Topic: Resurrecting my HP-29C
I hope you forgive me for emailing you directly. I can post to your HP calculator forum if you prefer.
I've been reading your posts on your Jacques Laporte Website. I'm an HP calculator collector and electrical engineer. I've finally acquired an HP 29C with the dreaded "all zeros" display and no key response.
In general, the calculator case was very clean. After opening it up I noticed some minor areas of green oxide on the pcb. I then conducted a pcb cleaning using mild vinegar and warm water on the lower half of the pcb (around the ACT and RAM ICs).
After blowing off the water with compressed air and drying the pcb, I reassembled the calculator to find that I now have NO display whatsoever and no response from the keypad. I've hooked it up to a bench power supply at 2.5vdc on the battery pins and I can see that the DC-DC power supply is generating +4.0 vdc, -11.6 vdc and +6.0 vdc, so the power supply seems to be working. I looked at the input current on the +2.5 vdc and it's measuring 180mA with no LED's on. From what I've read, this is too high. So it appears that one (or more) of the IC's is consuming too much power.
Some suggestions I've seen for next steps for troubleshooting an HP 25 (does troubleshooting the 29C behave the same as the 25?):
1. Check the tank circuit on pins 13 & 14 for the oscillator for the ACT chip.
(I checked the LC oscillator today and found that it was oscillating at ~ 790kHz and was a sine wave.)
2. Unsolder the RAM and ROM chips (5 IC's!!!) to find out if the ACT chip will drive the display and do basic math.
(Will this approach work on the HP 29C?)
3. If the ACT chip is alive and driving the cathode driver and anode driver chips, resolder the ROM chips and check the transcendental functions.
Since all the zeros lit up on the LED display when I first powered up the HP 29C with a charged NiCad battery pack, I'm assuming that the cathode driver and anode driver/ROM IC's are ok. Or is that a wrong assumption?
I'm wondering if I have blown CMOS RAM IC's, how hard is it to find replacements for these? I would imagine that I'd have to find another HP 29C to cannibalize the CMOS RAM from it.
Do you know what manufacturer originally made these 16-pin, side brazed ceramic IC's? I'm assuming that these RAM IC's eventually moved into some sort of 16-pin plastic DIP package since ceramic is very expensive.
I'm drawing a schematic of the HP 29C since I haven't found one any on any website. When I'm done, I'd be happy to share it with you unless you already have it.
If possible, I'd like to NOT unsolder 5 IC's just to test out the ACT chip and to try and isolate the problem to either the RAM or the ROM chips. I guess I could start with just unsoldering the RAM chips OR the ROM chips and find out if it works at that point.
Any thoughts or ideas on troubleshooting would be helpful.
P.S. Your discussion on the HP 25, 35, 67 & 97 power supply is very interesting. I'm trying to draw the schematic for the HP 29C power supply, but I don't want to remove the daughter board from my HP 29C. Much of it appears to be the same as your HP 25 power supply with the exception of some resistors and capacitor values that were changed by HP.
I have repaired two HP-97 desktop calculators and have done the "gummy wheel" repair of the magnetic card readers. I've also repaired an HP 67's gummy wheel and they all read and write to magnetic cards.