MCT Serial board mystery - Can you crack the pinout code?

[drlegendre .]

Howdy,

On Sat, Nov 15, 2014 at 5:37 PM, Tothwolf <tothwolf at concentric.net> wrote:

>
>> What happened with the charred area between the uarts and the edge
> connector (hopefully not conductive)? Did that pair of tantalum capacitors
> burn up? Nichicon PW series low-esr aluminum electrolytics make for an
> excellent radial tantalum replacement.
>

Ah, that info must have been part of a different discussion.. and it might
also be a clue, since the board was (apparently?) operating in +some+ form
prior to the repair..

Those charred areas locate the 12V zener diodes & dropping resistors that
form the +/- 12V supplies for the board. As received, the board was already
like this, and the offending components had been simply cut away - this all
happened long before my time with it.

Now obviously, I didn't initially know what was supposed to be there. But I
started tracing the board and found that one set of traces went to the -12V
supply input for the original 1013 UARTs as well as the GND / -Vcc
terminals of the 1458 op-amps. Now, the present UARTs no longer require a
-12V supply - that need was eliminated with the 1014/1015, so they would
have been fine without the zener reg.. But the op-amps most certainly do
require a bipolar supply (if they're going to do real RS-232 conversion,
right?).

So that tells me that whatever was going on previously wasn't using the
board as originally designed.. because the never bothered to replace the
+/- regs once they killed them. I went ahead and fixed them, this time with
5W zeners and 1.2W resistors. Now I'm thinking.. I need to see if there's a
hack anywhere on that board to supply single-supply 8V or 5V power to the
op-amps.. but I think they need more than 5V, don't they? (time to look at
the datasheet)