Tascam 388 no right channel

Hello all,

My 388 just started a new issue where the right channel is not producing sound. I've searched here and other places, and it seems like this has been an issue for others but I haven't found any solutions.

What is happening:
The right channel is not producing sound across the board. Headphones, monitors, playback, playback on mix mode, recording when panned right both line in and mic. However, the right VU meter is functioning. And when I switch to mono the right side will produce normal sound. If I start in the center and pan right the sounds taper off.

What I've done:
I have a parts machine. So far what I've switched out: right and left bus PCB, monitor PCB, the green strip that ties the channel pcbs together, right fader and main fader.

I've had the machine for a while so I don't think it's user error. If anyone has any thoughts or might be able to give me a hint what to look out for I'd really appreciate it. I'm stumped.
 
1. Is the issue present at the L-R main outputs jacks as well as the MONITOR out jacks and the headphones?

2. Do you have the same symptoms if you connect a stereo source to the STEREO BUSS IN jacks?
 
I'll definitely check these when I get home from work.

Any chance a fuse could be an issue? I replaced them about 5-6 years ago when I got the machine. Just trying to be proactive since I would have to order them online.

Also, before the right channel issue started, I was having intermittent static on channel 8 during playback for a while. In case there might be any relation.
 
Fuses protect entire power rails. If a fuse is bad, nothing powered by that power rail will function. The right and left channels are powered by the same power rail. A fuse is not the issue.

I don’t believe the channel 8 issue is related.

I suspect you will find both channels are working at the STEREO out jacks both balanced and unbalanced, and that audio input connected to the STEREO BUSS IN jacks is A-OK at the STEREO OUT jacks, but missing the right channel at the MONITOR OUT and headphone jacks.
 
Ok, now the machine is throwing a curve ball. I was able to test everything. What is happening now is that both channels are coming through with sound but it seems to be a mono signal. The sound seems to be stronger when centered and slightly lower in volume when panned left or right. The main meters are reading correctly. The strange thing is it's even happening in my mixdown deck. It's connected to the unbalanced stereo outs. The left and right meters are moving correctly when I pan, but monitoring through headphones of the mixdown deck I'm getting the same weird mono-ish anomaly.

Come to think of it, it did this briefly the other day when I was trying to figure out the problem. But only briefly before going back to the no right channel issue.
 
I still need to know if this is happening in the STEREO OUT jacks as well as the MONITOR and headphone outs.
 
Sorry I forgot to be specific. This new problem is actually happening in all manners tested except through the monitor outs. The monitor outs are the only thing that after playing correctly when I pan. Everything else is doing the weird mono thing.
 
Do you have a multimeter?

Do you have any weird stuff going on with indicator LEDs, like them turning on when they shouldn’t?
 
Okay. Well, sit tight. No promises. Your issue sounds like a switching logic issue, and I know of some common problems with the 388 due to a particular logic IC, but this is manifesting differently, and the fact it is not consistent between which sets of outputs is a conundrum because it means multiple PCBs may be in play. When I get a chance I’ll study the schematics.

Does the MONO switch have any effect at all on your output when it’s doing the weird mono thing?

[EDIT]

Don’t do any more shotgun swapping out of boards...we need to isolate the problem, and shooting in the dark may be introducing more problems which complicates isolating one issue and systematically addressing the problems as a whole.
 
Ok, so I checked this morning. When I first fired it up it was back to no right channel sound. So I pulled out my headphone cable and plugged it in to my mixdown deck and it was back to the weird mono thing. I then plugged the headphones back into the 388 and it was still doing the mono thing.

Anyway, yeah, when I press the mono button the volume basically just gets louder, but does the same thing, where it's louder at the center and a little quieter when panned either way. With the mono button pressed and when it's not, it never actually pans when you rotate the knobs. It just gets equally quieter on both channels. Does that make sense?
 
Yes it does, but what are you monitoring exactly?

Could you tell me, with as much detail as you can, where the material you are monitoring that is doing the weird mono thing comes into the 388, what those channel settings are, to where you have the channels assigned, and then those summing busses what their settings are? If it is tape playback you are monitoring how are you sourcing it, and what are those settings etc?
 
I'll try to be as thorough as possible. I have mostly been monitoring through tape playback and simply using the vertical monitor section. I have tracks that had already been recorded and I was using various tracks through the monitor section to test. I also tested the initial problem on mix mode and tried panning on all 8 channels and the issue was consistent.

When I tested the stereo bus in yesterday, the issue was also the same in all manners tested. Weird mono problem with everything except the monitor outs, which were functioning as they should.

