Tascam M-312 channel distorting and cutting in and out

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The Groke

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Hi everyone. My Tascam M-312 has two channels with intermittent problems. They both occasionally do not work at all, and when they do work they are distorted about 80 percent of the time, but sound OK the other 20 percent.

My first guess without opening anything up is that there is a bad op amp on each of those channels.... but is there something else I should check? Would a bad op amp only be bad some of the time (and it does not seem to be a temperature issue since the problem can be at first power up or after a long period of time) Any thoughts on this?

Also... can anyone (I'm hoping Sweetbeats can chime in here, since he's the resident expert on these boards?) advise me on the easiest way to access the channel cards for servicing. Is it just a matter of removing the bottom plates and unscrewing the knobs for the respective channels? My desk is really packed in tight, and I would like to know what I'm in for before I go trying to pull everything apart. The service manual is somewhat vague on the order of disassembly.

Thanks in advance - D
 
Hi Ghost - Thanks so much for your reply. I did see that thread, which is what inspired me to post. But I didn't think my issue really sounded like a "bad connection" issue. I guess that's what you are implying though :) I'll take your suggestion and just open it up and reflow/resolder and see what happens. Are these cards a pain to get at, or is it trivial? I can of course just try it and see, but I like to know how many hours of pain I am in for before I go ripping the studio apart. And it's always good to have some tips in advance so that I don't break something else in the process of going in.

Thanks - DT
 
I've never opened my 312B's when I owned them, so I can't offer any direct advice. I suspect opening up the bottom will give you access though.

Luck.

Cheers! :)
 
The M-300 series are pretty easy to work on. I have the M-320B. This is something I love about Tascam consoles from that era... nicely spaced individual channel cards. The first thing I would try is to swap a known good channel card with one that's acting up. That will help you narrow it down real fast and it often is a connection issue that will be fixed simply by removing and reconnecting a channel card. These consoles are 25+ years old and over time oxidation can build up causing intermittent bad connections. Op-amps don't fail on those boards as often as other components, but its possible.

Another thing you can try to narrow things down before you start pulling channel cards is to check the signal from the channel insert point with the EQ switched out and then also switched in. With EQ off you're hearing only the op-amp in the pre circuit and not those in the EQ circuit.

If that checks out ok next turn the EQ on so you’re hearing the EQ circuit. Remember to monitor all this from the send half of the insert point, AKA send/receive point for that channel on the back of the console. Test using both the XLR mic input and the line input because they have separate pre circuits with different op-amps.

Finally monitor signal from the Direct out for the offending channel because that is post-fader and you’ll now have the channel fader op-amp in the mix. That should get you started in the right direction. Keep us posted and we can move on from there.

Edit: Oops, almost forgot. Yes, to get to the channel cards remove the bottom panel. If you need to remove a card, first remove the rotary knobs and unscrew the nuts for that channel. You may luck out and not have to get to that point however.
 
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Hi Beck, thanks for the info. This is exactly what I needed. I'm kind of curious as to how the cards connect, but I'll figure it out once I get it open. Now the trick is finding time to do so... but I will be sure to report back on this forum. Thanks - DT
 
Hi Beck - Well, it took me two hours to get the dang thing out of my set up (It is set up in back of my desk, which is the only place it fits... weird, but there you go). The good news is that after popping open the bottom plates and reseating all of the connectors, the problem vanished. So, perhaps it was just oxidation, as you said. Of course, the problem was a bit intermittent before, so I'm hoping it holds up. But thank you VERY much for pointing me in the right direction. I'm sure I would have wasted a lot of time trying to debug the circuit otherwise.

best - DT
 
...and thanks to Ghost as well, for basically saying the exact same thing :)
 
And now, the weirdest thing happened. After not using the mixer for two weeks, I turn it on to discover that now ONLY channel 5 works (which was the channel that gave me the most trouble before). All of the other channels have super low volume. Bussing them over to 1-4 does not help. This is bizarre...
 
The Groke said:
And now, the weirdest thing happened. After not using the mixer for two weeks, I turn it on to discover that now ONLY channel 5 works (which was the channel that gave me the most trouble before). All of the other channels have super low volume. Bussing them over to 1-4 does not help. This is bizarre...
Click to expand...

That is odd!

Perhaps something is amiss with the solo system, soloing that channel and muting all the others?

Cheers! :)
 
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