Noisy Marshall JMC 800 Lead (2203)

[rbotn]

Hi.
I am repairng guitar amplifiers and I have a problem with a JMC 800 (2203). The owner of the amplifier has short-circuited two pins on an output tube. There is sound at the amplifier, but there is a noise in the pre-amp, mostly at high input. There is no noise when the master volume is turned down. Attach a sound-file with the noise.

 

[Chris-in-LA]

So, what have you checked or replaced up to this point?

 

[Drinkingdeath01]

Sounds like every other JCM800 that I've ever owned.

 

[ApathyForum]

Sounds like every other JCM800 that I've ever owned.

Doesn’t sound like any I own.

 

[rbotn]

So, what have you checked or replaced up to this point?

I have change both preamp tubes with new one, no result. I sounds like the noise is comming for the high input. When I pull out V1, the first tube, the amp is silence.

 

[Chris-in-LA]

I have change both preamp tubes with new one, no result. I sounds like the noise is comming for the high input. When I pull out V1, the first tube, the amp is silence.

Both inputs run through v1. Do you have any experience working on amps? Do you have a voltmeter? Have you opened up the amp for a visual inspection?

 

[marshallmellowed]

Hi.
I am repairng guitar amplifiers and I have a problem with a JMC 800 (2203). The owner of the amplifier has short-circuited two pins on an output tube. There is sound at the amplifier, but there is a noise in the pre-amp, mostly at high input. There is no noise when the master volume is turned down. Attach a sound-file with the noise.

Try swapping components "C" and "M", should sound perfect!

 

[StingRay85]

V1 is shared by both inputs. High input uses V1A while low bypasses it. So there's a massive difference in volume right there. Based on the very limited info you gave, it can be a lot of things. Maybe this should be moved to the workbench subforum

 

[JP2036]

Thats nothing.

 

[rbotn]

Both inputs run through v1. Do you have any experience working on amps? Do you have a voltmeter? Have you opened up the amp for a visual inspection?

Yes I have experience working on amps, but this is a tricky one. There is nothing to see trough visual inscctions. The voltage around V1 and V2 is whitin 10%. The noise is the same with or without cable in the inputs. Are there anybody who have bad experiences with the jack inputs?

V1 is shared by both inputs. High input uses V1A while low bypasses it. So there's a massive difference in volume right there. Based on the very limited info you gave, it can be a lot of things. Maybe this should be moved to the workbench subforum

Thanx, I will also post on that forum too.

 

[StingRay85]

Clean the inputs is for sure where you should start

 

[dro]

Are you using a Craftsman model 1019 Laboratory edition signature series torque wrench?

 

[Ufoscorpion]

Are you using a Craftsman model 1019 Laboratory edition signature series torque wrench?

No point in even tackling the job without one in my opinion.

 

[Chris-in-LA]

If the voltages look ok and you see no damage, you might need a scope to track down that noise.