Almost Dead HP-81

[fxsuarezc]

Hello!

Some time ago, i bought (locally) an HP-81 with the portable case, manuals, etc.
The Calculator is "almost" dead.
When turned on, the drum printer rotates and
a very bright "0" appears on the screen, the keyboard is inoperative.

I bought the "manual" (actually, it's the printer's manual)
and check +15V, -12 and +6V are OK. No noise on the voltages.
(I have a 10Mhz scope to check)

The Clock signal is about 800Khz (almost 1Mhz but it's not xtal so i think it's OK).
Some pins in the father/display board are pulsing.
I was able to find the sync pulses from the printer are OK.
The clock pulses from the father board are OK.

Some "clock" signal from 8 pin chip labeled "1127-3910-7" (pin 8)
appears funny, doesn't like me
(appears to be a clock signal but the pulses doesn't start fr om 0 to 6V, are from 5 to 6V)
and this signal repeats in many chips in the motherboard.

The "SYNC" test point in the board it's always high.
The "ENABLE" test point is always high (no pulses).

What is this 8 pin chip? (clock driver?)

There is any chance to fix this beautiful machine?

Thanks a lot!

[Dave Frederickson]

Correct. The "1127" chip is an 1820-1127 clock driver. At least that's what it looks like from the schematic. The HP-46 and 81 Service and Repair manuals are on the Museum flash drive.

[wojtek]

You have not written if the calculator did work in the past or not.
Anyway I suggest start off with:
- take off and put in all the parts sitting in any kind of connectors
-if this not help look carefully at all soldering points
-if this not help try to find a service manual for this machine

[fxsuarezc]

(03-10-2016 12:30 AM)Dave Frederickson Wrote:  Correct. The "1127" chip is an 1820-1127 clock driver. At least that's what it looks like from the schematic. The HP-46 and 81 Service Manual is on the Museum flash drive.

Thanks!!
I bought the CD11 and CD23 and this schematic was not there.
I will check the signals again.
If this chip IS bad, where can i get it?

Thanks again!

Francisco Suárez.

[fxsuarezc]

(03-10-2016 12:31 AM)wojtek Wrote:  You have not written if the calculator did work in the past or not.
Anyway I suggest start off with:
- take off and put in all the parts sitting in any kind of connectors
-if this not help look carefully at all soldering points
-if this not help try to find a service manual for this machine

0) I don't know, i bought the calc with the case in a flea market for the equivalent
of U$20, and that is how it came.
1) Done.
2) Done.
3) Got CD11 and CD23, not very useful. Now i have the schematic, i will check
again.

Thanks!

[Dave Frederickson]

(03-10-2016 01:03 AM)fxsuarezc Wrote:  

(03-10-2016 12:30 AM)Dave Frederickson Wrote:  Correct. The "1127" chip is an 1820-1127 clock driver. At least that's what it looks like from the schematic. The HP-46 and 81 Service Manual is on the Museum flash drive.

Thanks!!
I bought the CD11 and CD23 and this schematic was not there.
I will check the signals again.
If this chip IS bad, where can i get it?

Thanks again!

Francisco Suárez.

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

[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!

[fxsuarezc]

Hello!

I wish to thank you for all the data.

Finally i replace the 1061 cathode and the clock driver chips, pulled from a
badly rotted HP45, and the calculator works perfect.

It's really fun to watch (and listen) the printer give the results when TVM functions
are used.

Thanks again!

[PANAMATIK]

My deep respect and congratulations for having accomplished this repair.

Bernhard

[bshoring]

Congratulations ! Your hard work paid off and you have a machine many of us would pay dearly to have.