2203kk (Kerry King Signature JCM800) noise issue

[Mr_bibbles]

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

The Knob fluxuates between green and red depending on the setting, it functions as a noise gate without problem... EXCEPT....

When you have the threshold set right at the point where it starts gating, it will "pop" in and out of gate mode, the light will flash with it it in time to the gating.

It WOULD be normal operation but there seems to be a slight little pop and crackle in the noise, which causes the gate to turn on and off quickly. If the noise was a flat, constant, hiss, it would probably be fine. That's why I was hunting for just excessive hiss earlier, but If it's not present at the booster but it IS present after the gate, that would mean it's likely from the THAT's circuit, yes?

[strato56]

With the gate constantly open (LED = green ), the hiss is also present at the output from the THAT ?

[Mr_bibbles]

With the gate constantly open (LED = green ), the hiss is also present at the output from the THAT ?

There is still the strange fluctuation at the gate when the LED is green, and if you engage the gate (RED) it will mitigate the noise at pin 12 of the that.

BUT I've found the point in the circuit where the noise is the loudest- Pin 7 of IC3 (Ic3b OUTPUT) creates a humming signal with the same peaks and fluxuations that are causing the gate to act funny. I checked both Diodes, they test OK, as does the resistor values in that circuit. Put a new Cap on c19, no change. I traced it back and found...

Noise is also present on pin 4 and 5 of the THAT, "RMS OUT/CT". It seems louder after IC3B, but it's definitely the same noise. There is NO noise on pins 1 and 2. I have -15volts at one side of R22, but the other side (and pin 2) read almost nothing, 14mv. On the other negative supply, pin 10, I have the correct -15v

With this information, can we assume there is an issue with the RMS detector portion of the THAT4301? Since it's eating the -15volts, does that mean the Chip itself is broken?

[strato56]

pin2 should be around 0V, so that's ok.
Make sure no noise enters the RMS- detector : check at R30 en C20

[Mr_bibbles]

Confirmed, only normal amplifier noise at r30/c20. So... since RMS out is the very next step in the chain.....

[strato56]

OK, so with no signal present at the input of the RMS detector, still a fluctuating signal at the output of the RMS detector .
This might be a long shot but :
the circuit around IC3B is a "non linear capacitor" ( got that from THAT application notes ); for testing purposes , you could replace it with a fixed capacitor from pin CT to ground ( cf the datasheet ).
The RMS detector may not have the right time constants for guitar signals, but you might be able to find out if it is working properly.

[Mr_bibbles]

OK, just to make sure I understand-

In the 2203KK, the circuit from CT OUT to the IC can be disconnected at the junction of c24 and c25, and then we go straight from CT OUT to ground with a 10uf electrolytic capacitor. When this is complete, check for noise at the CT out??

[strato56]

pin5 = CT = release C24 & C25 and put 10µF to ground. (CT = capacitor for time constant )

pin4 = OUT = measure the noise there.

There is no "CT OUT"pin.

see also page 4 of the datasheet from reply #12

[Mr_bibbles]

pin5 = CT = release C24 & C25 and put 10µF to ground. (CT = capacitor for time constant )

pin4 = OUT = measure the noise there.

There is no "CT OUT"pin.

see also page 4 of the datasheet from reply #12

Sorry for the delay, was out of town on tour.

Awesome Thank you! Substituted the 10uf electrolytic to ground as shown on the datasheet, it seems a little off but it's working MUCH better, no spikey noise, no fast blinking engagement / unengaging. So this means something in the control circuit (Capacitor for time constant) is muck? Probably one of the leaky films? I'll start replacing things one by one and let you know

[Mr_bibbles]

pin5 = CT = release C24 & C25 and put 10µF to ground. (CT = capacitor for time constant )

pin4 = OUT = measure the noise there.

There is no "CT OUT"pin.

see also page 4 of the datasheet from reply #12

Alright, so I ended up shotgunning every single capacitor and diode from that circuit, but there is still no change. Measured all the resistors and metered out the connections to make sure we didn't have an open line. The gate does work better, but for some reason it's still a little noisy at pin 4 of the THAT, and extremely noisy at the out of the dual JFET icb3. At this point I'm at a loss.

When I swapped the 10uf to ground in, it seemed to work ok but was a little off- like there was a constant hum rather than an erratic one. If all else fails, can I use a 10uf permanently? Does anyone have any clever ideas of where this might be coming from? Do we just have a bit of noise off IN the THAT that's being amplified by ICB3????

[Mr_bibbles]

Bumpity bump! Strato56 wan Kenobi, you are my only hope.

[strato56]

If you lift one side from R3, thus removing the RMS detector from the circuit. Does the threshold pot have enough range to open and close the gate ? If so, let us know your findings.

[nickb]

Bumpity bump! Strato56 wan Kenobi, you are my only hope.

The purpose of IC3b is to give a different time constant for slow and fast signals. It will work with a 10uF instead but that is not what the designer intended. If you raise the gate threshold higher can you stop the "hiccup" effect? The fact the noise gate is there tells you noise was a problem.

[Mr_bibbles]

If you lift one side from R3, thus removing the RMS detector from the circuit. Does the threshold pot have enough range to open and close the gate ? If so, let us know your findings.

Thank you man I really do appreciate your help! With R3 lifted, the threshold pot has no effect from one end to the other.

Nick- The hiccuping can be stopped with the threshold, but it doesn't really sound like the audio definition of "noise", it has a strange sort of chatter to the signal. I was able to minimize it with some new components but I'm hesitant to just wipe my hands of it and say I'm done. I'll poke around the RMS circuit in to make sure it's not something there, I'll replace that input resistor cause i already have it out too.

[strato56]

Thank you man I really do appreciate your help! With R3 lifted, the threshold pot has no effect from one end to the other.

Meaning the gate stays closed ? Red LED ?