Debugging a MXR M-118 analog delay

[VPIF]

Hi.

I have an old MXR M-118 analog delay that only passes the dry sound wether engaged or bypassed. The volume of the dry signal decreases as i turn up the "mix" while engaged. I have replaced all the SAD1024 chips and the NE741N, but to no effect.

This is the 230v version, so I feel a bit reluctant towards fiddeling around inside the circuit with the power chord plugged in, but I guess I will be able to measure the voltages of the chips without jamming my pinky in the transformer.

So, to the actual questions:
-Is there anything in particular I should look for (voltages, visual checks and so forth)?
-There are seven (!) trimpots in the circuit. Would it be a bad idea to fiddle around with these before I know exactly what function each of them serve?

Schematic: http://smg.photobucket.com/user/mhammer/media/mxr_analogdelay.png.html
Will add gut shots later, if required.

Thanks!

[Govmnt_Lacky]

This should help you to help US help you get it fixed! 

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

[Mark Hammer]

The unit has a number of CMOS chips onboard (40xx numbers) that are essential to having any audible delay sound.  If one of them is blown you won't get anything.  There are also several trimpots on board, some of which couldlead to no delay sound if they have been budged from their optimal position.

Which delay chip does your unit use?

[theehman]

I would start by confirming voltages are correct per the schematic then audio probing the inputs/outputs of the delay ICs to see where the signal is stopping.

[VPIF]

This should help you to help US help you get it fixed! 

http://www.diystompboxes.com/smfforum/index.php?topic=29816.0

I'm familiar with the debugging checklist, but as this circuit is far to complex for me to understand (I've only ever buildt overdrives) and because there's a 230V transformer in there I am a bit hesitant - which probably means I should try to find a qualified tech...

Posted by: Mark Hammer 

Insert Quote

The unit has a number of CMOS chips onboard (40xx numbers) that are essential to having any audible delay sound.  If one of them is blown you won't get anything.  There are also several trimpots on board, some of which couldlead to no delay sound if they have been budged from their optimal position.

Which delay chip does your unit use?

There are five CD40xx chips. These are, unlike the BBDs and the clock (?) NE741N, not socketed. By delay chip you mean the BBDs? Those are three Reticon SAD1024A.

Posted by: theehman 

Insert Quote

I would start by confirming voltages are correct per the schematic then audio probing the inputs/outputs of the delay ICs to see where the signal is stopping.

I will get the voltages of the twelve (!) chips asap.


Here are some gutshots: One of the box caps has a brown, sticky substance on top. I guess it would be wierd if a film cap should have burned? I can get most of it off with my fingernail.

http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/2013-06-17+20_07_00_001.jpg.html
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/2013-06-17+20_07_32_001.jpg.html
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/2013-06-17+20_07_41_001.jpg.html
http://www.aronnelson.com/gallery/main.php/v/Schematics-etc/2013-06-17+20_07_48_001.jpg.html