Marshall 5210 circuit design questions. Guitar noise

[mrjones2004x]

So I’ve got my 5210 back from Marshall and all the pots and jacks are new and working great. I cleaned up the tolex today with some convertible roof cleaner and it worked amazing but I have a query.

There is noticeable static type noise when adjusting the volume pots on my guitars. I asked the Marshall tech about this and he said the amp circuit design causes this.
I have noticed all guitars do it but the worst are my pcb gibsons.
I’ve read before about dc noise ect. Is there a way I could get rid of this issue?

 

[Jon Snell]

As there is no DC blocking capacitor on the input, if the first IC has a DC offset on it (which is quite likely) placing a 2.2uF 50v electrolytic capacitor between the input socket and R1 will cure your dilema.

 

[mrjones2004x]

As there is no DC blocking capacitor on the input, if the first IC has a DC offset on it (which is quite likely) placing a 2.2uF 50v electrolytic capacitor between the input socket and R1 will cure your dilema.

Thank you. I don’t know much about amp circuits but I’ll look in to your recommendations.
I know how to work safely as an electrician and can try this myself.
I did take this pic when I had the chassis out.
Can you help me understand where I would need to place the cap?

 

[thetragichero]

lift the end of r3 that's connected to r2. place 2u2 (1uf, whatever. I'd prefer multilayer ceramic over electrolytic because they're tiny and nonpolar. if you're in the us i have 1uf 50v axial mlcc i get pretty cheaply that i can throw in an envelope for you) cap between the lifted end of r3 and the vacated solder pad

 

[mrjones2004x]

lift the end of r3 that's connected to r2. place 2u2 (1uf, whatever. I'd prefer multilayer ceramic over electrolytic because they're tiny and nonpolar. if you're in the us i have 1uf 50v axial mlcc i get pretty cheaply that i can throw in an envelope for you) cap between the lifted end of r3 and the vacated solder pad

Hi, in England.
I’ve got a bunch of caps here just can’t work out what’s what as my meter seems to be dead. Not all of them are readable to look up the values unfortunately.
Is the cap value important in this position?

I know I’ve definitely got a 0.1uf

 

[thetragichero]

you want it reasonably big enough to not cut bass. op amp input impedance *should* be large enough for 100nf cap. give it a try and if you don't like it go bigger

 

[Tatzmann]

All good info up there, but
its not normal behavior, had alot of 80s transistor channelswitchers come through,
they can oscillate at very high gain settings
sometimes, but the pots were always quiet
after cleaning and there wasnt any noisy interference with the guitarpots.

I'd clean every pot and contacts, that should resolve it. Who knows how long the new pots
were sitting on a shelf at Marshalls.

 

[mrjones2004x]

All good info up there, but
its not normal behavior, had alot of 80s transistor channelswitchers come through,
they can oscillate at very high gain settings
sometimes, but the pots were always quiet
after cleaning and there wasnt any noisy interference with the guitarpots.

I'd clean every pot and contacts, that should resolve it. Who knows how long the new pots
were sitting on a shelf at Marshalls.

It did this before the new pots and jacks were installed. I asked him to look at the guitar noise. Just said the circuit design would cause this. I didn’t ask if it could be modified. He checked all circuitry and solder joints as part of the service.

Also to note it does this noise on the clean channel also

 

[Pete Farrington]

lift the end of r3 that's connected to r2. place 2u2

Will that work for both channels?
The schematic and photo are too low res to be much use for me.

 

[thetragichero]

based on what transistors the footswitch controls it appears so from that schematic. r2 is 1m to ground off the input jack and r2 is limiting current to the op amp input

 

[blademaster2]

So I’ve got my 5210 back from Marshall and all the pots and jacks are new and working great. I cleaned up the tolex today with some convertible roof cleaner and it worked amazing but I have a query.

There is noticeable static type noise when adjusting the volume pots on my guitars. I asked the Marshall tech about this and he said the amp circuit design causes this.
I have noticed all guitars do it but the worst are my pcb gibsons.
I’ve read before about dc noise ect. Is there a way I could get rid of this issue?

