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Solid State Amplifiers => Amplifier Discussion => Topic started by: gdeig on August 20, 2013, 12:01:54 PM

Title: Marshall 5210 gain squeal question...
Post by: gdeig on August 20, 2013, 12:01:54 PM
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
Title: Re: Marshall 5210 gain squeal question...
Post by: Roly on August 20, 2013, 12:37:27 PM
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.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 20, 2013, 01:45:12 PM
Thanks, I've looked but couldn't find anything.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 20, 2013, 03:58:12 PM
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.
Title: Re: Marshall 5210 gain squeal question...
Post by: J M Fahey on August 20, 2013, 05:10:20 PM
Quote
I believe that it may be a Capacitor going bad
Quote
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?
Title: Re: Marshall 5210 gain squeal question...
Post by: DrGonz78 on August 20, 2013, 07:23:13 PM
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.
Title: Re: Marshall 5210 gain squeal question...
Post by: Enzo on August 20, 2013, 07:48:25 PM
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.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 20, 2013, 08:40:07 PM
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!
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 21, 2013, 11:11:23 PM
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?
Title: Re: Marshall 5210 gain squeal question...
Post by: Enzo on August 22, 2013, 01:03:44 AM
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?
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 22, 2013, 11:02:08 PM
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?
 :-)
Title: Re: Marshall 5210 gain squeal question...
Post by: Enzo on August 23, 2013, 12:29:08 AM
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.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 23, 2013, 12:39:44 AM
????
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
Title: Re: Marshall 5210 gain squeal question...
Post by: Enzo on August 23, 2013, 03:24:46 AM
And yet one guitar does it and the other guitar does not.   The amp has no idea which guitar is plugged in.
Title: Re: Marshall 5210 gain squeal question...
Post by: phatt on August 23, 2013, 05:09:33 AM
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.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 23, 2013, 06:36:21 AM
Hi Phil,
I understand that sometimes an electro mechanical oscillation can be triggered by cheap pickups. but this doesn't explain why the exact same  oscillation tone can be heard with no guitar
Title: Re: Marshall 5210 gain squeal question...
Post by: phatt on August 23, 2013, 07:40:49 AM
Opps,, I may have goofed. :-[

just re read.

Yes but you say it does not happen with the Jackson (Active PU) that presents a different situation over a passive PU.

Either way with nothing plugged in and it squeals says that something is wrong in the circuit.

tricky one. :'(

Phil.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 23, 2013, 10:44:29 AM
Would it help anyone if i recorded a short video of what it is doing?
Title: Re: Marshall 5210 gain squeal question...
Post by: g1 on August 23, 2013, 11:49:29 AM
this doesn't explain why the exact same  oscillation tone can be heard with no guitar
Ok, if i short out the input or there is nothing plugged in (same thing) there is no squeal.
  These 2 statements can't both be true, which is it?
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 23, 2013, 11:56:35 AM
let me clarify, if the cable is plugged into the amplifier opening up the internal grounding circuit on the input jack but no guitar at the end of the cable. I hope that clears things up
Title: Re: Marshall 5210 gain squeal question...
Post by: Kaz Kylheku on August 23, 2013, 05:21:05 PM
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.

That feature of the schematic is the most striking to me. I do not understand the jack hookup that is going on there, but if I squint my eyes, it looks like a positive feedback loop waiting to happen.

Feedback over several op-amp stages can easily create a situation whereby enough phase shift accumulates to create a phase shift oscillator.   And oscillators are, of course, sensitive to gain: they need just the right amount of it. (Or in practical terms, enough of it: the surplus gets clipped.)

I second Enzo's idea of disconnecting this bullshit hookup from the jack.

If it doesn't solve the problem, you can reconnect it (if you're so inclined), so it is worth a shot.


Title: Re: Marshall 5210 gain squeal question...
Post by: Enzo on August 23, 2013, 11:51:19 PM
It isn't a BS hookup, it is like so many other things a good idea that could in some instances cause unwanted trouble.

Marshall uses the Cliff jacks.  They have tip ring and sleeve contacts, then under each of those on the far side of the jack, are cutout contacts.   This is the sleeve one, the one closest to the bushing.

