Tascam 388 - weak right channel

T

twainband

New member
hello HR

I'm having an issue with playback on my Tascam 388
All signal is roughly 7db quieter when panned to the right than it is when panned to the left
The problem appears to be global - measured the signal from Stereo Out, Monitor Out and Headphone jacks 1 & 2.
The signal is both audibly and visibly (according to Stereo L & R V.U.s quieter) quiter in all cases.
Switched bias pcb cards, which had no affect.
All cards have been recently cleaned and reseated.

i think this has been a longstanding problem, but i'm just becoming aware of it... tracking does not seem to be affected.
also, when i put the machine into Mono, both left and right channels play beautifully.

i've seen this problem occur a number of times on the forums, but the threads generally devolve into some hairier issue, and i haven't been able to deduce the basic issue that leads to unequal stereo feedback.

if anyone could point me in the right direction they would make really make my day

btw this is my first post , thanks everyone for being kind to the newb

cheers

mat
 
(btw when i say "all cards" i mean those accessible from the top pannel... indiviual channel cards, bias pcb cards, reel servo, and pwr supply)
 
Hi.

So let me ask you this: does the problem exist even when you connect a test tone to your STEREO BUSS IN jacks, and monitor off the RCA STEREO OUT jacks?
 
wow , sweetbeats - you're a legend. glad to have a chance to say thanks for the wealth of info you've provided on the forums. my deck would definitely not be running if it weren't for some of your posts.

to answer your question: no. my signal generator is just a phone app, which i plugged into STEREO BUSS IN jacks via 1/8" to rca Y cable. connected monitors directly to RCA STEREO outs. the tone registered equally on the STEREO L/R V.U.s and to my ears sounded balanced. tbh, i have never used these inputs before; the signal was a little dirty at first but seemed to clear up as i ran the tone through.

thanks again for the quick response. cheers, mat
 
Hey, Mat…thanks for the positive comments and glad my content has been helpful in some way. That’s great.

Okay. So that’s pretty telling that the problem doesn’t manifest when your source is the STEREO BUSS IN jacks. If you look at the Block Diagram you’ll see those jacks connect just ahead of the STEREO MASTER fader booster amps. So that tells us that amp stage, the master fader and everything after it is okay, and that it is upstream.

What’s your skill level with tracing tone through a circuit using a good multimeter or an oscilloscope? I’m not saying that’s the only thing to do next…I have a less elegant solution you could do, assuming you are able to desolder and remove a chip and put a new one in…
 
ok, i'm seeing that path on the pictogram (for some reason, the actual block diagram is missing from both my physical manual and the pdfs that are online, though it's listed in the ToC). would that suggest the problem could be somewhere between Monitor Logic Control and Stereo/Monitor Logic ?

i'm comfortable desoldering/replacing components
but
my experience tracing circuits is limited to checking continuity - I use a multimeter called AstroAI DM6000AR to do that. i don't own a functioning oscilloscope (bought a really cheap one on ebay but it doesn't seem to work).
 
Okay. Fair enough. Your multimeter looks to be decent for general purpose stuff, and nice you can measure capacitors and test transistors too, but just be aware it’s not rated for audio measurements across the audio spectrum. Like, it’s not rated to be accurate outside of a range of 40Hz to 400Hz when measuring AC volts. So you probably don’t want to use it for calibrating audio levels. But for lots of other general purpose stuff, it’s fine, even for tracing signal.

But let’s just jump to what I suspect might be the issue, and yes I suspect it’s an issue with the 4069 hex inverter IC U104 on the BUSS B PCB. This is the electronic switching IC that automatically switches the source of STEREO buss depending on the status of any L-R assign switches, etc., and there is a definite pattern of these failing in the 388 and other period Teac/Tascam devices. And the failure manifests in a lot of different ways. Your issue is a new one, but there’s not much in between where we tested at the STEREO BUSS IN jacks and that IC. You could check the solder joints for P108 on the BUSS B PCB (that’s a red 3-pin connector) which connects to P113 on the MONITOR PCB (a white 4-pin connector)…so check those solder joints and exercise those connections. There’s other signal tracing you could do to verify it’s U104, but the part is readily available and cheap…$0.69 at Mouser https://www.mouser.com/ProductDetail/Texas-Instruments/CD4069UBE?qs=gqbMQSs93zN4MVMbMFTI6g==.

Just to verify am I correct that the issue also exists when the source of the STEREO buss is the monitor mixer?

