'78-80 MXR NoiseGate distorted & low output when gate's on

[kdude34567]

There's a thread some time ago about this that I posted this info to, but it appears no one is following that thread any more & no one is responding, so I thought it might be better to create a new thread...

I have an original 1978-1980 grey MXR NoiseGate that hasnt worked for years. I finally got around to repairing it & had some issues. Then I found the thread on this forum about the MXR Noise Gate, folks seeking the schematic, doing replica builds, mods, and/or troubleshooting problems. In that thread, several people were having the exact same issue as I'm having now.

Long story short, is I got the pedal working & it is gating. But I'm having the exact same problem as others had that is discussed on pages 3 & 4 of that thread. It's where when the gate is engaged it's filtering down or squishing the incoming guitar signal & it's at 1/2 the normal volume & a bit distorted. When the gate is off, the guitar input signal is full level & proper.

It should be noted that I changed a few components on my stock pedal to those discussed in that thread. It wasn't working right before I changed the components & it started gating after I did. So the folks on that thread were onto something (Does that mean the original MXR design has some flaws in it? Because the pedal never did work 100% correctly since the day I bought it back in the late '70s as a teenage rock guitarist).

Anyways, here the components I changed:

1M --> 1.8K (between JFet gate & 150K)
22K --> 680 (between 5.1V Zener diode & Vcc)
680 --> 4.7K (between -IN of U1 or U1a going to gnd)
1uf --> 2.2uf (same as the 4.7K)

Also, it appears that all the opamps, transistor, & JFet are all working, and I measured all the resistors & caps, which appear to be good and at expected values. So my guess is some resistors and/or capacitors are off in selected value & need to change to fix the problem. But I'm not sure what & where values should be changed.

I was wondering what the guys on that past thread did to come up with a solution that fixed the issue?

If that info is not available or forgotten about, then starting fresh from the beginning on this thread is fine too

[kdude34567]

Update:

I reverted the 4.7K & 2.2uf connected between U1/U1A (-IN) & ground back to stock values (680 ohm & 1uf), as the output was just too stifled using the 4.7K & 2.2uf. Really choked output at 1/2 volume & much reduced gain. With the stock values this improved considerably, and the output is hotter and at about 3/4 volume with less gain reduction.

Should I continue lowering the resistance & capacitance values below stock values for these components tied to the (-IN) of the 741 opamp to keep following this pattern?

Though I still have 2 other issues:

1) There's a continuous pumping noise, whether the gate is engaged or not, motorboating around 5-10Hz.

2) The gating effects cuts in & out, like dropouts, while its gating. Making the the gate effect useless.

Any idea?

[kdude34567]

Update for anyone who has a late 70s MXR NoiseGate or has built a replica, here's how I solved the problems listed in this topic thread:

-installed a 10K trim pot in series with R7 680 ohm
-installed a 20K trim pot in series with R2 680 ohm
-installed a 1M trim pot in series with R11 1.8K ohm

Adjusted all 3 trim pots until everything started working, gating was functional, output level was nearly 100% w/o being choked down & a little distorted, and when gating stop cutting in & out. The result of these test adjustments seeking the best sweet spot w/best performance & full sound was the following:

R2 was best @ ~20K-22K ohm
R7 was best @ 680 ohm
R11 was best @ 1.8K ohm

Installed fixed resistors & removed trim pots:

R2: 22K ohm between zener diode & Vcc
R7: 680 ohm between U1/U1a (-in) & C5
R11: 1.8K ohm between Q2 JFET gate & 150K

In a prior thread someone had mentioned R2 should be 680 ohm instead of the stock 22K to get the zener to work. In my case, this was not true & it only causes additional problems. Going back to the stock 22K solved a lot of problems & got the pedal working properly again. However the change from stock 1M to 1.8K as suggested in earlier threads did help & improved the JFET responsiveness. So I did do this modification.

Now the pedal is working properly, gating well, the pumping sound has stopped, intermittent gating has stopped, and the low output w/a tad of distortion has improved significantly. The output with the gate engaged is nearly full & almost 100%. The output is still being choked down a tiny bit, but it's almost negligible. Just a tiny bit of gain is being lost. Not much, but a little.

Over time I'll continue to investigate that output gain loss, and will report if I find anything of merit. But it's livable now & not too bad at all. Something i can deal with.

If anyone else has solved the output gain loss issue when the gate is engaged, I would welcome any input or reports of a solution.