That contact is the sleeve cutout contact.  Nothing cosmic, when a plug is in the jack, that opens.  When the jack is empty, it closes and grounds off the output of the preamp.   So it prevents the self-noise of the preamp from getting to the power amp.

Plugging a cable into the input with nothing at the far end is just asking for noise.
Title: Re: Marshall 5210 gain squeal question...
Post by: Roly on August 24, 2013, 10:10:17 AM
Might I observe that one difference between a passive guitar, active guitar, and open circuit lead is impedance?  The active guitar will present a low impedance to the input thus shunting any stray feedback, while a passive or open lead will leave the door open.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 26, 2013, 08:05:40 AM
Roly, I 100% agree about the reason my active pickups won't squeal when plugged in. But it still didn't explain why it's doing this to begin with, it's not "normal" to this amp.
This is why i was thinking that I've got a cap going bad allowing some stray "frequencies"  to feedback in a loop.

I guess I just might be forced to start changing out one component at a time.
Where would you start??
I've already changed out the "gain" channel op-amp and the 330mF cap (for that circuit) and there's not much change.

Does anybody know how the "automatic" EQ gain operates on this circuit, apparently, it's suppose to be unique to this series of amplifiers. I'm thinking that there is a possibility it might have something to do with this.

This has been a fun and frustrating discussion.
 :-)
Title: Re: Marshall 5210 gain squeal question...
Post by: Enzo on August 26, 2013, 10:52:27 AM
The amp may well have something wrong with it, but I surely don't follow your diagnosis.

Quote
a cap going bad allowing some stray "frequencies"  to feedback in a loop
doesn't sound like anything I can imagine, no offense it just sounds like a made up phrase.

Did you cut the trace to the input jack sleeve cutout as I suggested?   If you have a layout or crosstalk issue, you can replace every part in the amp and the issue will remain.  That is why we don;t throw parts at the problem.

The only 330uf caps I see are in the mute circuits.  They switch to ground.  If one gets leaky or shorts, it just shunts your signal to ground.  If the cap opens or dries up, then the mute won;t turn the channel off.

I am not sure what automatic EQ gain is, but the circuit is conventional.  This is a basic small amp, the preamp has four op amp gain stages.  All the EQ around them is common RC tone shaping.   I suspect the automatic part is marketing speak.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 26, 2013, 12:37:03 PM
Thanks Enzo
No i haven't cut that trace yet, I'll try that.
The "automatic" gain EQ thing is in the manual, but like you, i couldn't see anything that could be automatic. I think your right... Marketing.
And yes, I'm kind of grasping for straws. It's just weird.
Title: Re: Marshall 5210 gain squeal question...
Post by: Roly on August 26, 2013, 01:03:00 PM
Quote from: gdeig
I guess I just might be forced to start changing out one component at a time.

This is called the "blunderbuss" method of faultfinding and it never works (but it does keep a lot of techs employed undoing the damage).

By tests and deduction you work out what the problem is, then you just fix that.

Cut the track @Enzo suggests, where it comes off R16&R18.

There is a bit of a thing about fingering capacitors, but electrolytics aside, caps are generally about as reliable as most other components in amplifiers.

Now in the two supply rails, +/-15V, for the preamp op-amps there are a couple of electros, C46 and C47 1000uF, and if one or both of these have dried out and gone low value it is possible they could allow a feedback path, so you could try locating these and tacking a known good electro across each.  The value isn't critical, a few hundred uF at sufficient voltage rating (15 or more volts), and see if this makes any change to the symptoms (even if it doesn't actually cure the problem).
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 26, 2013, 03:55:04 PM
LOL Roly!
Ok, I'll take that advice!
 :lmao:
Title: Re: Marshall 5210 gain squeal question...
Post by: g1 on August 26, 2013, 08:13:03 PM
  We still come back to the problem that some pickups do it and some don't.  And the problem only occurs at highest gain settings.
  When it squeals with the guitar connected you said it stops if you turn down the guitar volume a bit.  How about if you turn down the master?  Does it stop squealing with high gain settings when you turn the master down?
  The idea that it is not the pickups because it does it with a cord plugged in but no guitar connected is not logical.  Any high gain amp will squeal under such conditions, it is to be expected and perfectly normal.
  I am not saying there is nothing wrong with your amp, only that so far there is no proof this is not normal feedback.  If it is really a problem with the amp, I can't understand why it would be pickup selective.
Title: Re: Marshall 5210 gain squeal question...
Post by: Roly on August 27, 2013, 08:46:01 AM
I'm leaning in this direction myself.  I'm suspicious of that line going from the input socket to part way down the preamp chain.  It suggests to me that they may have had instability problems during design and that this was their 'fix', but I'm none too impressed by it.

