TASCAM M-308 repairs & upgrades

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Ego Tripper

Well-known member
Joined
Jun 12, 2010
Messages
61
Location
Lombard, IL
Hello all-- long-time reader, first time poster :)

I bought an M-308 a few months back for $90. It was pretty dirty but passed signal on all channels. I brought it home, cleaned the exterior & electronics, then found that channels 5 & 6 have a constant white noise on them. The level of the noise can be adjusted with the trim pot, but it's on both the mic and line settings.

So I've just been using the mixer for very basic tasks while avoiding channels 5 & 6. Now that it's summertime and I have time to actually do something about these noisy channels, another problem becomes apparent: after using a few channels for their pres (and using direct outs to Pro Tools) on toms during a drum tracking session, I found that after every tom hit came a short burst of noise.

Now, I've read all the info on the M-300 series I could find, and it seems that they often need a recap at this age. Based on what I know of capacitors, I could see the transient issue with the toms being caused by faulty caps, but what about the constant noise on channels 5 & 6? Does anybody have any ideas what could be causing either of these issues?

I really like the board and I'd like to go about upgrading the opamps once it's fixed... something with a lower noise floor and great transient response... I just want to be sure it's fixable.
 
do you have the schematics ?

If not, try to get them, it makes everything easier.

check if operating voltage in all the channels is correct.

For the noise on the two channels : This might be a broken op-amp. At least I´m pretty sure that this mixer does not feature any discrete input stages. Look for an IC near the input. Since the other channels are working, just swap the IC with another one from a working channel and see if it helps.
While your at it, install IC sockets this will make further experiments a lot easier.

 
You know, it's funny you mention bad op-amps. I was just thinking in the shower this morning about where the noise is occurring in 5 & 6's signal path, and it is most likely the op-amps! Installing sockets isn't a bad idea, either.

I guess this means I'll just go ahead and swap out the op-amps on all the channels, since that'll likely fix channels 5 & 6.

My biggest issue is still the transient-triggered-noise that appears to be present in all the preamps' circuitry. Now, since it doesn't appear to affect the line inputs, and the signals coming through either input flow through the same signal path-- with the exception of their respective portions of the op-amp (right??)-- then the problem must lie in the preamp's portion of the op-amp ICs, right?

In case you couldn't tell, I don't have a schematic. I find it difficult to shell out $50 or so for a manual when I'm perfectly capable of operating a mixer and just need a simple schematic. :-/
Can anyone with a schematic verify my assumptions about the only difference between the signal paths of the mic and line levels being within the ICs?
 
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