Let me know what you think.
 
ok - sounds like a plan. going to order some parts today (channel 1 wouldn't record for the longest time, but a recent cleaning/reseating of the card seems to have done the trick. however, it's pretty noisy, so planning to recap) . meanwhile i will take a look at those connections

regarding your question about the STEREO buss source : i patched the Monitor RCA out into the Stereo Buss In and ran a tone through two channels. the problem persisted, but it wasn't as dramatic... only a 2 dB reduction on the right channel as compared to the left ( i tested this by panning the signal from left to right on a single channel, and also by comparing the stereo signal into two seperate channels). not sure if i understood the question so sorry if that was superflous

on a side note - how would you recommend i round out my tool kit? would it be most practical to pick up a seperate oscillator (like this) or upgrade my multimeter to one that is capable of measuring audio signal dependably ?

many thanks, m
 
So that’s not what I meant when I was asking about the monitor mixer output. U104 controls a set of 4 FET switches that determines what you hear in the stereo mix. Like when you depress any L-R assign switch there is a control signal that goes to U104 and then U104 sends the appropriate voltage to a pair of the FETs that closes (i.e. connects) the signal path between the L-R buss to the STEREO MASTER buss…that’s why when you depress any L-R assign switch it automatically switches to that L-R buss as the source for the STEREO MASTER buss…the same is true if the CUE/PGM switch is in the depressed position (i.e. “PGM”), now the source of the MONITOR mixer is the PGM groups, and as long as no L-R assign switch is depressed, if you turn up the level of any of the 8 MONITOR mixer channels and have something assigned to that corresponding PGM group, you will now hear that PGM group signal in the STEREO buss (assuming you have the “STEREO” button depresses in the monitor source select switch and above the PHONES level knob. So I wasn’t intending for you to externally patch anything. That won’t tell us anything new because we already tested what happens with a source connected to the STEREO BUSS IN jacks. SO…

1. Make sure all the L-R assign switches are up/off
2. Make sure the PGM/CUE switch is depressed (i.e. in the “PGM” position)
3. Make sure the STEREO button is depressed above the PHONES level knob to select the STEREO buss as the source for the STEREO OUT jacks
4. Connect a tone generator to any line input (let’s use channel 1)
5. Turn up the LINE trim, raise the fader, and assign the channel to any PGM group (let’s use PGM 1)…set the PAN fully counter-clockwise (hard L)
6. Raise the level of the MONITOR mixer channel 1 and set its PAN control hard L
7. You should see your test tone on the L meter and hear it at the L output. Note the level on the meter.
8. Now set MONITOR mixer channel 1 PAN hard R. You should now see your test tone on the R meter and hear it at the R output. Note the level on the meter.

Now post what the levels were here in this thread.
 
ok - sorry for my confusion there.

i ran the test and the levels were pretty close

L meter: -6.5db
R meter: -7db
 
So can we go back to your original post, and you said you were getting uneven playback. I should have asked this…so when you are playing back, what is the signal path? Are you using the monitor mixer and the PGM/CUE switch is in the up (CUE) position, or do you have input channels sourcing RMX and you have L-R assign switches depressed?
 
Okay. Yeah, I am reasonably certain U104 needs replaced, and the fact you can get relatively even signal strength on both the L and R buss when sourcing the output of the MONITOR mixer tells me it’s not the connections between the BUSS B PCB and the MONITOR PCB. Honestly if it was me I would just shotgun U104. If that doesn’t solve the problem it is, IMO, really good PM on a 388 at this point anyway, and we can continue the search if it doesn’t work. It is always possible or it is Q103/104, the FET switches, but I bet not.
 
ok, roger that. i'm all in favor of PM over regular M. making a Mouser order today. I've got the components list here so may just grab Q103/104 while i'm at it. will post back with results as soon as i get U104 in.

again, thanks so much SB! this thread has been really enlightening. i'm something of a slow study, and the thoroughness is really apprecaited.
 
It Worked !

apologies for the long lag - got slammed with work and also did something foolish that required taking the deck apart again.
in any case, i replaced U104 and while the back was open, recapped most of the electrolytics and ceramics on the channel 1 card, which was alternately noisy or just not tracking at all. just went and played back a few things and the stereo balance is perfect. additionally, the machine just overall sounds great in a way that i can't quantify... could just be the excitement of getting the right side back, but it really seems like some extra 'clarity' has emerged. and track 1 is working now.. not sure if that could have been complicating the issue.

i'm really grateful for your patience with this.. i think this is the best my 388 has sounded, at least since i've owned it.. cant thank you enough
 
well, i really had no idea what was wrong with the card. my diagnostic skills are pretty much limmited to visual problems... there weren't any componenets that had obviously failed, but a few of the ceramics looked really crusty/brittle, like tiny cracking between the lead and the cap... so i figured i'd swap em out while i was in there..
 
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