1989 jcm800 2210 - constant hum in power amp - SOLVED! sort of :)

Overtone85

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Long time lurker, first time poster.

I have a couple of plexi/jose builds under my belt and I am servicing my friend's 2210.
It had original LCR caps.
The amp came with a failed PI tube so I replaced it. When I turned it on it had a background hum, loud enough to be very annoying whem playing the amp at bedroom volume.

Also, the hum is most noticeable when all volume controls are at zero, gain at zero on the dirty channel. The hum affects the clean channel as well.

I thought it was the filter caps so I proceded to replace them with F&T caps. I also reformed the new caps properly with a current limiting resistor until the voltage drop was under 2v.

I cleaned the pots, input and output jacks.
I replaced the bias caps with new 10uF 100v electrolytics
I replaced the main 3300uF 6.3VV filter cap on the switchign circuit supply.

The power tubes are healthy and I rebiased them at 65% plate dissipation.

I fire the amp up and the background hum was still there. So I started removing all the preamp tubes one by one, starting from v1. Nothing was changing until I got to the PI.

With the PI in the amp, there is still hum. When I pull the PI abnd just leave the power tubes, the hum goes away.

I scoped the B+ line. Except the residual ripple after the first filter stage, the DC is flat and clean after the choke.

I found a thread in another forum where a guy had the same amp, same exact problem, ( no PI, no hum ) and he discovered a loose ground lug under the board.

So this morning I lifted the board and I did not find any loose ground lugs, they're all pretty solid and continuity meausres fine.

So I proceded to reflow all the solder joints under the board. Found some dirty connections on the reverb return tube socket and reflowed them. I reflowed the ground wire that is part of the shielded wire going to the fx loop and then to the chassis bolt that is fastening the reverb transformer.
I also cleaned the chassis where the pots nuts are supposed to make contact with it.

After all of this the hum is still there.

If I jumper the 2 grids of the PI, the amp is completely silent.

The wires going to the FX loop recovery stage are a bit microphonic when I tap them.

I measured all the preamp tubes with my Orange valve tester and they all seem fine. Pplus, if the hum is still there even with all preamp tubes pulled , they must not be the cause.

The only components I have not replaced yet are the small electrolytic on the preamp cathodes and switching circuit.

I forgot to mention taht all of this happens both with guitar plugged in and not plugged in.

One last thing. If I keep the amp in plying mode (off standby) and turnt the power switch OFF the hum goes away instantly and I just hear the hiss of the preamp fading as the filter caps discharge.

Apart from all of this the amp sounds great.

Any suggestion are highly appreciated.
Cheers
 
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Overtone85

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I also forgot that I already cleaned all the input and output jacks and pots with contact cleaner.
How about the rca jacks on the reverb tank?

I tightened the the outer contacts of the rca jacks, sprayed contact cleaner on the tank sockets, plugged the jacks in and twisted them a few times. Doesn't also the hum is still there with the reverb off. When I turn the reverb effect up I hear a different hum, but that's a different problem altogether, bore like a 100hz buzz. The hum I hear sounds more like 50hz

I am in UK by the way.
 

Gunner64

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In my experience even if the output tubes are technically healthy, they can drift into a mismatch situation, that can cause hum.. I would try another matched set of output tubes.
 

neikeel

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I have also been through a similar debugging exercise, on one occasion it was a modded amp with a ground loop, on another it was as Gunner suggested a mismatched set of tubes. I presume that the screen resistors are in spec and the output sockets are clean and scorch free?
I would be interested to know exactly how a preamp socket fails (broken pin, scorched, failed tension) in normal use but I'm sure it should be explored.
Have you scoped the PI output couplers (or more correctly decouplers!) are you getting leakage across one of the caps onto the ouptut grids? Not bad enough to cause redplating -just hum?
 

Overtone85

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Thank you all for the suggestions.

Today I've replaced all the other small electrolytics on the board. The 22uF Cathode caps, 22uF/100uF on the switching circuit and footswitch jack, the 10uF under the board.
I also replaced all the rectifier diodes.

There is a little improvement, but you can still hear the hum.

I discovered that when I crank the reverb the background hum is partially cancelled and I hear more of the 100hz buzz from the reverb circuit.

I still havent tried a new set of power tubes though.

I will check the coupling caps at some point.
 

Overtone85

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I just tried new power tubes and they do not solve the problem. Also the screen resistors all measure in spec. 2k as written on the component.
As far as coupling caps go though. if they were leaky I would imagine they would hum even when the PI tube is pulled out. But the hum stops when I do that.
 

