MXR Noise Gate Line Driver True Bypass addition and Repair?

[Zaos]

Hey everyone!

Preface with parts I've replaced.

Replaced both 2sc1849's with a matched pair of 2N5089's, didn't seem to help with the problem.

Pinouts were matched between the new and old transistors.

What's confusing, is that it kills your gain when you set the threshold for the gate. Also, when you try to bypass it it completely removes all gain.

Could one of the LM741CN circuits be scuffed?

[Mark Hammer]

I had one of these 40 years ago.  Admittedly, I did not open it up at the time, except to replace the battery.  But my recollection is that the stompswitch was a latching type, yielding an obvious click when you stepped on it.  Yet all the schematics I've seen posted around show it as a momentary switch.  I'm not seeing how the state of the attenuating JFET could be held steady for very long.   So which is it?  Is the switch SPST latching or momentary?

[Zaos]

It's a latching switch

Afaik you'd only have a noise gate while standing on the pedal itself if it was a momentary right?

[Slowpoke101]

Welcome to the forum. I hope that you will have an enjoyable time here.

The MXR noise gate pedal is a bit of an odd one. The foot-switch merely defeats the gate action. When the gate is open signal will pass through (un-muted ). When the gate is closed no signal will pass through (muted ). It isn't actually a true bypass effect as it's always in-line (more like a buffer ).

It also can be a bit funny to operate. Place it in your signal chain between your guitar and your amp. Usually the foot-switch is non-latching - but there can be exceptions to this. The threshold pot operates backwards...You start with it fully clockwise (maximum ) and slowly turn it counter-clockwise until it mutes any signal going through it. At that point the gate is "closed" and the signal is muted. If you now play something on your guitar the signal should now "open" the gate (un-muting ) and you should hear yourself playing. When you stop playing it should "close" the gate muting any noise that may be present.

[Zaos]

Thanks for the welcome I hope to build quite a few of the cool builds I see on here!

So I'll try to clarify more what exactly happens!

Checked switch based on continuity.

Switch on:
Pedal sort of works. It does work in terms of gate action, but as soon as it you approach the part where it starts reducing the noise on your amp it also kills the gain coming out of the amp?

Switch off:
Turns the guitar into an acoustic. Jokes aside, it completely kills the gain out of the amp (guitar I guess). Turns an ac/dc tone into a simon & garfunkel sound!

[Zaos]

Quickly checking the circuit, looks like one of the LM741CN's is bad. Input voltage is about 8.8V and output is 4.5V. Other one is steady, but I'll replace both for peace of mind

[Zaos]

Just an update in case anyone else has this problem, replacing both of the opamps (LM741CN) fixed it, and it works perfectly now!