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Solid State Amplifiers => Amplifier Discussion => Topic started by: lordraptor1 on February 06, 2020, 06:22:04 PM

Title: marshall G30R CD clean channel volume problem
Post by: lordraptor1 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
Title: Re: marshall G30R CD clean channel volume problem
Post by: joecool85 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?
Title: Re: marshall G30R CD clean channel volume problem
Post by: phatt 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.
Title: Re: marshall G30R CD clean channel volume problem
Post by: Enzo 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.
Title: Re: marshall G30R CD clean channel volume problem
Post by: g1 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
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 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


Title: Re: marshall G30R CD clean channel volume problem
Post by: phatt 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.
Title: Re: marshall G30R CD clean channel volume problem
Post by: g1 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.

Title: Re: marshall G30R CD clean channel volume problem
Post by: Enzo on February 08, 2020, 06:48:40 PM
Then the issue COULD be crosstalk.
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 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
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 on February 08, 2020, 11:12:05 PM
(https://flic.kr/p/2iqT8tu)
Title: Re: marshall G30R CD clean channel volume problem
Post by: phatt 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.
Title: Re: marshall G30R CD clean channel volume problem
Post by: phatt 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.
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 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
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 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?
Title: Re: marshall G30R CD clean channel volume problem
Post by: g1 on February 09, 2020, 02:40:52 PM
Ok, looking closer at the schematic, the reverb thing doesn't matter.  The reverb gets it's input signal after the switching IC, so it shouldn't be able to bypass the clean vol.

But I'd still like to establish that this is the correct schematic.  You do get reverb on the clean channel, don't you?
Here's a pic as yours didn't show.  The black line above the OD ch. controls show which are for OD channel.  Reverb is not included, but is over to the right, as it is 'global' and supposed to work for both channels.  Is this pic like your unit?

(https://www.ssguitar.com/index.php?action=dlattach;topic=4765.0;attach=8660;image)

If the schematic is correct and the volume pot is not the source of the bleed, the only other place I can see would be the 2120 switching IC.

Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 on February 09, 2020, 11:57:53 PM
yes that is what the G30R CD looks like, sad i cant get pics to post even when it was resized to 253 KB.  yes the reverb works on both clean and distortion channels. i could have told you the reverb had nothing to do with it as i had unplugged the spring "tank" from the board and it had 0 effect on the clean channel volume neither did multiple brand new pots or a wire from pots ground lug direct to chassis.
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 on February 11, 2020, 10:03:46 AM
Ok, looking closer at the schematic, the reverb thing doesn't matter.  The reverb gets it's input signal after the switching IC, so it shouldn't be able to bypass the clean vol.

But I'd still like to establish that this is the correct schematic.  You do get reverb on the clean channel, don't you?
Here's a pic as yours didn't show.  The black line above the OD ch. controls show which are for OD channel.  Reverb is not included, but is over to the right, as it is 'global' and supposed to work for both channels.  Is this pic like your unit?

(https://www.ssguitar.com/index.php?action=dlattach;topic=4765.0;attach=8660;image)

If the schematic is correct and the volume pot is not the source of the bleed, the only other place I can see would be the 2120 switching IC.

thanks for the 2120 ic, i have also been messaging marshall about the issue to which i just now get a reply back from marshall:

Joanna Green <joanna.green@marshall.com>
To:
lordraptor1@yahoo.com

Feb 11 at 2:03 AM

Hello Fabian,

 

Regarding your question, it could be IC3 at fault as this is switching between the two channels.

 

Kind Regards

 

This email (including any attachments) may contain confidential information,
​privileged information, or constitute non-public information. Any use of this
​information by anyone other than the intended recipient is prohibited.
​If you have received this email in error, please inform the sender
​immediately and delete this information from your systems.

Marshall Amplification plc, registered in England, Registered number: 805676.
​Denbigh Road, Bletchley, Milton Keynes, MK1 1DQ, England.

JOANNA GREEN​
RETURNS/CUSTOMER SERVICES MANAGER
+44 (0)1908 375 411
DENBIGH ROAD, BLETCHLEY, ENGLAND, MK1 1DQ

 
This email (including any attachments) may contain confidential information,
​privileged information, or constitute non-public information. Any use of this
​information by anyone other than the intended recipient is prohibited.
​If you have received this email in error, please inform the sender
​immediately and delete this information from your systems.

Marshall Amplification plc, registered in England, Registered number: 805676.
​Denbigh Road, Bletchley, Milton Keynes, MK1 1DQ, England.

they to seem to think that IC3 ( this is the 2120 im assuming you were talking about in the schematic) COULD be at fault causing the issue. 

Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 on February 11, 2020, 06:09:42 PM
well good news bad news, good news i ordered the 2120D op amp i needed, bad news they are manufacturer discontinued and mouser and digikey were out of stock and not ordering any more of them bummer, all other outlets have a MOQ of at least 50 pieces o.O.  and to be honest im not even sure this will fix the issue with the amp but i have tried everything else i could think of. 

so i at least got the opamp i need on order before there were none left in the USA, i mean sure there are a lot of ones listed from chinese sellers but personal experience and word of mouth suggest a lot of those "china" sellers are selling fake op amp chips ( guess china has no shame in that they will even fake ICs.
Title: Re: marshall G30R CD clean channel volume problem
Post by: g1 on February 11, 2020, 08:33:44 PM
Yes, I meant IC3.  NJM2121D is a higher spec'd chip that will work fine there.  Mouser has stock.

Agree that it will not necessarily fix the issue.  Two other IC's that are used for both channels (so could possibly cause the crosstalk Enzo mentioned) would be IC1, and IC5, both are type 4558D.
Title: Re: marshall G30R CD clean channel volume problem
Post by: Jazz P Bass on February 12, 2020, 02:00:15 AM
I would recommend 'grounding' the clean channel wiper.
That should kill the signal.
As yet, I have seen no resistance measurements of the circuit.
I would look at the junction of C49 & C 22 to ground.
What are the readings when the clean volume control is swept from min to max?
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 on February 12, 2020, 02:43:54 AM
I would recommend 'grounding' the clean channel wiper.
That should kill the signal.
As yet, I have seen no resistance measurements of the circuit.
I would look at the junction of C49 & C 22 to ground.
What are the readings when the clean volume control is swept from min to max?

been there done that, grounding clean wiper had no change, C49 and C22  were tested and fond good but having had parts test good and be bad in the past i tried replacing C49 and C22 which again had no effect so i reinstalled the original parts.  only the op amps remain, and after talkign with marshall which only took them a week to respond to e-mail ( yeas that is sarcasm if you dont get it), they state it COULD be the IC3 2120D.  for those interested in this tidbit find and buy 2120S ICs while you can as they are manufacturer discontinued.
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 on February 12, 2020, 02:46:12 AM
Yes, I meant IC3.  NJM2121D is a higher spec'd chip that will work fine there.  Mouser has stock.

Agree that it will not necessarily fix the issue.  Two other IC's that are used for both channels (so could possibly cause the crosstalk Enzo mentioned) would be IC1, and IC5, both are type 4558D.

already found a seller in the US that actually had NJM2120D so ordered some 4558 woulld have to wait till more $$$ presents itself.  i will look into the 2121D
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 on February 12, 2020, 02:49:58 AM
Yes, I meant IC3.  NJM2121D is a higher spec'd chip that will work fine there.  Mouser has stock.

Agree that it will not necessarily fix the issue.  Two other IC's that are used for both channels (so could possibly cause the crosstalk Enzo mentioned) would be IC1, and IC5, both are type 4558D.
just looked at the 2120D on mouser but like the original 2120D it has also been manufacturer discontinued.

makes me wary of marshall  tube amps from teh JCM900 to tsl line which all used the 2120D's in them.  i mean really manufacturer discontinuing parts and i havent seen anything from marshall in the way of a service bulletin regarding replacements for the ics should they fail.
Title: Re: marshall G30R CD clean channel volume problem
Post by: phatt on February 12, 2020, 07:00:13 AM
I would recommend 'grounding' the clean channel wiper.
That should kill the signal.
As yet, I have seen no resistance measurements of the circuit.
I would look at the junction of C49 & C 22 to ground.
What are the readings when the clean volume control is swept from min to max?

Bingo  I Agree   :tu:
What you think might be ground may not actually be at ground.
That is why we use meters. 8|

I'd add to that,, and suggest measure the DC resistance from volume wiper to the track ground on PCB and then also to the main filter caps common/ground point,, and then again to Chassis/Case.
They should ALL be within a bees dick of of Zero Ohms.

THEN and only then will you know it's not the volume control.

Frankly I'd suggest save yourself the pain and $$$,,just leave it and pull the plug half out as that will ground the input and kill all sound. How hard is that?
Phil.

Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 on February 13, 2020, 08:37:35 PM
I would recommend 'grounding' the clean channel wiper.
That should kill the signal.
As yet, I have seen no resistance measurements of the circuit.
I would look at the junction of C49 & C 22 to ground.
What are the readings when the clean volume control is swept from min to max?

Bingo  I Agree   :tu:
What you think might be ground may not actually be at ground.
That is why we use meters. 8|

I'd add to that,, and suggest measure the DC resistance from volume wiper to the track ground on PCB and then also to the main filter caps common/ground point,, and then again to Chassis/Case.
They should ALL be within a bees dick of of Zero Ohms.

