M-312 mixing board troubles

[Erik]

I recently picked up an old M-312 board from a local shop. Got it to my buddy's place for a test drive & discovered some worrisome kinks in it that I wouldn't mind some pointers on.

First off, when one plugs a pair of headphones into the jack on the front, you're greeted only by some low ground hum. It is at a constant volume, with no other sound coming through, & the twiddling of knobs has no discernible effect on the sound.

Secondly, we can't seem to get signal to go anywhere. Admittedly, this may be ignorance on our parts, but we each have significant experience with studio boards so I feel like we should have gotten something. We tried a few different channels, & had no success on any of them. We tried both mic & line inputs. We dimed the trim knobs. We sent each signal through every aux. We got no movement on the meters, & no sound out of the board. Is this simply user error, or is something horribly wrong happening within the electronic guts of the board?

Lastly, two of the meter backlights don't turn on. This really isn't that big of a deal to me, but I feel like it bears mentioning.

If anyone has advice or methods of repair, I would be incredibly appreciative. Thanks.

 

[SkywaveTDR]

It all depends on the shop that worked on it. Some do not know the first thing about this kind of stuff and will just clean off the face plate. Headphone should not have a hum or should have output- you need to take it to a real Technician. meter lamps last a long time but they can be affected by shock. They are not that hard to change- they are not backlights as these are not LCD displays.

 

[SkywaveTDR]

The amount of circuits in the unit are far too many to give you a sure fire solution. One thing that has been a problem is the buss bars that run from channel to channel. Where you got the unit was there any warranty and if it was serviced what about that place? I hear too many stories of people taking items to shops and NOT getting their stuff fixed and leaving sums of money with these crooks. IF they don't fix something then there is no reason to allow them to get paid for that as it is the same as stealing and is fraud. I am always getting stuff in from customers of Just Service and Deltronics in Chicago, I also get stuff from allover the USA.
Yes I fix mixers to a certain size limit. I have worked on M312 and others all through the past 40 years.

 

[SkywaveTDR]

There are buttons that have to be pressed to get a signal through. As a person who has checked these kind of units before you place a -10dBV signal in to the line in and look for it at the direct out- if that is missing then either the normalized effects insert jack is dirty or circuits may be damaged. Just because a device has 1/4" jack on it does not mean you can hook an amp speaker out into this input- it will blow out the chips. I have had that done on a Porta studio.

 

[wkrbee]

The first thing you need is a M 300 manual .The block diagram is easy to use for non-schematic reading folks to verify you have routing set up correctly.