Tascam LA-40 Info Needed

[MENTION=204989]yoshi420[/MENTION] I was finally able to locate my LA-40 Service Manual and found some info in it that may be helpful...maybe more than what is in the Owner’s Manual...or maybe not...they may have included this info in the Owner’s Manual. But the short of it is the switches are on the main PCB inside, accessible after you remove the chassis cover. I’m attaching a couple pics to this post.

[MENTION=130918]skywaveTDR[/MENTION] the thing about the original LA-40 though is it is one of the most versatile converter boxes I’ve seen, with different jack type options, the switchable level, and the high level output amp is not garden variety straight opamp-based...it is actually a semi-discrete opamp/BJT line amp. An opamp drives a complimentary pair of output transistors running with gain and higher than garden variety power rails for greater headroom (i.e. +/-20V vs the more typical, for this echelon of gear, +/-15V). This amp circuit is derived from, and I believe developed for my prototype “Tascam M-__” console. The LA-40 circuit is nearly identical to the same balance amp circuit found in the Tascam 58-OB 1/2” 8-track tape machine. In both cases it is a tamed version of what’s in the prototype console (which is a mildly more complex circuit powered by +/-35V rails). The M-50 12x8 mixing console 2-channel “BALANCE AMP” is the only other place I’ve found the exact same line driver circuit as the prototype console, and so far the LA-40, 58-OB and 122B cassette deck are the only period devices I’m aware of that use the concept. I suspect it’s use was pretty limited...suspect it was dropped due to cost considerations. But I will say the outputs on my prototype console that use the full monty +/-35V circuit sound great. So...my point is I think the LA-40 has a bit more going on than the many other offerings one can get...more Swiss Army I/O options, and higher headroom. I can verify the later iterations from Teac do not offer the same (i.e. LA-40mkII & mkIII, LA-81, LA-81, etc.) and again I don’t think that’s because it wasn’t a good amp design, but it was cheaper to default to a more garden variety full opamp design.

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I have a LA-40 mki that turns on but no audio signal is passing through. I wonder if you can share with me the service manual in order to track the problem with my unit.
 
The first thing I would do is remove the top cover and exercise all the level adjust switches the span from left to right across the rear portion of the PCB. And also verify your connections are appropriate. If the problem is global across all 4 channels, it could be user error, or, if not, a problem with the power supply. So:

1. Verify you have things hooked up correctly and maybe even share the setup here (and note the the XLR jacks are pin 3 hot/high, which, depending on your individual setup, can be problematic for signal continuity)

2. Exercise the level adjust switches, and

3. Check the +/-15V supply outputs…like just measure for DC volts from pin 4 to ground and pin 8 to ground of any of the 5532 or 2041 audio opamps.
 
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