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Author Topic: Marshall 5210 gain squeal question...  (Read 15459 times)

gdeig

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Re: Marshall 5210 gain squeal question...
« Reply #15 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

phatt

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Re: Marshall 5210 gain squeal question...
« Reply #16 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.

gdeig

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Re: Marshall 5210 gain squeal question...
« Reply #17 on: August 23, 2013, 10:44:29 AM »
Would it help anyone if i recorded a short video of what it is doing?

g1

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Re: Marshall 5210 gain squeal question...
« Reply #18 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?

gdeig

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Re: Marshall 5210 gain squeal question...
« Reply #19 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

Kaz Kylheku

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Re: Marshall 5210 gain squeal question...
« Reply #20 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.


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Enzo

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Re: Marshall 5210 gain squeal question...
« Reply #21 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.

Roly

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Re: Marshall 5210 gain squeal question...
« Reply #22 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.
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 #23 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.
 :-)

Enzo

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Re: Marshall 5210 gain squeal question...
« Reply #24 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.

gdeig

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Re: Marshall 5210 gain squeal question...
« Reply #25 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.

Roly

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Re: Marshall 5210 gain squeal question...
« Reply #26 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).
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 #27 on: August 26, 2013, 03:55:04 PM »
LOL Roly!
Ok, I'll take that advice!
 :lmao:

g1

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Re: Marshall 5210 gain squeal question...
« Reply #28 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.

Roly

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Re: Marshall 5210 gain squeal question...
« Reply #29 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.   ;)
If you say theory and practice don't agree you haven't applied enough theory.