In the meantime, round up the usual suspects - if only to eliminate them.   ;)
Title: Re: Marshall 5210 gain squeal question...
Post by: phatt on August 27, 2013, 09:22:36 AM
For fear of adding yet more confusion  :-[,,,,,there seems to be 2 schematics for this amp. :o

On this page you will find both;
http://www.drtube.com/library/schematics/69-marshall-schemas#JCM800SS

*Enzo* mentioned that mute cutout thingo,, but the schematic posted by *gdeig* on page one joins the mute at R16 yet on the other schematic it goes to efx (as *Enzo* noted) so although the circuits are very similar they are different. Maybe more than one pcb for same amp,, which does suggest to me they where trying to fix design flaws.

I do remember not so long back a posting regarding a squealing reverb return in a SS Marshall amp and that ever so clever Mr Fahey noted it was a bad design. I think they inadvertently built an oscillator while trying to subdue the input for the reverb. (or something along those lines)
So yep maybe little R&D is spent on pumping new models out the factory door.

Still,,,
I'd be interested to know if *gdeig* got hold of another passive pickup guitar to see if it reproduced the same problem.

I have 2 almost identical cheap strat copies (passive PU's) and one squealed it's head off while the other did not. Upon inspection one was wax potted the other was not.
Phil.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 27, 2013, 04:19:11 PM
OK, I'll try and answer some questions.
1: Yes, really, the squeal "noise" will happen without a guitar plugged in to the cable, but with a cable, this opens up the tip-ground on the jack.
2: When I turn the Master Volume up or down, it has no effect on the squeal noise.
3: Yes, I have tried several "passive" pickup guitars and they all do it as well, except it seems to be less prone on the neck pickups (again, the squeal is not overall volume or guitar placement dependent).
   When I turn the volume down on the "Gain" (on the amp) or the "Volume" (on the guitar), just a little (just a few degrees), it will stop.  In other words, the wiper on the pot is not making direct contact and is touching the resistive material inside the pot (Hope that makes since).
4: When I plug in my Effects pedal, it seems to decrease... if I plug my Jackson "Active" pickups in directly, it stops immediately.
I can only attribute this to the impedance of the guitar pickups... maybe.  But again, it still makes the noise with or without a guitar.  Thats why I can't believe that the noise is being generated by a guitar.  It is a different sound than a " guitar feedback" and it is also different than cheep pickups that sound like microphones when you tap on them.
5: And lastly... Yes, there are two main schematics of this amp "5210" out there.  My amp is aprox. 1983-84.  The Marshall 5210 vs2 came out around 1988 (easier to read, but not labeled exactly the same).
Yes, I've noticed as well that they made several changes to the amp... mostly in the High Gain channel.
I do know that this series of amplifiers was Marshalls first attempt at Solid State and they made several mistakes in design...  But it does sound pretty good!
 :)
 
Title: Re: Marshall 5210 gain squeal question...
Post by: g1 on August 27, 2013, 04:33:54 PM
 Have you tried Enzo's fix from reply#6? 
Is this a problem that was definitely not there before?  Or a problem that may have always been there?
Title: Re: Marshall 5210 gain squeal question...
Post by: Kaz Kylheku on August 27, 2013, 06:52:40 PM
Everything points at the squeal being controlled by the source impedance at the input. When you turn down the guitar's volume, there is less impedance. Neck pickups typically have less impedance (e.g. 8K versus 16K, when we're talking typical matched-pair humbuckers, wired in series). Output of effect pedal: low-ish impedance, though sometimes they have some passive components in the output so that it is not zero.  Output of active pickups should be close zero impedance since it's just the output of an op-amp.

Bottom line, reducing the impedance at the input kills the squeal. This strongly suggests that the input stage is part of a feedback loop, and the conditions for oscillations are broken when the input is grounded, or put closely enough to ground so that the gain around the feedback loop drops.

