Almost Dead HP-81

[fxsuarezc]

Hello!

(03-10-2016 01:13 AM)Dave Frederickson Wrote:  Hi Francisco,

The Repair Manual is on CD24, just to make sure. The clock chip is mentioned on Eric's site, https://www.brouhaha.com/~eric/hpcalc/chips/. You might be able to find a replacement part by salvaging an HP-35, 46, or 80. Also, I've purchased spare parts from Sphere, but they don't have this one.

The HP-80 in the above link is being sold by a Forum Member, so you can try asking for a MoHPC discount.

Good Luck,
Dave

Ok, the calculator was missing an entire clock signal. These older processors need bipolar-bi-phase clocks (similar to Gottlieb System-1 pinball machines). The clock is generated
in the Anode Driver in the father/display board, but in 0-5V level. The clock driver
translates to -12-6V signals.
I ended replacing the clock driver chip with an LM293 dual comparator and some
resistors in a piggyback board.

Those chips are HEAVY load, i ended with 820Ohm pull up resistor in the LM 293.

The result? Well.. it's working almost perfect now!

The printer works perfect, and the calculator appears to be functioning OK in the printer.
BUT the cathode driver in the display board does not work
All the numbers are overlapping. Both control signals in the chip are OK. (Well, pulsing with
some logic).
When i first turn on the calculator, a very bright "0" was shown on the screen, maybe the
chip burn out with no control signals.

Some photos attached.

Thanks a lot for your info!