HP27 Restoration (Lots of Corrosion)

[calc-calcs]

(04-08-2022 10:38 PM)teenix Wrote:  It might be worth replacing the tantalum capacitors in the power supply. Also, there is a 2u2 power up reset capacitor hanging off pin 18 of the ACT chip. If this is faulty, it may not reset properly.

cheers

Tony

I compared the ripple readings of the power supply rails to the ones in the HP21 and they seem very similar, so I would expect that what I saw on the HP27 power rails is normal and the caps should be fine. Besides Tantalum being so stable rarely fails due to ESR or degradation, they usually just like to pop or burn up if voltage stressed or with reverse polarity.

But since the last repair session the HP27 seems to have changed its mind and now upon power on only rarely wants to show any activity on the display at all, perhaps just one digit to the right now and then, but mostly just a blank display, I decided to follow your suggestion about taking a closer look at that reset cap.

The cap reads 2.19uF and 1.7 ohms ESR. I compared that to a new 2.2/16V Tant and the readings where almost identical, 2.12uF and 3.1 ohm ESR, so actually a bit worse than the original. Even so just for the heck of it substituted the old cap with the new one but no change in the calculator, other than I managed to break one of the very fine legs of the original cap but managed to reattach it, green beads and all, so the little guy is back in place.

That not withstanding, the voltage on that 2.2uF reset cap (and pin 18 of U3) is strange for a reset circuit. On power up and with the DMM connected it starts at zero and the voltage very slowly then starts to raise until it reaches about 0.6 volts at which point it only seems to keep climbing very little, like the DMM is actually placing a load (10M) on the charge current. All in all it takes the cap around 45-50 seconds to get to to around 0.6V. If I then disconnect the DMM and wait a couple of seconds and reconnect it the voltage will have risen to around 0.7 - 0.8V and after a couple more measurements will then appear to settle at about 1.2-1.3V. Looks like the cap is very slowly being charged by a minuscule current. I even checked at the actual pin of the IC in case there might be a bad contact with at socket on pin 18, but the voltage was exactly the same ruling that out.

Can someone please confirm the expected behavior of the Reset pin 18 of the HP27 ACT?

Also given the change in behavior of the calculator from one session to the next checked again the system waveforms, but they are still present and look ok, except for the SYNC line (ACT pin 20) which looks odd, but since I had never tested it before no previous reference so not sure if that might be related to the change in behavior. Also checked for SYNC at the other expected nodes on U4 RAM/ROM pin 13, and U2 Anode Driver pin 18 and get the same waveform, so no open traces here.

Will post some updated scope shots including the SYNC line on the next post.