me without mentioning that little detail! I had him bring me the original choke and swapped them out with no change in hum. I have now reinstalled the Mercury. So, signal tracing finds my hum at both sides of the choke, at all four output tube 1k resistors, and at the plate of V3b which is fed directly from the choke. Suggestions? How can I isolate the source of this hum. Choke is correctly installed.

[big_teee]

Were any grounds moved or added?
Grounding is always a big issue with high gain Marshalls.
Or possible corrosion of existing grounds?
T
** Some gut-shot Pics would be nice?

[jvm]

Hey, big-tee. I don't see any wiring changes compared to online gut shots. I cleaned every ground I came across when I changed the filter caps. Here is a shot of the wiring pertaining to the choke. The two large black wires are the choke leads. One on the filter cap at left and the other at the post on the pc board

[gtrplayr1976]

Is that where the choke is suppose to be wired ? Schematic looks different . I see it now. Never mind.

[Chuck H]

So, signal tracing finds my hum at both sides of the choke, at all four output tube 1k resistors, and at the plate of V3b which is fed directly from the choke.

And this is to say that you have checked and there is no hum present at the plates of any other preamp tubes then, correct?

If the hum is 50/60Hz then it's filament circuit or mains related. The rectified B+ should be twice that frequency. And that is what the decoupling/filter caps are supposed to remove. I don't know why you would have 60Hz on your HV supply unless your rectifier is bad or there is a mistake in the wiring. There is still the issue of the low filament voltage which may be related.

How many AC volts are on your HV supply line at the offending choke node?

If the hum is 100/120Hz then make sure whomever installed the choke before you did indeed see to it that there is a decoupling node at both ends. If you were simply replacing caps and it wasn't there when you started it would be easy to miss that. Still...

WHY THE LOW FILAMENT VOLTS??? That the filament volts are low with a new PT is indicative. They are not supposed to be 5.5V. Find that problem and you may find the hum issue also. In fact I think it's likely. I wouldn't bother with one more thing regarding the hum problem before I solved for the low filament problem.

EDIT: Here's a hint, if the hum is 50/60Hz and only appears on all supplies from the one choke node, try to determine why the filaments are related to that node. They aren't schematically. But are they in the real world?