You know, is it possible that someone in the past tried to service the amp and replaced the input jack with the wrong one?
Title: Re: Marshall 5210 gain squeal question...
Post by: Enzo on August 27, 2013, 07:12:27 PM
They do something like that on a lot of their higher gain amps.   I don't get the impression is is  dodge.  If you pull the input, the input to the preamp is grounded, but all the gain stages are still free to generate their own noise, which would be coming out the speaker.  So empty input jack mutes the output of the preamp.  Amp silent.  The amp isn;t unstable, just noisy.    In SOME cases, not most, it CAN act like an antenna when the line is not grounded.  The potential for feedback is there.  We don;t know it is the case here until we break the line.

Yes, there are at least two revisions of this amp.  On the other one, the mute cuts in half way along the preamp.  That is how they did it on the AVT150 for example.  In some amps they mute the entire preamp, in some they only mute the OD channel, since it is the one that makes noise.
Title: Re: Marshall 5210 gain squeal question...
Post by: Roly on August 28, 2013, 06:00:22 AM
Quote from: Kaz Kylheku
Everything points at the squeal being controlled by the source impedance at the input.

Concur; particularly from comments in reply #31.

Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 28, 2013, 09:26:02 AM
Kaz,
Yes, the input jack has been replaced with a Switchcraft "ground switch"input jack (not a Marshall replacement).
Hhmmmmm.
No, i haven't had a chance to cut the trace yet... kids soccer and other stuff.
I should get a chance tonight.
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 28, 2013, 09:56:31 AM
I was just struck by a revelation!
Original Marshall jacks are insulated from chassis ground, the Switchcraft  is not.
Possible ground loop that is being amplified by the "Hot" channel.
Maybe...
I'll find out tonight!
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 28, 2013, 08:37:59 PM
talk about a wild goose chase!!
The switchcraft input jack was it!
The  board wasn't grounding properly on it's mounting post and the amp was grounding the chassis through the input jack causing a ground loop.
It sounds far fetched, but i could duplicate and repeat this condition.
I'm going to order the correct jack.
Thanks to everyone who participated in this thread!
Sorry if i drove a few of you nuts.
 ;-)
This was a very interesting learning experience.
Thanks again!
Title: Re: Marshall 5210 gain squeal question...
Post by: Roly on August 29, 2013, 09:43:19 AM
Kaz gets the ceegar this time!

(http://www.columbo-site.freeuk.com/cigar2/cigars43.jpg)

Quote from: gdeig
It sounds far fetched, but i could duplicate and repeat this condition.

Far from being far fetched one of the things I would impress on my techs doing industrial service was that if they couldn't reproduce a fault on demand they couldn't be really certain they had found it.  This is not always possible, sure, but in a case like this it is proof positive you have found the problem; and as it so often is, in the end it's something simple and stupid.

Well done.   :dbtu:
Title: Re: Marshall 5210 gain squeal question...
Post by: gdeig on August 29, 2013, 12:57:07 PM
This is a fantastic site!
Title: Re: Marshall 5210 gain squeal question...
Post by: custimguitarman on January 05, 2015, 07:11:41 PM
Hello,
I am new here. Just picked up a 5210 that works for a short time and then goes into a 60 cycle hum at full volume. Now the master can be turned down to quiet this to nothing. The pre amp volume does nothing. My understanding of electronics is growing but I am not quite where I would like to be.
It seems to me that something is amiss with the preamp section of the circuit or the transistors have latched on.
I realize this is an old thread. Just thought I would try.

Mike
Title: Re: Marshall 5210 gain squeal question...
Post by: Roly on January 06, 2015, 12:26:30 AM
Hi custimguitarman/Mike, welcome!


Read this thread carefully from the beginning and apply some of the checks.

If it has Fx Send/Return or Pre Ou Main In sockets, try patching these together with a good known lead.

Many of these problems are cause by dirty/oxidised sockets.

Your hum isn't a high pitched squeal, so start a new repair thread for this one.
Title: Re: Marshall 5210 gain squeal question...
Post by: custimguitarman on January 06, 2015, 06:06:51 AM
Thanks. I will.