HP-67 Reads cards but does not write

[Trevor Gale]

Right! I do have both a digital storage 'scope and an analog storage 'scope (the analog one is a Tektronix!) - but before I coupled that up I thought that I would further inspect the interface board for any reason behing the differing bias levels for the two amplifier channels - after all, to change one of these levels only required a very high resistance... I wanted to double-check things like leakage resistance of the new capacitors and such.

I had replaced these tantalum electrolytics on the board of course, but here's the rub - as I normally do with new work these days, I use lead-free rosin-cored solder (in various guages, in this case very fine). Any rosin that is left over after the cleaning e.g. with isopropyl alcohol can present a high resistance - and with one of the capacitor connections being close to the CRC chip, this turned out to be the case.

This chip is soldered into the board with very little space between the bottom of its package and the board itself - so my short-brushing to clean away the rosin did not reach under the chip. I cut a small strip of paper card, soaked it in isopropyl alcohol, and managed to slip this in and out underneath the chip, rubbing away the left-over rosin. I could see that this was working by looking at the discolouration of the card when I extracted it from underneath the chip. I performed this with a second piece of card, and a third, at which point I detected no further discolouration on the card.

I re-situated the board in position, re-adjusted the eccentric spindle to what I thought was the last good working position (I had made a small mark on the side of the screw head), and placed a battery set to hold in position and turned on the calculator. That worked of course (I would be surprised if it had not);

Keeping the battery in as before, I measured the bias voltages - these had changed to 1.42V on BOTH channels, therefore I was perhaps on the right trail!

To check that it was still at least writing correctly I then wrote in a very short program and saved it to an empty card. This read perfectly well on my HP-97.

I then wrote a slightly different program with a different start label ("D" instead of "A") on the HP-97 and wrote that to a new card. I placed that card into my HP-67 and to my amazement (and glee!) it read in perfectly.

With this having been achieved, I took some of my earlier-written and more complex, longer programming already saved to cards and read many of these into the HP-67 with no problems - I checked the operation of the programs just to be sure - and I also re-wrote new empty cards with some of these programs (many using both sides of a card) in both directions between the HP-97 and this HP-67 - with again no problems with either function in either direction.

I am overjoyed to have found a solution to this problem, but at the same time I thought I would examing the possible results of using this lead-free solder in other high-impedance situations and compare it so some of the older Pb-Sn leaded solder I still have. I found that the residue which can be left by the rosin-cored lead-free solder can be an order of magnitude less in ohms than the flux residue left by lead-tin solder.

In my other occasional hobby interest, restoring vintage radios using valves (tubes in U.S. language) I also use lead-tin solder when making joints or replacing components, since it 'wets' the component leads and solder tags far easier and cleaner than does any lead-free silver-containing solder.

I suggest that as and when we have to repair boards or replace capacitors in our HP calculators then we do so using lead-based solder - if we can get some since it is banned from sale in some countries. The solder we would use and the calculators we have are not toys and any miniature amount of lead we will use is not going to be chewed at by young children.

In any case I would like to take the opportunity to thank everyone who made suggestions and/or ideas to assist me, especially Tony who clearly has a good in-depth knowledge of our classic and other HP calculator models. I wish you all a fine weekend.