HP-27 Repair/ACT/Panamatik LP?

[Maximilian Hohmann]

Hello!

I could have asked this directly to Panamatik, but maybe it is of interest to other members as well?

Finally I was able to find an HP-27 in excellent cosmetic condition, one of the last remaining gaps in my collection of LED calculators from HP. Presently it is not working unfortunately, so I wonder what to do. Below are some cloeseup pictures inside and out. There is a stain of solvent (or a burn mark?) on the display cover, but I guess I can polish that out. The PCB traces are in excellent state apart from a couple under the keyboard.

Since this is the first 27 I ever saw two things caught my attention compared to other woodstocks:
There is a piece of black wire soldered onto the board in the top left corner. Is this original from HP or the result of some later repair? (see picture below)
A piece of the ACT chip housing is broken off (what can have caused that apart from brute force - overheat maybe?) exposing an interrupted golden trace underneath. May this be the cause of the malfuncion?

What can I do to determine wherher an ACT chip from Panamatik will restore the calculator or if another LP board will be required?

Regards
Max

[teenix]

The keypad pcb could be tested with an ohms meter to test the continuity of the discoloured tracks. but could be repaired easy if open circuit. Probably caused by battery gunk.

The under side of the main pcb looks ok, although on the top right corner there looks like a defect between pins 1 and 2 of the 5061 0430 chip

The bottom side looks like corrosion around the 5061 0459 chip so might have more under it.

Can't help with the wire.

It looks like someone has gripped the chip ends with needle nose pliers and gouged the ceramic. For what reason, escapes me. The solder connections for these chips look normal under the board so it doesn't look like they were de-soldered first.

I'm not sure if the gold connections have been damaged and would be hard to know without knowledge of the chip.

Maybe one a Bernhards upgrade kits would save the day.

cheers

Tony

[Maximilian Hohmann]

Thanks Tony

for your analysis! I couldn't finish this straight away because the company called me for a flight but upon returning this morning I was able to revive the calculator :-)

(10-07-2018 04:07 AM)teenix Wrote:  Probably caused by battery gunk.

I guess this was the problem: Battery gunk creating connections where none should have been. All the PCB traces tested OK, even the suspicious ones. After removing the gunk around the ACT chip with a needle and some vinegar and rinsing and drying the board it now seems to work. The piece of ceramic broken off the ACT does not seem to matter. The only problem is the the "1" key which does not register but I hope that this can be remedied with some contact cleaner applied through the holes under the keys. I am very reluctant to take the keyboard apart - better a working HP-27 with no "1" key (one can work around that with some arithmetic...) then one with a totally broken keyboard!

Regards
Max

[teenix]

(10-09-2018 06:55 PM)Maximilian Hohmann Wrote:  The only problem is the the "1" key which does not register but I hope that this can be remedied with some contact cleaner applied through the holes under the keys.

It might be possible that the through hole via on the keyboard pcb is open circuit due to corrosion. See attached image.

This looks to be a direct keyboard matrix connection to the "1" button.

If so, it might be difficult to repair without a keyboard dissection.

cheers

Tony

[teenix]

I was just thinking, if I was going to test the via for a problem without keyboard disassembly, I would get some small gauge wire and poke it though the via just enough so it doesn't quite touch the "1" button contact when not pressed. Then temporarily solder the other end to a clean part of the gold circuit trace that connects to the via.

If you press the "1" key you should see the wire move up and down.

Reassemble the calculator and try the "1" key. If it works then that was the problem and you have to figure out a proper fix.

cheers

Tony

[Zaphod]

The 'chip' on the 466 chip looks to me deliberately done , at manufacture time.... and not a 'chip', but filed (by a round needle file - by the looks of it) to electrically disconnect the lid/cover from the lower level connection to the inner substrate.

The wire also looks like a post-production addition/mod.

If a via is broken it could be fixed by inserting a very short wire strand (tinned) and soldered so the solder draws in through the via and reaches both sides. One would have to be careful to ensure the length of wire.

[Maximilian Hohmann]

Hello!

(10-09-2018 08:54 PM)teenix Wrote:  ... I would get some small gauge wire and poke it though the via ...

I followed your advice till here and used a resistor wire bent into a small hook to poke around the hole behind the "1" key. Together with a drop of contact cleaner and followed by some compressed air spray this was enough to bring the key back to life without further disassembly. It is not perfect (right now it misses about one keystroke in five) but sufficient for me. I have a working HP-27 :-) Took me 20 years to find one that fits my budget...

Thanks again for the advice
Max