Gunner64

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I just tried new power tubes and they do not solve the problem. Also the screen resistors all measure in spec. 2k as written on the component.
As far as coupling caps go though. if they were leaky I would imagine they would hum even when the PI tube is pulled out. But the hum stops when I do that.
Have you cleaned the tube sockets? Especially the p.i. and powertube sockets? I would hit them with some cleaner just to rule them out as the culprit.
 

Overtone85

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Have you cleaned the tube sockets? Especially the p.i. and powertube sockets? I would hit them with some cleaner just to rule them out as the culprit.

The first thing I did before servicing the amp was cleaning the top of the chassis (it was full of gunk). Then I sprayed contact cleaner in all the sockets and then used one of those inter dental brushes to scrub the preamp sockets.
What I didn't try yet is to retension the socket contacts. Although I don't hear any changes when wiggling the tubes around.

When I have time I will try disconnecting the input cap at the PI to see if the hum is in the signal before it. I hope it's not a socket becoming conductive.

I have to say that something must have been spilled on this amp. Maybe the classic pint of beer on top of the head? It's not dirty inside though. I'll post some pics
 

Marcell

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Isn't it the groundloop? Same issue when you plug it in a not earth ground wallsocket.
 

Chris-in-LA

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I just tried new power tubes and they do not solve the problem. Also the screen resistors all measure in spec. 2k as written on the component.
As far as coupling caps go though. if they were leaky I would imagine they would hum even when the PI tube is pulled out. But the hum stops when I do that.
Have you checked your coupling caps to see if they are leaking DCV? I don’t think I’d keep swapping out parts hoping that you can fix it. You might need an oscilloscope to see where the noise is actually coming from.
 

Sg-ocaster

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I would be interested to know exactly how a preamp socket fails (broken pin, scorched, failed tension) in normal use but I'm sure it should be explored.
In my case I think I wore it out rolling tubes......i had about 25 differant ones I rolled through and rerolled my favorites to get my favorite combo.....even deoxit wouldn't help.....i had to wiggle the tube to stop the hum till I replaced the socket.
 

Overtone85

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Sorry for the silence. I have been away from this amp for a while, just tried to have a go at it again.

In case I haven't mentioned yet, I have confirmation the hum is in the power amp only. I pulled out the PI and send the preamp output to a solid state power amp, no issues, just normal expected noise.

So again I pulled all the preamp tubes but the PI and power tubes. If I tap the wires coming from V4 they are a bit microphonic.

I tried to disconnect the input wire at pin 2 of the PI and the hum becomes stronger, I also tried to disconnect the 100k plate resistor at V4 B with the same result. With this resistor disconnected I tried to tap around and found that the wires from V4 were not microphonic anymore, but the wires going to the PI socket were. If I bend the board up by pulling the shielded wire near the reverb tranny the hum decreased but then I found it was mostly because of the PI wires moving. So i tried to retension the PI socket and I broke one contact. I replaced the socket with a new belton 9pin now, AND THE HUM IS STILL THERE!

I had already tried a new set of power tubes, with no luck.

My next step would be to lift the PI couplers to check if they're leaky.
By the way somebody had already tried to debug this amp I found solder blob on the feedback resistor and the V4 plate resistor I mentioned above, where they snipped the component and then soldered it back in place.

So far the only thing that reduced the hum by a very small amount was lowering the feedback resistor to 50k. Bypassing the feedback resistor just gives motorboating.

If the PI tube was faulty when I got the amp ( it was white inside ) maybe something else in the circuit caused it to fail?

R
 
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Overtone85

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Does the standby switch have good continuity?, Deoxit in here will make connection worse by the way because of all the grease in there.

I presume you have read my recent thread, although I don't think the pot's fitted to yours have this problem but you never know, https://www.marshallforum.com/threads/70s-super-bass-hum-fixed.116702/

Thanks for the suggestion, When I pull the board up I will check the pots again. I just tried to jumper the grids of the PI (with all the other preamp tubes pulled out) and the hum goes almost silent. So something is happening in the PI circuit for sure. next step I will disconnect the heaters from the V4 socket downstream just to rule it out and then disconnect the input coupling cap to the PI. I hope that doesn't change anything so I know for sure it's in the PI.

I will report with my findings soon.

R
 

Overtone85

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A little update

Disconnecting the heaters from v4 socket did not improve.

