Tascam M-312B Channels 3-6 Cutting Out

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ecs787s

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Hey Guys,

I'm trying to figure out an issue I'm having with my M-312B board. When I pass a signal through any of channels 3-6, they cut out by tapping on any of the lower buttons(closer to operator) such as the bus select or fader buttons. If I start by passing a signal through and they are not working, all I have to do to get them to work is tap on the lower pots or buttons and bam, there they are. It's almost like a loose connection. I've taking the bottom of the board off to check and see if everything is connected too. All looks fine.

Any ideas? Check out my video of me reenacting the problem!

jenericcreative.com/video/MVI_2294.AVI
 
I had a similar issue with one of my M312B mixers back in the day, ( I had two of them), and also found that tapping the top panel in key spots temporarily fixed the issue. I never got around to getting the issue properly fixed as the tap trick usually lasted long enough to get through a recording session. But yeah, I'd suspect an oxidized connector somewhere which would require unplugging harnesses, spraying a bit of De-oxit and re-connecting back the harnesses and or re-seating channel modules. Looking at it visually is not an effective way of spotting an issue unless there is glaringly obvious rust or rodent remnants...yuck.

Good luck!

Cheers! :)
 
If it was me I'd take a look at possible bad solder joints on that connector PCB that runs across the bottom of the channel cards.

Tip it up with the bottom panels off and see if, by dickering around with the connector PCB you can make it cut in and out and if you can then pull it and re-flow the solder joints.
 
Thank you both for your suggestions. I've tried both options with no success. I think the problem is the on button. Can I deoxit that?
 
I also failed to mention that just touching the on button with the softest of touch will make it cut out or cut back in. A couple of Panning pots on the affected channels also don't pan properly. Could it be something with the proximity of those pots and the on button? Makes no sense to me.
 
ecs787s said:
Thank you both for your suggestions. I've tried both options with no success. I think the problem is the on button. Can I deoxit that?
Click to expand...

You could glob some in there I suppose or you could try exercising the button on and off several times in rapid succession to try to scrape the junk away. And that would be the far easier route too. Plus that same trick can be tried on pots too, usually with good success before you go to the de-oxit option.

Cheers! :)
 
The thing is, to inject the DeoxIT properly you really need to get the channel card out. In my experience the cleaner doesn't really get down to the contacts in the switch when injecting from the top. On the side of the switch housing there are joints where the case snaps together and I've found that by holding the straw right up to one of those joints and giving a gentle squirt of the cleaner I can fill the inside of the switch and the lotsa click-click-click. Same goes for the pot. DON'T try injecting from the pot shaft. There is grease in there to keep dust and dirt and beer from getting into the guts of the pot. All you'll do is potentially force grease, dust, dirt and dry sticky beer into the guts of the pot. With the card out you can get to some little holes on the body of the pot. A nice jet of cleaner, and the lotsa sweep-sweep-sweep, let the cleaner drain, and the another jet and let drain to rinse and then try it out. While the card is out I'd check/reflow the solder joints on the pot and the switch.

NOTE: YOU MAY KNOW THIS ALREADY BUT DO NOT USE THE SAME DEOXIT FOR THE POT AND THE SWITCH. THE SWITCH NEEDS THE "D5" PRODUCT FOR METAL-TO-METAL CONTACTS, THE POT NEEDS THE "F5" FADERLUBE.

if all else fails PM me. I have spares.
 
Thank you both for your suggestions! We have fixed it!

So, with no success getting the channels to cooperate by lots of button pushing and sweeping with cleaner, I pulled out the cards like Sweetbeats said and reflowed the solder joints on the Channel On buttons and the balance pots.... Put them back in and they work like a charm!! Thank you both so much for your suggestions. I'm still new at this and was amazed that the solder joints where the problem. They looked find, but I figured why not give it a try and melt the solder and add some new solder on top. I did notice that the amount of solder that Teac used was very minimal and almost looked like it didn't have much surface contact.

Sweetbeats, I've read through your Tascam 48 Story on one of these sites since I just acquired one a few months back. Very informative. By any chance did you happen to come across my thread about the LED issue? Maybe you might have an idea since you've worked on it so much!

Tascam 48 Dim record and spooling LED - no edit LED
 
Glad that worked!

Often the break in the solder joint is not from the solder blob to the PCB, but between the blob and the tail of the component, in this case switches and pots...really hard to see. Remember that we are dealing with electricity here and electrons *always* take the path of least resistance to ground. A broken solder joint may not cause a complete break in continuity, but if it is enough to raise the resistance at that point in the circuit such that another path is least resistive, *poof*, there goes your audio.

I'll look at your other thread but honestly I've been avoiding it because the title tells me that it is a logic issue, and folks that know me know that I don't fancy IC logic. Its beyond me. I'll take the big clunky relay logic on either of my tape machines any day over IC logic.

Like I said I'll have a look but don't think I'll be able to contribute. My 48 ran like a top...pretty much had no issues with it so there was very little digging/troubleshooting/repair to do.
 
I see you got it fixed, but just so you know, I posted a link to the owners manual over in the Tascam forum.
 
Thanks man, that is very generous of you! Have a good weekend.
 
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