Tascam M520 L/R imbalance issue

[Matt C]

I've got an old Tascam M520 console in my studio and I've noticed lately that the stereo master output is a bit lopsided (the left channel is always louder than the right).  Since opening this thing up and trying to fix it will be a bit of an undertaking (and I won't be able to do any studio work until it's finished), I'm trying to plan ahead a bit and hoping for a bit of advice here. 

I think I've found the chunk of the schematic where the problem must be coming from (a handful of opamps, with associated resistors and coupling caps), but I'm not sure what is actually causing the problem.  Maybe a resistor has drifted in value?  maybe some of the opamps aren't matched very well? maybe the stereo fader isn't very precise?  obviously hard for anyone to give a diagnosis without the schematic, but is there anything obvious that I should be looking for?  I'd be fine with replacing every component in this section of the mixer, it wouldn't be that tough, but hopefully I can be a bit more analytical than that.

my other question is, the opamps in this section are all NJM2041DD (which looks like they are no longer available) and TL072P.  I'm still pretty ignorant when it comes to opamps, so if anyone has ideas for different/better/more readily available chips to drop in there as replacements, recommendations would be great. 

thanks!

 

[pucho812]

rupert neve once said the best IC chip opamp was a tl072. swapping opamps although the in fad thing to do does not always yield good results and sometimes can cause oscillations and other audio problems. Swap with caution. As far as the stereo buss being heavy on one side I would suspect component failure, has the board ever been recapped? it might be due.

 

[Matt C]

yeah I'm not too worried about "upgrading" the opamps in the circuit, mainly with finding a suitable replacement for the apparently hard to find NJM2041DD chips. I guess I will just start comparing datasheets?

I don't believe the board has ever been recapped.  it's something i've been meaning to get around to, but obviously it's a bit of an undertaking and I haven't decided if the board is really worth it or not.

 

[abbey road d enfer]

my other question is, the opamps in this section are all NJM2041DD (which looks like they are no longer available) 

You can reasonably replace them with NJM4580D or NJM2043D (available from Profusion plc)
The gain difference is almost certainly due to a passive component. You'll have to trace the signal.

 

[Matt C]

I'm starting to think the master fader itself is at fault here.  What is the best route for replacements?  I need two 60mm 10k audio taper stereo faders.  I might call the Tascam parts department to see if they still have spares, and I think mouser carries one or two ALPS replacements (RS**N and RS**1 series). 

I guess I'm looking for a little guidance as far as the quality and availability of faders since I haven't shopped for them before.  is it worth searching out a different brand or model?

 

[abbey road d enfer]

I'm starting to think the master fader itself is at fault here.

Do you still have the same level discrepancy with the fader fully up? If yes, that indicates the fault is elsewhere.

 

[Matt C]

I'm starting to think the master fader itself is at fault here.

Do you still have the same level discrepancy with the fader fully up? If yes, that indicates the fault is elsewhere.

thanks for the tip.  just double checked this and with the fader all the way up, the channels are balanced.  with the fader a bit lower, there is roughly a 10% difference in voltage between the two channels.  looks like it's time for a new fader.

 

[abbey road d enfer]

I'm starting to think the master fader itself is at fault here.

Do you still have the same level discrepancy with the fader fully up? If yes, that indicates the fault is elsewhere.

thanks for the tip.  just double checked this and with the fader all the way up, the channels are balanced.  with the fader a bit lower, there is roughly a 10% difference in voltage between the two channels.  looks like it's time for a new fader.

10% is just about 1dB. Tolerances on "standard" faders (not P&G) is often like 2dB.

 

[Matt C]

hmmmm....maybe I'm chasing my tail here.  thanks for the reality check.