SOLVED Tascam M308 distorted bus outputs and a big THANKS

I thought I would publicly honor Mr. Sweetbeats for helping me get my Tascam M-308 mixer back in working order, because while I'm super handy with a soldering iron, I just don't have the skill to figure out schematics.

Just a little rambling history, if you will.... the original problem was distorted outputs on bus channels 7 and 8. This is the video (it's 2 years old; goes to show you the priority):


I pretty much tucked the mixer away under a shelf for the last couple of years as I just didn't have the time or patience to deal with it. But as I was thinking recently about some of the gear I own and what I was going to do with my half-finished projects, one of the more annoying but seemingly-worthwhile was my Tascam 38 and M308 mixer. I bought these within days of each other about 9 years ago. I felt a little taken on the 38 because I drove a considerable distance to pick it up, and even though it was cheap, it wasn't that cheap... I basically picked up a unit that I confirmed would power on and needed most VU bulbs replaced. Belt was shot and left parts of itself smeared all over the inside. I'm not that smart. It even came with some old sticky 456... bonus.

The M308 on the other hand was in very good condition locally. I felt good about scoring that one. Unfortunately, other than knowing it was an 8-channel mixer that should be compatible with the 38, I didn't know anything about bussing and was using it in the most basic way. So, if there was a problem with the bus channels, I never would have known it. But something tells me I blew em out not knowing how to use it... So, I used it as a keyboard mixer for a little while and then it sat for years under a shelf unused.

A couple years ago I busted it out with the 38 to try giving it a go again, this time with more knowledge than before, and trying to sort out the issues with the deck. But what I found were issues that turned out to be the mixer. As if the headache couldn't get worse, I gave up on it again. This is when I shot the above video.

Cut to present day; I need to make a decision with this extraneous, heavy stuff I have sitting around. Do I get it fixed? Do I sell it for parts? Do I care? In the interim I had purchased a Tascam 388 for next to nothing in impeccable shape, so I was getting my analog fix that way, but I still wanted that 1/2" deck... so I pulled it all out and gave it another go. This time, I got some incredible help from Mr. Beats, who offered his help purely out of interest and kindness. He listened to the problem, dealt with my irrational and shotgun diagnostics, spent his personal time helping me by going through the schematic and pretty much nailing the issue... I'm incredibly grateful. I was able to get one huge headache out of the way so I could get to the 38 and figure out its issues, and not have to fight with a malfunctioning mixer in the middle of it.

The actual problem, since it would be silly to not mention the fix, was a blown op-amp on both of those channels, and coincidentally the same one. Somehow, something overloaded and blew U203 (NJM2041DD) on both cards and Beats was able to figure out by just looking at the schematic that it was the problem. Pretty impressive... Unfortunately, that particular chip is out of production, and getting a drop in replacement seemed harder than it should have, so I was fortunate to have Mr. Sweetbeats send me some pulled chips to try and sure enough, they were good and they fixed the problem.

So, as much as this is a very public 'Thank You' to our resident guru Sweetbeats, it's also acknowledging the forum as a wonderful place to give and take information on what most would consider a niche area of recording. I can't even begin to express how fortunate, not only I, but all of us are that we have this information at our fingertips and that we have regular people who give up their precious time (or what little of it they, and most of us, seem to have) to help others fix problems on equipment that will probably never benefit them in any way, shape or form. And I think it's so cool to know there's a community of people who pretty much have your back. It's nice to know that in this weird world we live in, people still like to help each other.
 
Sincere thanks for the background, for sharing our success and for accolades. I really appreciate and am just glad we got it going! :D

To dovetail with your account of the process, for me one of the biggest clues was being able to *hear* the behavior in the video with the playback of the Journey tune...the really diminished output except on peaks and the. Super loud and crunchy on the peaks reminded me of what an Tascam MX-80 mic pre I used to have was doing that ended up being a blown opamp. So opamp(s) was on my list of suspects, and then I used the block diagram coupled with your detail of where/when the symptoms were present...that narrowed it down to that amp stage (the group fader booster)...the clues made it apparent it had to be right in there somewhere...and then I drilled down using the schematics to identify the part and verify the location. So you provided all the clues, I did a little sleuthing, and then you did the work to swap the parts and the good fortune is...it worked!

Another M-300 lives!! :guitar:
 
Here, here! Cory is an invaluable resource on this forum, as I personally experienced last year when he spent hours and hours remotely diagnosing the problem with my 388. If I remember correctly, the problem was also a bad opamp. With his patient guidance, I was able to make the repair and get it running again. It was truly one of those moments in which your faith in humanity is restored.

Of course, that kindness extends across many forums and members here, and I've been helped many times by others as well. It's really quite a community we have here.

Oh, and congrats on getting the 308 on its feet again! :)
 
Sincere thanks for the background, for sharing our success and for accolades. I really appreciate and am just glad we got it going! :D

To dovetail with your account of the process, for me one of the biggest clues was being able to *hear* the behavior in the video with the playback of the Journey tune...the really diminished output except on peaks and the. Super loud and crunchy on the peaks reminded me of what an Tascam MX-80 mic pre I used to have was doing that ended up being a blown opamp. So opamp(s) was on my list of suspects, and then I used the block diagram coupled with your detail of where/when the symptoms were present...that narrowed it down to that amp stage (the group fader booster)...the clues made it apparent it had to be right in there somewhere...and then I drilled down using the schematics to identify the part and verify the location. So you provided all the clues, I did a little sleuthing, and then you did the work to swap the parts and the good fortune is...it worked!

Another M-300 lives!! :guitar:

Yes! Well done chap! I would be interested to know which ICs did the bizz? Looking at a data sheet for the originals it seems to me that the NE5532 would serve? Given that the 5532 is now some 30 years old perhaps I could suggest the LM4562 as an even better dual op amp?

We MUST keep fixing things or we are going to run out of planet!

Dave.
 
Hey, Dave...it was the PGM group fader booster amps...those are 2041s.

They used those there I think because of the output drive capability...25mA if memory serves. I think the 5532 likely has the same or better drive capability but the datasheets are dubious as to this spec with the 5532 (and I've looked at the Signitecs, JRC and Texas Instruments documents and none of them spell this out...they list the short circuit output current at a nominal 38mA though so I can infer *something* from that at least) so I'm not positive. Without having tested it I'm assuming, since Teac used the 5532 liberally elsewhere in the M-300 series consoles the 5532 won't work for those PGM group fader booster amps (BJT instead of JFET...sometimes it works...sometimes it doesn't, right?). But I've never put one in and checked the output offset, so my suggestion to Whiny if ever he *did* want to upgrade was to use the 2134...good part and has a 35mA max output current.

At any rate, I sent Whiny some used 2041 parts to drop in to get things going. :thumbs up:
 
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