I had popping and static sounds years ago on a 5210. See my other response, but to summarize I found that one of the tantalum coupling capacitors was reverse biased slightly (cannot recall which). Replacing it with a bipolar electrolytic cap fixed it permanently. The circuit was assembled correctly so I regard this as a design flaw. I can imagine that problem being the source of your troubles also.

 

[blademaster2]

So I’ve got my 5210 back from Marshall and all the pots and jacks are new and working great. I cleaned up the tolex today with some convertible roof cleaner and it worked amazing but I have a query.

There is noticeable static type noise when adjusting the volume pots on my guitars. I asked the Marshall tech about this and he said the amp circuit design causes this.
I have noticed all guitars do it but the worst are my pcb gibsons.
I’ve read before about dc noise ect. Is there a way I could get rid of this issue?

As I indicated in my previous post, just to assist more: This sounds suspiciously like the noise I had on my 5210 when the output of the JFET (TR1, I think) had a dc bias that must have drifted over time and caused reverse bias voltage on the coupling capacitor that comes directly after it. Replacing that tantalum capacitor with a bipolar electrolytic aluminum cap of the same value (and reasonable voltage rating) would fix it if it is the same. It makes sense that it might change with volume levels.

In my case the amp was assembled correctly (so the Marshall tech was correct), but made popping and crackling noise superimposed on the regular sound. The above fix has lasted more than 15 years now with no sign of the noise since then.

 

[fazz]

As I indicated in my previous post, just to assist more: This sounds suspiciously like the noise I had on my 5210 when the output of the JFET (TR1, I think) had a dc bias that must have drifted over time and caused reverse bias voltage on the coupling capacitor that comes directly after it. Replacing that tantalum capacitor with a bipolar electrolytic aluminum cap of the same value (and reasonable voltage rating) would fix it if it is the same. It makes sense that it might change with volume levels.

In my case the amp was assembled correctly (so the Marshall tech was correct), but made popping and crackling noise superimposed on the regular sound. The above fix has lasted more than 15 years now with no sign of the noise since then.

Hi, good read thanks. I think I have the same noise. my amp is the 5212 1986. I am going to change TR1 I think it might be in that location. You said a capacitor change for the tatalum. What value cap did you you use please?
Thanks

 

[TassieViking]

The value shown on the schematic is 2u2, or 2.2uF if it is the one right after the first transistor T1.
The Mk2 version is very similar and that schematic is much clearer to read.

Make sure the PCB screws are not loose, some early SS amp became noisy if the PCB was not making a good contact to ground through the mounting posts.
Sometimes the wrong screw was used in a hole and the screw felt tight but the screw bottomed out in the post and the PCB was still lose and making a bad contact.

 

[fazz]

The value shown on the schematic is 2u2, or 2.2uF if it is the one right after the first transistor T1.
The Mk2 version is very similar and that schematic is much clearer to read.

Make sure the PCB screws are not loose, some early SS amp became noisy if the PCB was not making a good contact to ground through the mounting posts.
Sometimes the wrong screw was used in a hole and the screw felt tight but the screw bottomed out in the post and the PCB was still lose and making a bad contact.

Thanks. I needed a link cable on the send return. Also I replaced c12 with electrolytic 2.2uf 35v tested and noise gone. I have replaced the mains switch and added filter on transformer primary. Changed op amps previously using the same chips new and installed holders. Added earth to centre tap. Changed all the electrolytic s. The earth to centre tap cured hum. The c12 fixed the crackling noise. The sound is very good.

 

[TassieViking]

The return jacks can get a dirty contact on the tip switch, unplug amp and take lid off.
Stick a jack plug in the return jack and spray a small amount of Deoxit onto the contact, and then move the jack in/out a lot of times and it should get clean.
If you look closely at the jacks you will see the contact move when a jack is inserted, sometimes they need to be cleaned because dirt can get in.

I sometimes get a bit of stiff paper and put it between the contacts, spray a bit of Deoxit and move the paper back and fourth to clean them.
Its an old habit from cleaning contacts in industrial situations, but we had the right type of impregnated paper to do the polishing with.

Its good you got it working good again.
Cheers
Mick