This is a fantastic site!

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!

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!

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.

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!

LOL Roly!
Ok, I'll take that advice!
:lmao:

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, 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.
:-)

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

Would it help anyone if i recorded a short video of what it is doing?

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

?
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

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?
:-)

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?

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!