Buzzing on High Input of JCM 800 2204 Horizontal Input Version

  • Thread starter Thread starter Bardagh
  • Start date Start date
I re-read you initial report that seems to indicate most of the noise is coming from the preamp tube socket being loose on the pin of the 12ax7 tube rocking back and forth and also probably not making good contact which can make mechanical scratching noises as well as the noises you are describing.

Since you replaced the jack to no avail I would look closely at the V1 preamp tube socket and tighten the pins as tight as you can get them then resinsert the tube. If the pins are tight and you no longer have those noises when you move the V1 input grid wire around then it was the tube socket pins. You can then explore the other suggestions listed in this thread.

The LCR brand filter caps are of highest quality and if the amp is used regularly they can last over 40 years no problem. It's when an amp goes unused and the electrolyte dries out and the begin to degrade, bulge and leak.

Sounds like you are getting to the bottom of your issues so let us know how it all goes.
 
one thing I’m not sure of with this due to the inputs/pots being mounted on the pcb is where I should ground the shielded cable when I replace the input wire. Ideally it should be grounded to the input jack but how would you go about that with these jacks that just have the pins instead of lugs?

I’ve only ever seen people demonstrate doing this with the pre-84 versions with the chassis mount inputs.
 
Bardagh said:
one thing I’m not sure of with this due to the inputs/pots being mounted on the pcb is where I should ground the shielded cable when I replace the input wire. Ideally it should be grounded to the input jack but how would you go about that with these jacks that just have the pins instead of lugs?

I’ve only ever seen people demonstrate doing this with the pre-84 versions with the chassis mount inputs.
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For the shielding wire ground you can use the buss wire ground at the input jack lug no problem for grounding purposes.
 
harddriver said:
For the shielding wire ground you can use the buss wire ground at the input jack lug no problem for grounding purposes.
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There’s no lug though, this is the post 84 version where the inputs are PCB mounted. The jacks themselves only have pins for inserting to the PCB solder pads. There is no ground buss wire for the inputs and pots.

IF I understand these old boards correctly, the trace that runs all along the board by the faceplate is providing a connection to ground.

The input wire itself is not soldered to the jack. It is soldered to a pad at the end of the PCB by V1 just downstream of the input resistor. So really there are two questions - where to locate the new input wire and where to ground the shield.

The V1 socket screws do not currently have any ground lugs, but if I added one would there be any problem with me grounding the input wire shield there? Then I could presumably just swap out the old wire for the new one on the same soldering pad.
 
Bardagh said:
There’s no lug though, this is the post 84 version where the inputs are PCB mounted. The jacks themselves only have pins for inserting to the PCB solder pads. There is no ground buss wire for the inputs and pots.

IF I understand these old boards correctly, the trace that runs all along the board by the faceplate is providing a connection to ground.

The input wire itself is not soldered to the jack. It is soldered to a pad at the end of the PCB by V1 just downstream of the input resistor. So really there are two questions - where to locate the new input wire and where to ground the shield.

The V1 socket screws do not currently have any ground lugs, but if I added one would there be any problem with me grounding the input wire shield there? Then I could presumably just swap out the old wire for the new one on the same soldering pad.
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Since the jacks are PB mount just find the lug tabs on the chassis where the grounds from the PCB go to the metal ground tab. You should be able to add a black ground wire and run it up to your shielded wire ground solder them and heatshrink it and you'll be good to go. It's been awhile since I've looked at a Marshall PCB board from the horizontal jack years.

So the input wire would solder to the board connection and there might be a solder pad for the grounds on the board as well but there has to a black wire going from those solder pads to an actual metal tab mounted to the chassis. If it does not you could always drill one or use the mounting holes of the choke or the preamp filter cap clamp mounitng holes. The filter caps are still chassis mounted on those years correct??
 
harddriver said:
Since the jacks are PB mount just find the lug tabs on the chassis where the grounds from the PCB go to the metal ground tab. You should be able to add a black ground wire and run it up to your shielded wire ground solder them and heatshrink it and you'll be good to go. It's been awhile since I've looked at a Marshall PCB board from the horizontal jack years.

So the input wire would solder to the board connection and there might be a solder pad for the grounds on the board as well but there has to a black wire going from those solder pads to an actual metal tab mounted to the chassis. If it does not you could always drill one or use the mounting holes of the choke or the preamp filter cap clamp mounitng holes. The filter caps are still chassis mounted on those years correct??
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Yep, the filter caps and all the tube sockets are chassis mounted, and the ground terminals of a couple of the filter caps is where the two lone black wires from the PCB lead. It's honestly a little weird to me to have a board with no direct grounding to the chassis via stand offs or anything. The board is just free floating on the pot and input connections.
 
I have to redo the connection from the shield to the ground lug at the filter cap because I'm not happy with it, but functionally the one shielded input wire eliminates the buzz completely. Now with the buzz gone I can hear the other background noise in the signal. :lol: It may be a normal amount or the filter caps may be showing their age. I have some new F&T's on hand now so I might as well replace them.

The bias caps definitely did need replacing. Bias range now seems more normal and the matched tubes are biasing very closely now.
 
Bardagh said:
I have to redo the connection from the shield to the ground lug at the filter cap because I'm not happy with it, but functionally the one shielded input wire eliminates the buzz completely. Now with the buzz gone I can hear the other background noise in the signal. :LOL: It may be a normal amount or the filter caps may be showing their age. I have some new F&T's on hand now so I might as well replace them.

The bias caps definitely did need replacing. Bias range now seems more normal and the matched tubes are biasing very closely now.
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That's great to hear that it solved the BUZZZZZZZ. I agree if the bias caps are original it definitely is time.
 
harddriver said:
That's great to hear that it solved the BUZZZZZZZ. I agree if the bias caps are original it definitely is time.
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Just finished replacing the filter caps earlier today and while the old ones may have not yet been out of spec, there is definitely less background noise now. I would dare say it’s downright quiet now even with the preamp volume up and and an OD pedal engaged.

So I’d say case closed on the buzz and noise.
 
Excellent news... thanks for updating us here. It will be a good template for others if they have similar issues.

All you have left to change out are those bias caps and you'll be good to go for the next 20 years!:2thumbsup:
 
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