HP27 Restoration (Lots of Corrosion)

[calc-calcs]

Some progress made. This evening I set out to finish cleaning the board and test the power supply by itself. That shouldn't be too hard.

So pulled out both transistors of the power supply and pulled one leg out of the first string of adjacent components, the zener, the 680R resistor and 3x 1N914 to properly clean under them. Also the yellow 47uF cap and the small 2.2uF with the green beads on its legs, both located to the left of the ACT.

Tested those components and after cleaning as best I could resoldered them. Without the ACT and ROM chip, or display or keyboard attached powered it up and power supply seems to be working fine. These are the voltages (Spec value from schematic) and actual measured values:

Buss Spec / Measured
-------------------------------
Vcc 2.5 / 2.53
Vdd 6.4 / 6.44
Vgg 11.8 / 12.65
-V -2.8 / -3.06
Vdisp 4.9 / 4.91

Power consumption was about 40mA.

Then of course wanted more. I also did a bit of reality check and decided I am not going to go overboard with the cleaning, at least not yet, as it will be way too much work without even knowing if there is even any chance for the calculator to work. So just gave the traces a couple of additional rubdowns with a Q-tip and alcohol until there was no longer any blue residue left on the Q-tip, and also removed any obvious visible residues.

Then made a hand drawing of all the component side traces and started checking them for continuity and marking them down according to the test results. After that repaired all the broken traces with #30 Kynar wire which I had ordered a roll some weeks ago "just in case" so it came in very handy. The trace repairs also included one trace that takes ground up on the right side of the board (along the charge resistor and diode) and reaches all the way into the display drivers. Also had to repair two other traces that go up on the left side of the board and snake through the power supply and also reach all the way up into the display driver chips. These last ones where ISA and "01". Once this was done checked continuity of the repaired traces and all looked good.

Next installed low profile socket strips for both the ACT and the RAM/ROM chip. All good so far, but first the chips needed a bit of further cleaning off flux on the underside and straitening of the pins so that took another while. The ROM chip still even had a bit of crusty blue stuff on the inside back side of one pin. So in they went into their sockets. Then attached the display and keyboard. Time to power this up up and see what happens!

When I first turned it on nothing was shown on the display, current was about 45mA. But after a few seconds suddenly a combination of sometimes the right most digit flashing, sometimes the left side one. Sometimes all zeros would show. All this without touching anything yet. Then I tried the keyboard but no response to it. The flashing digits continued. Hmm looks like Panamatik is going to be selling me another ACT.

At this point decided to pull out my test HP21 and put the HP27 keyboard on it to make sure it was even working. The keyboard turned out to be just fine, except for the first 3 right side keys from the top down which did not work ([g], y^x, and CLX). All other keys worked just fine, no bounce or any other problems.

Next I put the HP27 keyboard back on its own board, and got a few flashes as before, but after about 10 seconds of leaving it alone, lo and behold suddenly "0.00" came up on the display!! And all keys started working as well, that is except again for the same 3 that did not work before while on the HP21 board. I looked at the schematic and actually those 3 keys are exclusive to one row, so since the keyboard had the exact same problem on both calculator boards it has to be a problem in the keyboard so will need to check those mating connectors as they had some blue stuff inside in a few of the gold plated holes although I managed to clean them ok or so I thought. Or perhaps is just the keys that need a bit of Deoxit, but it would be almost too great a coincidence that only these 3 keys of a specific row would have contact problems. Could be, but I think its rather related to one of the keyboard push in mating jacks. Will look into that tomorrow.

There must also be still something loose somewhere, I can sometimes make the display go haywire by moving or pushing the main board around a bit. Sometimes the display will momentarily go blank and a second later will at times come back on as "0.000" as if I had done a [f] FIX 3, and everything still worked. Another test I remembered doing when working on the HP25 is to hit the E+ key and display should advance by one each time. If not the RAM chip might be bad. Unless there is something else going on, this unfortunately also seems to be the case here. Not sure yet, will definitively have take a closer look at this.

But in any event I think I made a bit of progress today. OK its time to hit the sack. And of course added some pictures. Posts are not as much fun without them. But since there is a 5 picture limit per post, might upload a few more tomorrow on a follow up. BTW let me know if you want to see anything specific up close, etc.

Good night, or good morning....