MXR M134 Stereo Chorus - Troubleshoot

[MegaMattsson]

Hello!

I know this isnt a DIY-question but i need your help!

My MXR stereo chorus is behaving strange, signal loss and distorts
very easily if you run a synth through it.
I may have broken it by running a synth through a preamp then into a analog delay
(making the signal very hot...) and then into the pedal.

Does anyone have any clue how i can fix this?
Ill attach the schematic.

Thankful for help!

[Scruffie]

Can you provide voltage readings please? Hard to debug if there's a problem without.

Also, was it working properly before with a synth? The dip switches inside are for setting the input signal level to instrument/line.

As an aside, think you could do me a favour and post some gut shots?

[ElectricDruid]

I'd be very surprised if a hot signal had damaged it, to be honest. It'd have to be more like "mains voltage" hot, not "preamp hot". Anything below 10-20Vpp (which is a very hot signal) is just going to make the input op-amp clip like hell. So don't panic just yet (we can save that for later )

+1 voltage readings please.

[MegaMattsson]

It worked like a boss with synth before!

Do you want Voltage readings from all pins?

Transkribera a lot for your help!

[ElectricDruid]

You could start with just the op-amps, if you like. U1 to U6. They seem to be 4558 or 1458's. Just go around anticlockwise from pin 1 to pin 8 in order and give us the voltage on each one.

At some point we'll probably need to know about the delay line itself MN3008 and maybe the 4013 flip-flop, but that's for later. Let's start by having a look at the op-amps.

Thanks! I know it's a tiresome job, but it tells us a lot.

[MegaMattsson]

Thanks!

Ill do the reading friday and will get back with the results then!

[MegaMattsson]

Hi!

Here is what ive found:

U1
8,45
8,45
8,45
16,98
11,0
11,0
8,3
8,3
8,3
8,3
0,2
11,0
11,0
8,20

U2

0,91,5
04,6
09,5
04,7
04,7
0,7
0,65
002,9
0,5
000,4
0,7
004,8
0,46
0,7

U4

16,9
09,6
11,0
9,6
11,0
11,0
11,0
8,4
17,0
17,0
9,4
11,0
8,0
11,0
11,0
11,0

U5

9,4
9,4
000,4
4,9
4,8
4,7
0,002
9,5
4,7
9,5
4,7
4,8
4,8
4,9

U6

08,4
08,4
08,4
0000
17,0
06,0
06,0
06,0


I cant find U3, hope this is good enough!

//Johannes

[Scruffie]

I'm confused, does your stereo chorus have 14 pin op amps?

Does it look like this


Or this

[MegaMattsson]

The first one!

[POTL]

This circuit is from an old chorus, you have a new effect for which there is no circuit on the internet.
You can take some photos
1) Photo of the back side of the board
2) Photos on which the marking of microcircuits will be visible, including those hidden under the potentiometers.

[Scruffie]

As POTL states, you have a reissue, it bares little resemblance to schematic you've posted which is for the vintage unit.

That being the case, debugging is going to be trickier for us and you so I'm going to suggest you should try contacting MXR/Dunlop first to see what they say.

[11-90-an]

I was about to shout "IT'S TRACE TIME!!!" then I looked closer and saw that nearly everything was smd and would be much more involved... 

yea just go ahead and contact MXR/Dunlop as what Scruffie said...