Need a little help with an MXR M106 Noise Gate

[dbranger94]

Hello, I recently purchased a 1978 M-106 Noise Gate Line Driver. I found that the DC jack was shorted to the case and it had a bad protection diode (also shorting V+ to GND). I fixed both issues and restored the pedal to very good condition and it works now, mostly... As the pedal is clamping down on the signal to gate it, the pedal starts making a crackling/scratchy noise. Similar to the sound a pot makes when it's dirty. I have checked the Zener and it's 5.05V in circuit (seems a bit high). Both 741s seem to have the correct voltages on pins 2, 3 and 6 (around 1/2 Vcc). Any ideas as to where the issue lies? I suspect maybe one of the transistors are becoming unstable, but I'm not sure. I know more about op-amps than I do transistors. Thanks for any help.

 

[Diynot]

Sounds like a good use case for an audio probe. That can help you find where in the circuit the noise is originating from, then you can focus on that component and any components attached to it

 

[dbranger94]

I made an audio probe, but the darn thing pops and scares the hell out of me every time I touch a circuit. I mean it's only a capacitor in series, no idea how I messed it up. I've tried several cap values, but nothing works. I'm afraid I'm going to break the speaker I'm using to listen with. I've tried .1u, .01u, 100u, same results.

 

[jwin615]

1978-check the electro caps. Probably leaky(DC wise)

 

[dbranger94]

Doesn't have any electrolytic caps. They're all tantalum and ceramic. Not sure what tants do when they get old... I guess I could just recap the whole thing.

 

[dbranger94]

I measured all the tantalum caps and only one of six was still in spec. One of the 10uF caps was over 18uF. Looks like I'll re-cap it and see if it helps.

 

[dbranger94]

Well, I replaced all the tantalum caps. Then I swapped out the transistors, ceramic caps, and the op-amps to no great effect, so I put the old components back. Looking at the schematic I found below, I decided to try some changes. I added a 100pF cap across R13. This smoothed out the noise so it was more of an oscillation instead of static. I then played with the value of R14 by replacing it with a 5k linear pot and found that a value close to 5k was ideal. So, I replaced the 680 with a 5.1k resistor. This lowered the overall threshold value of the pedal, but helped quiet the noise. Then I replaced the 1uF cap at C8 with a 3.3uF and that made a small improvement, so I then tried a 10uF cap. This was a great improvement, but added too much of a delay to the attack, so I settled on a 4.7uF cap. Next I played with the value of the Zener diode at D2. It was originally a 5.1V diode, but I tried a 4.7V and a 5.6V. Both of which made things worse, so I put in a new 5.1V 1N4733 diode and it was as good as I've gotten so far. Still not perfect, but better. I have a suspicion that the problem lies somewhere around Q2, the 2N5952 FET. I'm pretty sure this is the actual "gate" portion of the circuit. Right now it's taking over a second to fully gate the signal, which I suspect is way too long and causing most of my issue.