2203kk (Kerry King Signature JCM800) noise issue

[Mr_bibbles]

Meaning the gate stays closed ? Red LED ?

Exactly

[nickb]

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.

The gain of the RMS detector is critically dependent on the current out of pin 2. In other words any noise on the supply will directly turn into noise on the detector output (but filtered by IC3b). A cap 100nF from pin 2 to GND will help, if that has anything to do with it.

A similar concern is the threshold voltage from VR7a which again comes off the supply but this time with no filtering at all. You cab try a 10uf 25V cap from the wiper of VR7a to GND, +ve to the wiper to see if that has any effect.

[strato56]

Can you give us the DC-voltages of pin4, pin7 and pin18 from the THAT, in the 3 states ( open, close and hiccuping ) ?

[Mr_bibbles]

Can you give us the DC-voltages of pin4, pin7 and pin18 from the THAT, in the 3 states ( open, close and hiccuping ) ?

There is minimal DC voltage on all pins regaurdless of gate position. Something in the 5-40mv range, low enough where it seems like a misreading from the meter.

Tried bridging the wipers you suggested with a cap, no change to the signal.

[strato56]

Weird. Pin18 should have a "high" state and a "low" state ( depending on gate closed or not ) ; how can it drive TR2 otherwise ?

[Mr_bibbles]

Weird. Pin18 should have a "high" state and a "low" state ( depending on gate closed or not ) ; how can it drive TR2 otherwise ?

You are smart! I'm sorry, I think my leads may be on their way out. I redid the measurements, got the following:

pin 4 - 379 MV, all positions

pin 7 Green -8mv
Half -.35V
Red -.65v

Pin 18 Green -280-310 mv, fluctuates quickly
Half .52v
Red 3.5v

[strato56]

That fluctuating pin18 : what voltage do you read at the wiper from vr7a ?
You could try expanding the treshold range by putting a 100K resistor parallel with R24.

[Mr_bibbles]

That fluctuating pin18 : what voltage do you read at the wiper from vr7a ?
You could try expanding the treshold range by putting a 100K resistor parallel with R24.

15+ on one terminal
A sweep from .93v to 15v in the center
.93 on the third terminal

The threshold works correctly and cuts off the noise, but the "chatter" is riding on TOP of the noise. Turning the gate to Red completely eliminated the noise and the chatter , but you can still hear the strange sound any time there's signal, even if the gate is off.


I've probed the noise (The chatter) in the following locations:

junction at the cathode of D1 and anode of d2.
ALSO present on the other side of d2, stops after r7, c10
Present on the NON-linked side of r31 and 32
Both sides of r4
pin 7 of ICb3

[nickb]

Tried bridging the wipers you suggested with a cap, no change to the signal.

OK, that eliminates power supply noise. Let's see if it's coming from IC3a. Can you lift one side of R30 or C20, whichever is easiest? If that kills it, try changing IC3.

[Mr_bibbles]

OK, that eliminates power supply noise. Let's see if it's coming from IC3a. Can you lift one side of R30 or C20, whichever is easiest? If that kills it, try changing IC3.

IT"S GONE!

The noise is not present on ANY pin of IC3 now, a or B, it's gone from the other surrounding points as well.

Earlier in the thread, we had probed the noise out r30/c20 and it was fine, so with this conclusion, it seems like IC3 is the culprit right? I'll get a new one on order and update everyone with this solution. Strato, and Nick, I really appreciate you sticking with me and making this happen.

[nickb]

IT"S GONE!

The noise is not present on ANY pin of IC3 now, a or B, it's gone from the other surrounding points as well.

Earlier in the thread, we had probed the noise out r30/c20 and it was fine, so with this conclusion, it seems like IC3 is the culprit right? I'll get a new one on order and update everyone with this solution. Strato, and Nick, I really appreciate you sticking with me and making this happen.

Don't dance for joy yet as I have a bad feeling that the problem is flicker noise from IC3. A replacement might be the same or worse but you should try it. If no better we might be able to substitute a variant with lower flicker (LF) noise. I'll have to research for one in case you need it.

PS: Also, while testing what has been plugged into the input? If nothing, plug your instrument in and turn its volume to zero and see what effect that has. If better, you should check the shorting contact on the input jack.

[Mr_bibbles]

Don't dance for joy yet as I have a bad feeling that the problem is flicker noise from IC3. A replacement might be the same or worse but you should try it. If no better we might be able to substitute a variant with lower flicker (LF) noise. I'll have to research for one in case you need it.

PS: Also, while testing what has been plugged into the input? If nothing, plug your instrument in and turn its volume to zero and see what effect that has. If better, you should check the shorting contact on the input jack.

Been testing with and without a cable; it's the same with or without a instrument plugged in. Just re-tested with the volume down to verify.


I thought NJM72BL was already supposed to be fairly low noise, is LF a specific type of noise?

[nickb]

TL072 are great for the cost. The one you have there might be a bad 'un so please try another. If that doesn't do it try an Analog Devices LT1169CN8 as they are specified for low flicker noise right down to 0.1Hz. Flicker noise (nowadays called 1/f noise) is low frequency phenomenon of semiconductors where the noise gets bigger with decreasing frequency.

[Mr_bibbles]

TL072 are great for the cost. The one you have there might be a bad 'un so please try another. If that doesn't do it try an Analog Devices LT1169CN8 as they are specified for low flicker noise right down to 0.1Hz. Flicker noise (nowadays called 1/f noise) is low frequency phenomenon of semiconductors where the noise gets bigger with decreasing frequency.

Thanks! The only issue is that it's a SIL package, not a DIP. I know I can run wires from a normal tl072 but a more permanent solution would be preferred. Is there a LT1169nc8 version that's single-in-line??

[nickb]

Thanks! The only issue is that it's a SIL package, not a DIP. I know I can run wires from a normal tl072 but a more permanent solution would be preferred. Is there a LT1169nc8 version that's single-in-line??

Bother... I had no idea. I think you're out of luck there. SIL's are few and far between. I suggest you get a turned pin strip for a socket and another as a carrier to hold the DIP package with the wires. That way you can try a few alternatives by simply plugging and unplugging.