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Author Topic: marshall G30R CD clean channel volume problem  (Read 1044 times)

lordraptor1

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marshall G30R CD clean channel volume problem
« on: February 06, 2020, 06:22:04 PM »
ok i have been working on a 1999 marshall G30R CD solid state amp and im almost finished with it only needing to replace 2 input jacks on it but i have ran into a rather perplexing issue.

ok so here is hte deal, it all works EXCEPT for a volume issue on the clean channel.  the issue with the volume is that when the pot is set to 0 ( i might add it is a brand new 1 meg pot as speced in the schematic) it still has volume. 

im wondering if anyone might know what might be going on?  could it be the 4558 op amp?, cap in the circuit? resistor in the circuit?  i have checked all the resistors in the clean section of the schem and they all seem to be good, i do not have the means to test caps or IC's.

i have been scouring the web for the better part of a week and all i can find of people with still having volume with the pot to 0 are all discussing guitar wiring.


please any help would be appreciated

joecool85

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Re: marshall G30R CD clean channel volume problem
« Reply #1 on: February 06, 2020, 08:19:26 PM »
You say it still has volume when set to zero...how much volume are we talking about here?  Barely audible?  Very loud?  Does the volume otherwise work properly, it just can't get down to dead quiet?
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phatt

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Re: marshall G30R CD clean channel volume problem
« Reply #2 on: February 07, 2020, 03:00:42 AM »
The most likely problem is the Volume pot is not grounded.
Several possibilities; the Volume pot itself might have cracked the ground pin,,or a cracked track,, a cracked or cold solder joint on the PCB.
Use plenty of light as these cracks are often hair thin and hard to see.
Phil.

Enzo

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Re: marshall G30R CD clean channel volume problem
« Reply #3 on: February 07, 2020, 03:02:29 AM »
And another important question. what problem does this cause?  When I want to play with zero sound, I just turn it off.

g1

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Re: marshall G30R CD clean channel volume problem
« Reply #4 on: February 07, 2020, 01:35:58 PM »
Agree that manufacturers don't tend to nitpick about non-playing faults, especially with practice amps.  I've seen other models do this and it is normal.
In this case, it looks like the clean vol. should be able to kill the signal, but only if the pot goes perfectly to zero ohms, which is not always the case, even with new pots.  So you need to verify the pot goes right to zero ohms (not just 'close').
The second place bleed could come through is the reverb circuit, so that pot has to be perfect as well.
As phatt mentioned, perfect grounds for those pots are also critical.
Lastly, if the 2120 switching IC has any internal bleed, the OD channel could be getting through while on the clean ch. with clean vol. at zero.

schematic:  https://www.ssguitar.com/index.php?topic=3132.msg23852#msg23852

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #5 on: February 08, 2020, 08:50:15 AM »
ok let me clarify a couple things, the volume pot was not only tested and does get to 0 ohms i tried several other brand new pots and all of them went to 0 ohms as well and the issue remains. 

i have reflowed every connection on the board by removing old solder and replacing it with new so i dont see that as the issue.

i checked for cracked/broken traces and i couldnt find any except on on the input jack positive side where the pad was completley gone but a tiny jump from jack pin to trace fixed that and that was on the positive side not the negative side

as far as how loud it is at 0 volume, you can hear it from across the room in a 10 x 20 room. 

enzo, i have a tendency to put my guitar down and walk away for a couple minutes and due to being a mainly tube amp guy i have grown accustomed to turning voume down instead of turning amp off all amps with a single power switch and no stand by regardless so reasons.

like i said been scratching my head on this, i would also like to point out the reverb pot is on the distortion side and everythign on the distortion side works as it should with no issues whatsoever as do the bass and treble pots on the clean channel, it is ONLY the clean channel volume pot that isnt doing what i expect it should do and i wasnt aware this would be normal on any solid state amp let alone a marshall and i was looking into finding and fixing this issue.

