Marshall 5210 gain squeal question...

[gdeig]

I have a mid-80's Marshal 5210.  Its one of Marshall's first attempts at a Solid State Amp. It has two channels, clean and hot which can be combined and a pretty nice Accutronic reverb.  I am having an issue with the "Hot" channel.  When ever I turn the "Gain" (VR-4) up  to 8 or 9 (out of 10), it will begin to squeal.  It is more prone to this when a guitar is plugged in, but it still will squeal without any signal input.  The pitch of the squeal will change with the "Volume" knob and the "EQ" knobs for that channel.
This is a completly Solid State Amp, No-Tubes.

I believe that it may be a Capacitor going bad, but I really don't want to change out a bunch of caps out just guessing.

Any ideas on which Cap and/or what you think it might be?

The Clean Channel is working just fine and the Hot Channel sounds pretty good if I back the "Gain" down just a little (to 7 or less).


Attached is a copy of the 5210 Schematic.


Thanks!
GD

[Roly]

Hi @gdeig, and welcome.

My memory fails me ATM, but I'm sure we have had a squealing Marshall problem not all that long ago.  Spend a bit of time with the Search function and see if you can turn up the thread.

[gdeig]

Thanks, I've looked but couldn't find anything.

[gdeig]

I'm about 99% sure its a bad cap creating an oscillation, but again, not exactly sure which one.  VR4 is the Gain (that triggers the squeal "tone"), VR5 is the Volume for the "Hot" Channel, VR6-8 are the EQ controlls for the channel. I'm suspecting C15 (333mf Cap.)  There are several polypropylene capacitors within this circuit.  What would you recomend to pull out first to help narrow down my issue?
Thanks.

[J M Fahey]

I believe that it may be a Capacitor going bad

I'm about 99% sure its a bad cap creating an oscillation

Well, I'm quite sure it's not.

Plug a shorted plug in the input jack, still squealing?

[DrGonz78]

Right there on the schematic it says "Mod from 12-9-86  s/n u29600 onwards c14 + c18 removed." Also s/n u29600 could read 029600 just can't tell really. Check if that mod is present and the serial number etc. It all relates in circuit to VR4 so it made me wonder. Let us know if your amp already has that mod too.

[Enzo]

I have no idea why we might think it was a cap either.

C14,18 are high end rolloffs, I am not sure how removing them would reduce oscillation.


Are your op amps original?   I have had Marshall SS amps of that era that expected 1458 op amps and became unstable with 4558 op amps in their place.  A tech at Marshall verified that as an issue.


Another Marshall trick was the input jack mute.  Look at the input jack, on the sleeve contact there is a cutout.  the factory drawing shows it going to the FX send jack.  When the jack is empty, presumably it grounds off the FX send.  The flip side of that coin is that it is a long trace from the output of the preamp right back to the input, and can act like an antenna.   I have stabilized such amps before by cutting that trace over at the FX send end.

[gdeig]

Wow, this is a great start, lots of great info.
Factory amp mod... Yes, its serial number is higher and i checked, it does have the mod.
And the OpAmp chips are original. I'll have to check to see what number they are.
That did occur to me, i just didn't think that it would produce a tone when the gain (vr4) was turned up.

I'll check the input Jack and let you know what i find out...
tomorrow.
Thanks for a great start!

[gdeig]

Ok, if i short out the input or there is nothing plugged in (same thing) there is no squeal.
The chips are 1458
I already replaced ic2b and switched out C15 (330mf), but i still have a squeal when the gain (vr4) is almost all the way up.
Any suggestions?

[Enzo]

OK, that helps.  PLug the guitar in and set the amp up so it squeals.  Now turn the volume control on the guitar to zero.  Does the noise stop?


Also, with the guitar turned back up, standing there it squeals.  Does turning sideways - face the guitar a different direction in other words - change the noise in ANY way?

[gdeig]

when I plug in my Jackson active pickups it will not squeal at all, but when I plug in my strat, it will squeal only when the volume is all the way up. when I turn the volume down just a little on the guitar, the squealing stops immediately.
standing in front of the amp and turning sideways has no affect on the squealing.
but selection of different pickups do, it will squeal the most on the bridge pickup, I assume this is because the bridge pickup has the lowest impedance.
okay, now what's next?
 :-)

[Enzo]

Well that pretty much tells us it is your pickups feeding back.   On a high gain amp, they can do it at not very high volume even.

[gdeig]

?
I can be across the room and the volume of the amplifier can be barely audible and it will squeal. It didn't sound like an acoustical feedback, but more like an electronic oscillation

[Enzo]

And yet one guitar does it and the other guitar does not.   The amp has no idea which guitar is plugged in.

[phatt]

Wax pot your Strat pickups which will likely go a long way to help fix the issue.
Squealing is not uncommon with cheaper pickups.
Phil.