Level dropping slightly on one channel

leddy

leddy

Well-known member
Hey folks.

One of my Tascam 22's is having some issues. The left channel level is fluctuating by a db or two. It will dip slightly every so often.

I ran some tones and checked that I was feeding it constant voltage (and the problem was not any of the gear passing signal to it), and I verified that it is happening on record and not playback, as the fluctuations are being printed on tape.

I tried a couple different rolls of tape as well.

Where do I go from here? Anyone seen this issue?
 
Is it random, or periodic? Does it correspond to any mechanical movements, e.g. revolutions of the pinch roller or the spool?

If it's random, does giving the machine a small knock help? (If so, it's typically a dry solder joint)
 
It appears random; about once every minute or so, the signal will drop quickly by about 2 db, then recover over a time span of about 1 or 2 seconds. I thought it was just tape dropout at first, but it is definitely isolated to the left channel, and it occurs on brand new tape as well as old.

Deck seems in good mechanical shape, I don't think it is that.
 
My hunch is a bad transistor, and/or bad capacitor on that side.

Have you run tests with both the front mic inputs and the RCA line inputs? The two circuits follow a different path up until the line amp. The mic input has a transistor that could be acting up. Both mic and line inputs go through the same capacitor before the line amp (op-amp JRC4558D).
 
That's great info.

I think I'll replace those caps on mine to make sure.

Redoing all the caps on the P/S board as well.

I have found, however, that the audio board connectors are very sensitive to connection and have cleaned them with Deoxit.
 
Beck said:
Have you run tests with both the front mic inputs and the RCA line inputs? The two circuits follow a different path up until the line amp. The mic input has a transistor that could be acting up. Both mic and line inputs go through the same capacitor before the line amp (op-amp JRC4558D).
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D'oh, why didn't I think to do this. I will test and report back.
 
Ok. Quick test shows that it still has the same issue using the mic inputs. You can see the left meter bob a little while the right one holds still (playing a 1khz tone). So you are suggesting it is a bad cap? I assume this would be on the record/repro amp pcb?
 
What should a tech charge to re-cap something like a Tascam 22 (I'm not experienced enough)?

Is this a worthwhile expense, or I should I try swapping out the board with one from a parts machine? I have two, but they both work. I hesitate to start ripping one apart for parts when it's still good. Perhaps I could locate one...
 
That’s a lot of capacitors. The cost would likely exceed the current market value of the unit, but full recapping isn’t normally needed. And it might not be caps, but a transistor or something else. The reason I suspect a cap or transistor is the cyclical nature of the level drop… approximately every minute or so you said.

Got a question though before getting too far into it. How have you verified that this is happening during record and not playback? Do you observe the drop durring recording, or have you played the recorded tape back on another 22-2 or flipped the tape to play it backwards… or have other tapes to play?
 
Beck said:
Got a question though before getting too far into it. How have you verified that this is happening during record and not playback? Do you observe the drop durring recording, or have you played the recorded tape back on another 22-2 or flipped the tape to play it backwards… or have other tapes to play?
Click to expand...

If I watch the meters while I'm recording a test tone, I can see the left meter dip a little and recover while the right needle stays still. Would this tell me for certain it was on record and not playback? Maybe not, since I cannot monitor from the record head on a 22.

I'm fairly certain it's on record. If I rewind the tape and play it, it happens in the same spot everytime. I can put the tape on another machine to verify though.
 
leddy said:
If I watch the meters while I'm recording a test tone, I can see the left meter dip a little and recover while the right needle stays still. Would this tell me for certain it was on record and not playback? Maybe not, since I cannot monitor from the record head on a 22.

I'm fairly certain it's on record. If I rewind the tape and play it, it happens in the same spot everytime. I can put the tape on another machine to verify though.
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Yeah that should be enough to tell, as long as you've got the meters set to monitor source. That does narrow it down to the input section. It wouldn't hurt to replace the caps in the signal path. But it could also be one of the three transistors (Q104, Q106 and Q107) in the input section before the record head. I would also replace the JRC4558 op-amps with NE5532A while I was at it. It's one of the best and easiest drop-in upgrades for the 22-2. It's even possible you've got a flaky op-amp. However those usually work or they don’t... but still possible. I’ve got two 22-2’s as well. One I’ve upgraded with new op-amps and metal film resistors in the signal path. It’s cleaner and less noisy than the original. The original isn’t bad at all, but I like to squeeze that little bit extra out of things when I can. :)
 
Problem has been solved. Here's what happened for future info if someone has the same issue:

I had a tech look at it. He replaced some IC's on the rec/repro amp card. It was only a few bucks in parts from Tascam plus his bench time. More than I paid for the deck, but he cleaned it out and tested everything else. It is working like new.

From what I can tell on the parts list, it was reference #'s U101, U102, U103, and U104.
 
I hope from the IC's you mentioned he did both audio channels as that would balance the system audio better.
 
Beck said:
I would also replace the JRC4558 op-amps with NE5532A while I was at it. It's one of the best and easiest drop-in upgrades for the 22-2. It's even possible you've got a flaky op-amp. However those usually work or they don’t... but still possible. I’ve got two 22-2’s as well. One I’ve upgraded with new op-amps and metal film resistors in the signal path. It’s cleaner and less noisy than the original. The original isn’t bad at all, but I like to squeeze that little bit extra out of things when I can. :)
Click to expand...

I just realized that the parts he replaced were the JRC4558 op-amps. I was looking for them on the parts list to get the reference number but did not connect them to your post until now. Not sure what Tascam sent him for parts without opening up my deck.

GCalo said:
I hope from the IC's you mentioned he did both audio channels as that would balance the system audio better.
Click to expand...

I'll have to ask him, but the audio has never sounded better.

Edit - Just to check I just recorded identical (mono) pink noise to both tracks and looked at a spectral display to compare. The left and right channels are within 3-tenths of a db of each other from 20hz to 20khz.
 
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