so at this point based on comments it is a bleed issue somewhere because it is not haveing any buzzing i associate with gound issues, nor is the clean tone disorted in any way it it just still has upt to a max of say 30% volume when the pot is set to 0



phatt

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Re: marshall G30R CD clean channel volume problem
« Reply #6 on: February 08, 2020, 09:16:44 AM »
OK then,
Try a temporary wire from the pot solder terminal (ground end) now wire that to the main common of the Power supply. (Usually the big filtercaps) Or just strait to the chassis. Does that fix it?
That is the only way you can be sure it's truly at ground.
Not all PCB's are well designed and depending how the tracks are laid down and how far around the ground track takes to get back to PSU common can effect the actual ground.
I got caught with something similar,, a reverb that would not turn completely OFF,, drove me nuts.  :trouble
Phil.

g1

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Re: marshall G30R CD clean channel volume problem
« Reply #7 on: February 08, 2020, 02:27:06 PM »
i would also like to point out the reverb pot is on the distortion side
Not sure what you mean by this, is the schematic wrong?  There is no reverb when using clean ch.?
Quote
not haveing any buzzing i associate with gound issues,
An imperfect ground at the volume pot would not necessarily cause buzz or hum, just would not shut the volume right off.
Try phatt's suggestion of the ground jumper for the volume pot, then try grounding the wiper of the reverb pot.


Enzo

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Re: marshall G30R CD clean channel volume problem
« Reply #8 on: February 08, 2020, 06:48:40 PM »
Then the issue COULD be crosstalk.

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #9 on: February 08, 2020, 10:59:59 PM »
tried different new 1meg audio pots problem remains

tried jumper from negative leg to chassis, problem remains.

tried new 1 pico cap jumping other 2 legs, problem remains

tried taking it to local music store to plug into store electricty to rule out house wire ground issue quieter but still problem remains

only thing i can think of at tis point is either the 4558 op am ic or the 2120 op amp ic ( or both)  closest to the clean channel control pots as shown in the schematic.

all electrolytics test good with little to no leakage on an ecr meter ( really good for 21 year old samwha electrolytics).

other than that i am at a complete loss.

Enzo, what i mean by reverb on distortion channel side is that the reverb potentiometer is on the far right side of the front faceplate close to the power switch where all the distortion channel control pots are.  the clean channel control pots are on the far left next to the input and foot switch jacks and consist of vol, bass, and treble.  the reverb works in both channels just that when marshall placed it on the front fp they put it on the side closes to power switch with all the distortion channel controls instead of on the left with the 3 clean channel controls.

tried to post an attachment but it keeps kicking the post out when i hit post

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #10 on: February 08, 2020, 11:12:05 PM »

phatt

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Re: marshall G30R CD clean channel volume problem
« Reply #11 on: February 09, 2020, 06:42:00 AM »
As for attachments,, just remember to check the file size limit.
It's shown under attachments.
"Allowed file types: txt, jpg, gif, png, zip, pdf, mp3
Restrictions: 12 per post, maximum total size 15360KB, maximum individual size 5120KB"
I often have to reduce images before posting.
Phil.

phatt

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Re: marshall G30R CD clean channel volume problem
« Reply #12 on: February 09, 2020, 06:52:38 AM »
Oh yeah,, Enzo mentioned Crosstalk,, Humm.
Try turning the Od Ch volume off as well as the Clean Ch volume.
Does the clean still bleed through?
Just a hunch? Phil.

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #13 on: February 09, 2020, 11:03:11 AM »
Oh yeah,, Enzo mentioned Crosstalk,, Humm.
Try turning the Od Ch volume off as well as the Clean Ch volume.
Does the clean still bleed through?
Just a hunch? Phil.

yes problem still exsists with clean channel volume with ALL of the distortion channel pots and the reverb pot set to 0

lordraptor1

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Re: marshall G30R CD clean channel volume problem
« Reply #14 on: February 09, 2020, 11:15:36 AM »
pic showing where the reverb pot is located on the front face plate.

enzo you mentioned "crosstalk" ok, so where whould that come from?  i have ruled out pots, jacks, resistors, caps, reverb "tank", and have ruled out grounding by jumping ground leg straight to chassis, and taking amp to local music store to rule out possible bad house ground.

so where else could this crossalk come from?  4558DD op amp?  2120 op amp?