THEN and only then will you know it's not the volume control.

Frankly I'd suggest save yourself the pain and $$$,,just leave it and pull the plug half out as that will ground the input and kill all sound. How hard is that?
Phil.

just to clarify i know exactly what and how to test this isnt my first rodeo.  and i will say this again i have tested EVERYTHING and i DO know which leg of the pot is the ground ( not hard to see if you know how to look at the traces on the PCB and see which leg leads to ground plane.  i did test pot and the wiper itself does 0 as does the other 5 brand new 1M pots i have.

fact is i have trouble shot this thing for over a week prior to even making a post here on the forum.  at the end of the day the only thing i have not checked or replaced that according to the schematic COULD be at fault are the ic's and based on the condition of the input jack ( and also the footswitch jack next to and share some traces) i have no difficulty believing that the faulty jacks may have cause an ic to fail.

i already have the 2120D ic coming,and i have a friend thaat says he might have one 4558 left in his stash he said i could have (yay for free ).  i normally dont mess with ss amps because of the headaches but i think after this im not only go back to my guns of refusing ss amp repairs but also refuse modern tube amps that have IC's in them. 

so much easier repairing, troubleshooting and building from scratch vintage, cloned vintage tube amps.
Title: Re: marshall G30R CD clean channel volume problem
Post by: Jazz P Bass on February 14, 2020, 01:13:51 AM
We;; that was a big help.
Title: Re: marshall G30R CD clean channel volume problem
Post by: lordraptor1 on February 15, 2020, 04:09:02 PM
We;; that was a big help.

what is not a big help is people not reading and understanding that i have already checked the ground and tried multiple brand new pots so ground was ruled out already, nor have they read or understood that all the other pots seem to work fine.  marshall finally got back and said it COULD be the 2120 ic switching op amp at fault so im going to change it.  other than that the only thing i could do would be start shotgunning ic's and electrolytics of course at that point it just isnt worth it when most the amp COULD sell for being that it is solid state and not tube is $200.  i just thought some ss guru would have some xp with this amp and would have some insights as to causes but seems like most point to ground followed by the pot itself at fault ( again both ruled out already).  i mean at this point im so frustrated and the check the ground, check the pot, and the may not actually be ground comments are not helping and are in no way useful and are in fact COUNTER productive.
Title: Re: marshall G30R CD clean channel volume problem
Post by: phatt on February 15, 2020, 11:55:26 PM
Sorry but it's the wrong forum for anger management issues.

We try to help sometimes we get it wrong,, but we don't have the amp in front of us. xP

If you are well versed in Valve circuits then you would know how important ground return paths are.
Yes every Common point may well be at Ground/Zero **BUT it's the path each section takes back to Main Common point that can make or break a design.**

Tiss why I suggest you take a wire from Volume Common back to to central PSU (where the 2 main filter caps join) and see if it resolves the issue. As you have a new chip coming I hope it works out for you if not then I'd go hunting Ground Paths.

I've worked on enough gear to know that many problems are ground related.
Just as wire layout is important in Valve gear it's no different with SS PCB layout. Unlike lead layout in p-p valve circuits which can be easy rewired,, PCB is Fixed once printed and resolving ground return path issues becomes a lot harder.

I fixed a mates Pro junior Valve amp,, The most sensitive audio trace ran right along side the screen supply trace. :duh
Hence the damn thing hummed like crazy. :trouble
I has to cut a few tracks and hand wire to shut the thing up.
So that was a Fender design layout stuff up.
Without a deep understanding of ground plane layout even the big name gear is often full of suspect design issues.
I wish you well and hope you resolve the issue.
Phil.
Title: Re: marshall G30R CD clean channel volume problem
Post by: phatt on February 16, 2020, 01:42:52 AM
Oh yeah,, just re reading to check what I may have missed.
You mentioned the input and switch sockets had some issues.
up shot is the control voltage that runs IC2&3 may not be working correctly which would likely cause the issue you have.
I'd be checking that before replacing chips. Control is around TR1.
hope it helps,
Phil.
Title: Re: marshall G30R CD clean channel volume problem
Post by: Enzo on February 17, 2020, 12:51:46 AM
Op amps don't fail making crosstalk, they simply amplify what comes to them.

It really doesn't matter where the controls are on the panel for the most part.  But maybe...

Crosstalk isn't a failure of anything.  Well other than the design.  Crosstalk simply means some part of the circuit is sensitive enough to pick up the signal from some other part.

An example.  Some poorly designed PA mixers, you can plug a hot signal into channel 1 and if you turn up channel 2, some of channel 1 will be leaking into it.  This is because the two channels are close enough together electrically.  it isn't something you can "fix".