Marshall JCM800 Lead (2205) VERY HI HUM---HELP!

[Pedro Vecino]

V2 is exclusively for the clean channel. It could be that his mute capacitor (0.22uF, usually a tiny blue ceramic below the PCB) was short or leak and let pass current to the grid.
If so, would generate hum.

[nosaj]

So what do you get if you pull v2 then inject signal at v3 grid. Lower volume probably but hum?

Nosa

[nevetslab]

That's something I haven't checked.....looks like C40. It's attached to the collector of Q3 within the CA 3046. All I see, DC wise on that grid is 0V, but worth looking at, once I get the amp back on the bench.

[nevetslab]

Do you mean inject signal at V3A...wiper of the Volume pot for VR5? Good question....I'll have to give that a try as well on Monday, when I muster up the courage for more abuse by that amp. Just finishing up swapping out the power tube sockets of a JCM 900. Good exercise for cursing at inanimate objects.

I'll advise what I find.....thanks.

Steven

[nosaj]

Do you mean inject signal at V3A...wiper of the Volume pot for VR5? Good question....I'll have to give that a try as well on Monday, when I muster up the courage for more abuse by that amp. Just finishing up swapping out the power tube sockets of a JCM 900. Good exercise for cursing at inanimate objects.

I'll advise what I find.....thanks.

Steven

Yes, so we can isolate it down.

[nevetslab]

New Buss Caps installed, no change

Removing V2 removes the problem,
[ATTACH]32700[/ATTACH] [ATTACH]32701[/ATTACH] .

Back from other maintenance chores, and ready for more abuse from this JCM 800. The replacement FT buss caps arrived, and I just got them installed. As I previously speculated....didn't make one bit of difference.

I was incorrect in my stating removing V2 removes the problem. It only leaves the Boost Channel, it being the worst offender, in circuit, and just as nasty as before.

Before I go and inject any non-60/120Hz signals into the amp, such as at the grid of V3A, I've still got V2 removed, and about to go probing thru V1 Boost Stages to see just where this hum source is coming from. I'll also be (once again) lifting the PCB up and out of the chassis to get to the bottom side of the board, allowing me to look again at the foil layout. And do some circuit isolation.

[Jazz P Bass]

Cheering you on.

[nevetslab]

V1B appears to be the offending stage. Clean up to that, and not seeing anything at the Grid pin 7, but at the plate, turning up the Gain control....all the hum/power supply ripple comes screaming thru. With the Gain control turned down, looking at V3As plate, it's clean. Gotta wrap this up to catch my bus home...will continue tomorrow.

Cheers.....Steven

[rf7]

Correct me if I'm wrong, but with the scope set at 5ms, isn't that 60Hz hum we're seeing in those pictures? I would mess with the heater wires in that amp and see if the issue changes.

[olddawg]

I played an outdoor gig in Las Las Vegas a couple of years ago where everything was run off of generators. The backline was provided by the promoter. By the end of the day the generators had killed three JCM800s and two old SVTs. Lol. All rented.

[Pedro Vecino]

I've seen in this and in many other Marshall that several Russian 12Ax7 (WA, WB, EH, Tung Sol...) generate abnormal hum at different levels in V1. If that's the case, I would put a Chinese o Slovak 12AX7 in in that socket to discard it.

[Jazz P Bass]

V1B cathode diode, D1, replace it.
Not that? Try bypassing the whole gain/ volume control.

[g1]

Maybe try disconnecting C11 too, just to be sure it has nothing to do with that switching IC.

[TimmyP1955]

I don't know if I've seen it in 800s, but 900s can be very picky about V1 - most tubes seem couple way too much heater supply into the signal, making replacement a real PITA. Thankfully, V1 usually lasts for years.

[Alex R]

does it go away if you ground the grid of the first stage? Sorry if I missed something but you did check the input jack tip is grounding?