My M312B

I

Instroverb

New member
Okay, so a few months ago I posted that I had bought a M312B off of Ebay. Some of the meter bulbs were damgaged in shipping but otherwise the board appeared fine. However, what I thought was a headphone jack issue turned into a whole other thing.

The machine inetmittently cuts out when ever I touch the metal faceplate or any of the buttons connected with monitor and talk back. There is also loud staticky noise when any of these buttons is depressed. I pulled the bottom off to see if there was anything obvious, but don't really know what I'm looking for. Other than this, the machine sounds and works great. I don't get any noise when I press buttons in the mixer or monitor (tape) playback, just monitor out and talkback. Thanks for any advice.
 
Cracked solder joints

This is not a big problem if you have the time and are handy with a soldering iron...

I have a 312b which had the same type problem.....

The thing to do is to set it on its side panel (on its end). Then take off the bottom panels. You should now have access to all the channel cards as well as the front of the mixer.

You will need a strong light. Starting where pressing causes problems and look on the solder side of those boards for cracks in the solder joints that connect the pots to the board. You will see a hairline crack that goes all the way around the lead.

Any crack is bad. You can test this by poking at the crack with an insulated tool. The goal here is to show that mechanical movement changes the signals.

Oh, I run a tone through the channels under test and listen via headphones.

Then you will need to reflow the solder on that joint. I recommend a 2% silver solder ( I admit to using some from radio shack ) in a fine guage (small diameter) and an Antex Precision soldering iron. The Antex M/3U is a fine British tool. www.mmnewman.com sells them in the US.

I would take the time to inspect all the pot solder joints for cracks as long as you have the thing on its side. Somewhere I might have photos.

Oh, of course this is a good time to de-ox all the connectors too.....

Regards
 
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