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

drlegendre . drlegendre at
Sat Nov 15 22:18:10 CST 2014

Quick update - I made an error.

Pin 3 (both i/o headers) isn't tied to the 1458 Vcc+ - they're tied to the
1458 Vcc-. So, they have a 1K pull-down to -12V rather than a pull-up to

So why is there a pin (3) on each header that's essentially a -12V @ 12mA
current source?

On Sat, Nov 15, 2014 at 8:42 PM, drlegendre . <drlegendre at> wrote:

> Howdy,
> On Sat, Nov 15, 2014 at 5:37 PM, Tothwolf <tothwolf at> 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)

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