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Thread: 2203kk (Kerry King Signature JCM800) noise issue

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    2203kk (Kerry King Signature JCM800) noise issue

    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?

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    Senior Member Guitarist's Avatar
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    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?

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    Quote Originally Posted by Guitarist View Post
    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.

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    Hi there,

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

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    Quote Originally Posted by strato56 View Post
    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

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    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.

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    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.

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

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

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    Some folks do. Can't hurt to try.

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    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...

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    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.
    Attached Files Attached Files

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    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.

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    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.

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

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    Quote Originally Posted by strato56 View Post
    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?

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

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    Quote Originally Posted by strato56 View Post
    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?

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

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

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    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.

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    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??

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    pin5 = CT = release C24 & C25 and put 10F 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

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    Quote Originally Posted by strato56 View Post
    pin5 = CT = release C24 & C25 and put 10F 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

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    Quote Originally Posted by strato56 View Post
    pin5 = CT = release C24 & C25 and put 10F 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????

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

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    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.

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    Quote Originally Posted by Mr_bibbles View Post
    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.

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    Quote Originally Posted by strato56 View Post
    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.

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    Quote Originally Posted by Mr_bibbles View Post
    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 ?

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    Quote Originally Posted by strato56 View Post
    Meaning the gate stays closed ? Red LED ?
    Exactly

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    Quote Originally Posted by Mr_bibbles View Post
    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.

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

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    Quote Originally Posted by strato56 View Post
    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.

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

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