Marshall jcm 2000 dsl 50 unresponsive eq

[PatriceK]

Hi , I got a weird problem with my jcm 2000 dsl ,
now the eq controls are unresponsive , the treble is acting as a volume control and when its at 10 , middle/bass pots does nothing , when treble is down , bass/middle seems to work .... also the master volume of both channel are very scratchy ..... HOWEVER .... when i press in the " tone shift " switch in... everything is back to normal and no more scratchy pots on both channels ...

anyone had this problem before ? dunno what to look for ...

 

[PatriceK]

Hi , I got a weird problem with my jcm 2000 dsl ,
now the eq controls are unresponsive , the treble is acting as a volume control and when its at 10 , middle/bass pots does nothing , when treble is down , bass/middle seems to work .... also the master volume of both channel are very scratchy ..... HOWEVER .... when i press in the " tone shift " switch in... everything is back to normal and no more scratchy pots on both channels ...

anyone had this problem before ? dunno what to look for ...

no one can help me ?

 

[rallye440]

I just took a look at DSL schematics and my wild guess is that your 470pF tone stack cap before the tone shift switch is shorted and the whole signal passes through the treble pot. Another 470pF is added in series with the first cap when you push the tone shift button in. It's time to take that amp to service.

 

[Tatzmann]

One of the problems i had with mine was that toneshift and the deep button didn't do shit.

It was a bad solder at a connector on the frontpanel/preamp board.

Check for bad solderjoints.

 

[PatriceK]

I just took a look at DSL schematics and my wild guess is that your 470pF tone stack cap before the tone shift switch is shorted and the whole signal passes through the treble pot. Another 470pF is added in series with the first cap when you push the tone shift button in. It's time to take that amp to service.

you must be talking about c19, c20 ..... visually they look good , but have nothing to test them

 

[PatriceK]

One of the problems i had with mine was that toneshift and the deep button didn't do shit.

It was a bad solder at a connector on the frontpanel/preamp board.

Check for bad solderjoints.

 

[PatriceK]

some of the solder joint were dry , fix them but issue still there

 

[Mitchell Pearrow]

To The Forum
I hope you get your amp sorted..
I have no advice for you.

Cheers
Mitch

 

[PatriceK]

To The Forum
I hope you get your amp sorted..
I have no advice for you.

Cheers
Mitch

thx !

 

[rallye440]

you must be talking about c19, c20 ..... visually they look good , but have nothing to test the

Yes, C20 in this picture.

 

[FleshOnGear]

I just took a look at DSL schematics and my wild guess is that your 470pF tone stack cap before the tone shift switch is shorted and the whole signal passes through the treble pot. Another 470pF is added in series with the first cap when you push the tone shift button in. It's time to take that amp to service.

I agree. This would also allow DC through to the volume pots, which would cause the scratchiness. @PatriceK, do you have a multimeter?

 

[PatriceK]

I agree. This would also allow DC through to the volume pots, which would cause the scratchiness. @PatriceK, do you have a multimeter?

yes i have a multimeter but dont have the capacitance option on it

 

[PatriceK]

yes i have a multimeter but dont have the capacitance option on it

maybe i could just put it in ohm fonction and see if its just bad ?

 

[FleshOnGear]

maybe i could just put it in ohm fonction and see if its just bad ?

That’s what I was thinking. With the tone shift button pressed in, you should get a really high reading, as there will be a resistance path of about 10.1Mohms through ground when the tone shift is in. If C20 has shorted, you’ll see something much lower.

You could also check for DC voltage with the amp on, which isn’t as safe because the amp is live. You are aware that tube amps can store high voltage even after the amp has been off for days, right? You’re aware of making sure the caps are discharged? Just making sure…

 

[PatriceK]

Happened to have a 500pf cap laying around , there was no reading on the 470pf , and i soldered the 500pf and .....Working again
Cannot thank you enought guys !!

 

[Neptical]

Dude...

First things first: KEEP AMP UNPLUGGED.

pull the board out ...spray pots, twist 6-10 times, wash, rinse, repeat. Easy peasy.

Check preamp and power tubes. Shake to check bad filament.

 

[PatriceK]

Dude...

First things first: KEEP AMP UNPLUGGED.

pull the board out ...spray pots, twist 6-10 times, wash, rinse, repeat. Easy peasy.

Check preamp and power tubes. Shake to check bad filament.

that the first thing i did

 

[PatriceK]

that the first thing i did

problem was a dead capacitor (C20)

 

[FleshOnGear]

Happened to have a 500pf cap laying around , there was no reading on the 470pf , and i soldered the 500pf and .....Working again
Cannot thank you enought guys !!

Great! Good job, and enjoy the amp!

Oh, wait a sec, not to be a wet blanket, but do you know what the voltage rating is on the cap you installed?

 

[PatriceK]

Great! Good job, and enjoy the amp!

Oh, wait a sec, not to be a wet blanket, but do you know what the voltage rating is on the cap you installed?

1kv