I haven't tested the line in or XLR inputs for the channels since it switched to the mono issue, but I can this evening. When it was the no right channel issue, the pan on each individual channel was giving no sound when panned right with both line in and XLR. So if I had an input on channel one and panned left there would be sound on track one in rec mode, but if I panned right there would be no sound of I tried to record on track 2.

If it would be helpful to try anything specific, I'll gladly try it when I get home.
 
Is it safe to assume when you are monitoring tape playback via the onboard MONITOR mixing section you are panning each track to taste using the pan controls, but they have no effect when the unit is exhibiting the weird mono issue except maybe to progressively diminish the level of the track somewhat when the respective PAN control is swept counter-clockwise or clockwise?
 
Well I’ve not come across an issue like yours before. What’s confounding is, the way the unit was designed, if you are having trouble with signal at the STEREO OUT jacks you should also have that same problem at the MONITOR OUT jacks and the headphones. It’s possible to have problems at the MONITOR OUT jacks but not the STEREO OUT jacks, but not the other way around. So the way your problem is manifesting is really weird.

There is a 14069 switching logic chip on the BUSS B PCB (maybe that’s the one you called the right buss PCB?), the one with all the PAN controls for the EFF returns as well as the monitor mixer channels. That logic chip, U104, controls muting transistors Q101~ 104...I can’t quite wrap my head around how that would cause your weird mono issue...no signal at all, yes, or one side only, yes, but not mono summing. But if it was me, based on past experience with that chip going belly up I’d shotgun it with a new part, and see where I was at.

I can’t recall if you answered this one: how does signal input at the STEREO BUSS IN jacks reproduce at the MONITOR OUT and STEREO OUT jacks?
 
The stereo bus in causes the exact same problem as what I've described for tape playback in every way.

The strange thing is with the stereo outs to the mixdown deck, the meters will correctly read the pan, but the headphone out of the mixdown deck is going the same mono thing. Not sure how the signal gets transferred to the deck via the unbalanced outs.

I did switch out the BUS B PCB with one from my backup deck. I guess it's possible that both are bad. But now I can't remember if it was when I switched the PCB that the new mono thing started. I'm going to switch them again and see where that leaves me. Is the IC chip something I could find a new one of and solder onto the board?

Someone also mentioned checking the post fader amplifier for the right channel. Would that be something to pursue?
 
Ok, I swapped the PCB bus B and it didn't change anything.

When I first turned it on after swapping the PCB, the same thing that happened this morning happened again. The issue was initially no right channel. Then after a couple minutes I pulled the headphones from the 388 and plugged then into the mixdown deck and then the mono issue started up. The mono then kept going afterwards.
 
Absolutely weird. I wish I could get my hands on it and actually trace the signal.

So when it’s doing WMT (weird mono thing) the issue is present in the stereo out jacks and in the headphones, but NOT the MONITOR OUT jacks??

I don’t know if I can explain how weird this is...the post-fade stereo sum branches and one limb goes off to the unbalanced STEREO OUT jacks and the balance amp and then to the balanced STEREO OUT jacks and the meter amp. The other limb goes off to the monitor and headphone amps. But all are fed from the same stereo sum. Your issue presents as a bad stereo sum everywhere but the MONITOR OUT jacks, but the headphone amp and monitor amp sit right next to each other on the same PCB, and have the exact same source...it’s just a 3-pin mini Molex connector. And the issue is present at the STEREO OUT jacks suggesting the cause is somewhere after the signal goes to the meter amp but before it goes to the STEREO OUT jacks and the monitor and headphone amps, but the monitor out is okay... :???:

You’re certainly welcome to replace the U104 logic chip, but it seems to me if that was the cause you would have the same issue at the monitor out jacks and the headphone jack...so just now something poked at my brain...I need to look at the operations section of the manual and refresh functional how the monitor mixer source is handled...there is auto source select circuitry there...

If you end up wanting the part here is a link to one place to get it:

CD4069UBE Texas Instruments | Mouser Europe

But, again, I’m not certain that is your problem, and it sure would be a crazy coincidence if both BUSS B boards were bad...AND it doesn’t explain some other things.

Oh and regarding checking the R fader booster amp you can do that, but the thing is when your 288 is doing the WMT you are getting audio in the R channel, just in a weird mono way, so the R amp is working.
 
Yeah I'm agreement it's unlikely that the chip is faulty in both PCBs. My goal for the summer is to one by one replace the caps on the various PCBs on this machine. When I'm ordering I'll order a backup chip since they are known for going out.

So right now I've been more into tracking, which really doesn't require an accurate stereo field. Would it be harmful to operate the machine before this issue is solved? I'm going to have a some time over the next few weeks where I can make a little noise without bothering the neighbors.
 
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