2203kk (Kerry King Signature JCM800) noise issue

[Mr_bibbles]

I have a 2203kk (Kerry King Signature JCM800) on the bench, the owner was complaining about a 'clicking / crackling" when the 'Beast' mode is on. The Beast mode seems to be a SS boost with a noise gate, but after a little investigation it seems like he was sitting on the threshold of the gate, meaning it was just turning off and on quickly while riding the noise.

https://drive.google.com/file/d/0B9W...Tc0/view?hl=en


https://drive.google.com/file/d/0B9W...DM0/view?hl=en


It SEEMS like the noise gate is working correctly, but I can't really tell because I'm not even sure the progression of signal in this amp.... I think the issue maybe be excessive hiss itself, which is causing the noise gate to 'sputter' more than usual. Without the beast mode engaged, there's more hiss than any JCM800 I've played, even at low preamp gain..

Yes, I've tried swapping the preamp tubes!
Checked all the KT88's and they're drawing appropriate current and are biased correctly.
Hiss is affected by all controls.

I traced the signal to the first preamp tube, and the excessive hiss is NOT there at r138, but IS there at r125. Replaced r125 just in case it was the plate resistor, no change.

What the goddang heck am I looking for here? Is this some awful byproduct of the solid state additions?

[Guitarist]

I was just working on an amp that I upgraded the output transformer in. I had hiss issues and when kranked, feedback as high ptched squeal. It ended up I had the OT wires reversed so it was sending back the feedback positive. Silly mistake on my part but the wire colors may've been reversed too;(

Did you mean the third plate resistor # r128?

[Mr_bibbles]

I was just working on an amp that I upgraded the output transformer in. I had hiss issues and when kranked, feedback as high ptched squeal. It ended up I had the OT wires reversed so it was sending back the feedback positive. Silly mistake on my part but the wire colors may've been reversed too;(

Did you mean the third plate resistor # r128?

Thank you for the reply-

The amp looks stock from the factory, and since I've probed the noise in the amp, i'm guessing it's got to be somewhere in the preamp.

Is r138is the 68k, isn't that a grid stopper? R128, not r138 is the 3rd plate if I'm not mistaken.

[strato56]

Hi there,

can you measure the voltages : HT1, H2 HT3 en the +15V and -15V ?

[Mr_bibbles]

Hi there,

can you measure the voltages : HT1, H2 HT3 en the +15V and -15V ?

Sure:

HT1: 395v
HT2: 260v
ht3: 252v
15: 14.84
-15: -14.9

[Mr_bibbles]

I want to guess and say that the Noise is coming from either the SS eq or the SS booster, at the front end. I'm having trouble tracing the signals since everything is on Molex's and the schematics are not clear about the signal chain to and from each section.

[Mr_bibbles]

So I probed a little bit more this morning,

I've found some definite noise / crackling issues that are centered around what I assume is the EQ / "beast mode" circuit. Does anyone have ideas for elements to check? I'm going to continue to trace as best I can but this is a toughie.

[g1]

Do you have freeze spray? That's what I would be hitting things with.

[Mr_bibbles]

I don't... can I use an upside down air duster can??

[g1]

Some folks do. Can't hurt to try.

[Mr_bibbles]

So I actually have no air duster on hand either, but my Audio Probe found no noise going into the "beast mode" circuitry, but DEFINATE windy/ wooshy / noisey issues after THAT4301 (pin 12 out?) I'll continue to root around...

[g1]

Attaching the THAT datasheet. One thing I forgot to mention earlier, the noise at the grid of the first tube (R138) may have just been too small to be noticeable, which I think is being confirmed.

[strato56]

If I see it correctly, the SS Beast Booster is completely in front of the amp.
SW1b connects the input jack directly to the first tube-half in the 0-position. From there it's a standard JCM800 .
IC3a is a buffer : should produce no noise.
IC2a amplifies with gain 22x , IC2b has variable gain between 1 and 10.
Please listen to the noise at outputs from IC2a and IC2b before they enter the noise gate.

[Mr_bibbles]

It seems like before the noise gate, everything is acting pretty well. There's noise, but nothing that strikes me as a malfunction. It seems like it's something that's coming from the THAT itself, and I'm really grateful for the datasheet but this is over my head.

But, lets take stock; So far, we have a normal signal coming out of the booster, but a strange wooshing/popping after the THAT. So something is malfunctioning here. Does anyone with a good handle on IC tech have an idea for where I should start checking? I checked for DC at the coupling cap C1 to make sure no DC was leaking in an altering the signal but to be honest I'm a little over my head.

[strato56]

Is the LED in the beast knob green or red; how does the circuit react to the treshold pot ?