Disconnecting the coupling cap into pin 2 of the PI did remove the hum but I got a buzz instead. I tapped and moved the wires around and found that the grid wire on pin 2 was picking up buzz from the heaters so I lifted it up. I also discovered that the Output tranny center tap and the chocke wire had a dodgy solder connection and the buzz decreased when i moved them away. So I have redone the solder joint on the HT-CT-choke turret and untwisted the wires to move them away. I also recut the ground wires of the rectifiers and the presence control because they seemed a bit weak. It did improve the buzz even more.

I reconnected the pi input coupler and the hum came back.

I retested the hum in the preamp, this time by keeping all the tubes in, hooking up the amp to a dummy load but using the fx loop send to play the preamp with a solid state power amp. No hum.

Then I've done something I should have done a long time ago (LOL). I plugged the amp back into the speaker cabinet and grounded pin 2 of the PI. NO HUM!

SO, I think something is going on with V4a, the fx loop recovery stage. I will try to bypass it and see what happens.

https://drtube.com/schematics/marshall/2210prem.gif
https://drtube.com/schematics/marshall/2210pwrm.gif
 
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Overtone85

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Update

It seems that i only get hum when the plate resistor on v4 is connected to the board. R30. When disconnected, the inout couplig cap to v5 pin 2 is floating and it picks up noise from the heaters, but it's a different buzz noise.
Pulling the tube on v4 doesn't change anything.
Replacing the couplig cap to the PI, C25, doesn't fix the hum.
I scoped the B+ line and there is no hum on it (besides, the preamp works well with minimum hum)

If I keep R30 disconnected and jumper the grid of V4a to the PI grid, I can play the amp without hum. This way v4b is still working for the reverb but I bypass the recovery stage.
I was thinking about replacing the v4a cathode resistor and see what happens, but I still get hum when the tube is pulled so I am not sure it's worth it.

I will try to remove C25 again and gator clip it to the PI off the board and see what happens.

I edited the previous post adding the power amp schematic.
 

zkrx

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Hey Overtone, I am debugging a similar issue with my Laney GH100L which has a similar output stage. In my case, desoldering the NFB wire from the secondary makes the hum disappear. Have you tried that? I still need to understand what's going on.

EDIT: i reread your last post and you're issue seems different.
 
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Overtone85

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So. I sort of fixed the issue and gave the amp back to its owner.

While trying to debug the circuit between the PI and the fx-loop/reverb recovery I reflowed a lot of solder joints under the board. After which I noticed that the sound at idle didn't just have a hum anymore but it also had louder hiss.
When V4 is pulled, the PI input wires become like an antenna.

Then I kept scouring the web for info and found that many people have issues with the master volume of these amps, complaining that it's very touchy, above '1' on the dial it sounds like it's on 10. Some replaced the PI with a 12AU7 and managed to make the master volume more usable.

When I scoped the output of the PI i realised that the PI is distorting the signal asymmetrically ALL THE TIME, and if I understood correctly this is by design. The lower portion of the sine wave is always soft clipped.
I also remembered that when the reverb is on 10 it drowns the dry signal almost completely.

So I thought:
1 - the fx send is before the recovery stage and has NO HUM.
2 - the recovery stage is too hot sending too much signal in the PI.
3 - the clipping PI is amplifying the noise.

I put a spare 12AU7 I had laying around in the PI, didn't help much with the hum.

I put the 12AU7 in V4, reverb driver and recovery stage, and it decreased the hum by A LOT. It made all the difference.

Next I reduced the V4 recovery stage plate resistor by soldering a parallel resistor on top of the board, in order to reduce the gain.
I tack soldered a resistor in parallel with the PI tail resistor to make the PI less asymmetrical.
I also tack soldered a resistor in parallel to the negative feedback resistor to reduce the PI gain still.
And finally i tack soldered a 4MEG resistor between the output of the PI input coupling cap and the presence control ground lug on the chassis, in the attempt to bring the input signal level down.

Now the amp has a very low hum when idle, you need to put your ear to the speaker to hear it, and when the guitar is plugged in, the hum from my Duncan JB and the preamp distortion is louder anyway. When rolling the guitar volume down, the hum is barely noticeable in the room.

I couldn't think of anything else, it's technically not a stock amp anymore but it's all completely reversible without damage to the amp. When cranking the lead channel to 10 and the master volume to 10 you can still overdrive the power tubes.
I hope this can be useful to others fixing a 2210.

Cheers!

R
 


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