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Author Topic: Marshall 5210 gain squeal question...  (Read 15460 times)

phatt

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Re: Marshall 5210 gain squeal question...
« Reply #30 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.

gdeig

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Re: Marshall 5210 gain squeal question...
« Reply #31 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!
 :)
 

g1

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Re: Marshall 5210 gain squeal question...
« Reply #32 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?

Kaz Kylheku

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Re: Marshall 5210 gain squeal question...
« Reply #33 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?
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Enzo

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Re: Marshall 5210 gain squeal question...
« Reply #34 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.

Roly

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Re: Marshall 5210 gain squeal question...
« Reply #35 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.

If you say theory and practice don't agree you haven't applied enough theory.

gdeig

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Re: Marshall 5210 gain squeal question...
« Reply #36 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.

gdeig

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Re: Marshall 5210 gain squeal question...
« Reply #37 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!

gdeig

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Re: Marshall 5210 gain squeal question...
« Reply #38 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!

Roly

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Re: Marshall 5210 gain squeal question...
« Reply #39 on: August 29, 2013, 09:43:19 AM »
Kaz gets the ceegar this time!



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:
If you say theory and practice don't agree you haven't applied enough theory.

gdeig

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Re: Marshall 5210 gain squeal question...
« Reply #40 on: August 29, 2013, 12:57:07 PM »
This is a fantastic site!

custimguitarman

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Re: Marshall 5210 gain squeal question...
« Reply #41 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

Roly

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Re: Marshall 5210 gain squeal question...
« Reply #42 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.
If you say theory and practice don't agree you haven't applied enough theory.

custimguitarman

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Re: Marshall 5210 gain squeal question...
« Reply #43 on: January 06, 2015, 06:06:51 AM »
Thanks. I will.