• jim
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Topic: Resurrecting my HP-29C

Hello Jacques:
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Do you know what manufacturer originally made these 16-pin, side brazed ceramic IC's?  I'm assuming that these RAM IC's eventually moved into some sort of 16-pin plastic DIP package since ceramic is very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.

Thanks!
Jim

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.

Re: Resurrecting my HP-29C

Hi Jim,

> I hope you forgive me for emailing you directly.

You are very welcome and I'd be glad to help you.

> 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.

The voltages are ok.
For the current, it seems too high but I have no 29c to compare. Remember though the RAM is powered continously...

> does troubleshooting the 29C behave the same as the 25?

Almost exactly.
Due to 'continuous memory' the dc-dc converter is slightly different. But if you feed the calculator with a regulated 2.5 DC power unit. There is no risk.

Check the ACT first.
pin 13 & 14 you'll find a LC cell (333pf, 150uH).
see : http://www.jacques-laporte.org/Woodstock/ws_HW_main.htm

You'll find a 800 khz signal there (almost sine).
scope photo here : http://www.jacques-laporte.org/Woodstock/ws_HW_main.htm

If ok, check after that the 2 phase clock phi_1 and phi_2 : 180 khz.
its a MOS clock with a big swing +6 -12V
photos here
https://plus.google.com/u/0/photos/1115 … 5588046897

Check the ISA bus and the SYNC signal (pin 10 & 20).
If all is ok, it signs a probably good ACT.

A steady '0000...00' display is not a bad sign.
Maybe could confirm first the ACT state (clock + SYNC)?

> 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.

I suppose that means the machine is using only the anode diver ROM.
It could be a good approach, if you have the patience and a good tool to desolder the chips.
But confirm first the ACT and next remove the RAM (which are very fragile).

> I would imagine that I'd have to find another HP 29C to cannibalize the CMOS RAM from it

Yes, that's the only way. I repared one 25C with 2 units bought on ebay.

> 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'd be glad to share information about the 29C circuit. I have drawn the dc-dc circuit from dead units on a 25C and 29C.
The big difference is in the continuous RAM powering : Vss or if  OFF, directly Vbatt (B1).

The RAMs are 'static'  because they must keep the data even without the clock  or (at least on the 25C that I have tested) the ACT is not powered coutinously.
That explains (IMO) the special PWO circuit : RAM PWO only if Vbat A2 AND B1 are missing.
What makes the 25C and 29C so fragile is the flaw in the dc-dc conception :  if the 2 nicads are not effective (clamping effect missing) then Vbat is rising > 5V.

The 'OFF' slide position is no use in the HP29C.

To know what has been burnt, you'd have to check the ACT and RAM.
ACT first, next remove one RAMs and check etc ...
ROMs are rarely the culprits ....

Give good me news.
I'd be glad if you could resurrect that one.

All the best,
Jacques.

photos/HP25C.jpg

  • jim
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Re: Resurrecting my HP-29C

Hello Jacques:
Thank you very much for your response.  I’m traveling the next few days on business, but when I have a moment I’ll check for the ISA, phi_1, phi_2 and SYNC signals on the ACT.

I agree that the RAM could be the issue.  I’m relatively good at de-soldering so that will likely be the next step is to remove one of the 5061-0469 RAM chips.   If removal of that RAM device doesn’t fix the issue, then I’ll remove the second one.

I noticed that the RAM is powered 100% of the time from the DC-DC regulator.  Its’ the only way this calculator could have continuous memory.

It took me a long time to find a good HP 29C  buy on eBay.  I’ll keep looking and perhaps I’ll find a “donor” HP 29C in the near future.

I’ll let you know how things are proceeding and what I find as I step through the process of troubleshooting this fine piece of HP history.

Also, I’ll try to sketch out the DC-DC circuit on the HP 29C and provide the values of components to you.  In looking at your papers on the HP 25C, it appears to be nearly the same, however I’ve noticed differences in the resistor values and the tantalum caps have different values as well.  I’m guessing that the transformer is identical since I see the red, green and copper windings.

Cheers!
Jim J.

Hi Jacques:
Today, on my HP 29C, I checked the Isa, sync, phi_1, phi_2 and data lines.  They are all clocking and the phi_1 and phi_2 lines appear to be functioning at the expected 180kHz.  Next will be the daunting task of removing one of the HP RAM chips, then checking for operation.  If that doesn't fix the problem, then I'll unsolder the 2nd HP 5061-0469 RAM IC.

Hopefully I can do this tonight since I'm traveling on holiday for the next five days.  I'll let you know what I find.

Cheers,
Jim J.

Hi Jacques:
Just a follow up to your reply, and by-the-way, thank you for your advice.

I carefully re-measured the waveforms of the 1st HP29C versus the 2nd HP29C that I was able to acquire on eBay. I compared the waveforms to a fully functional HP25 ACT IC.  The 2nd HP29C appeared to have a functioning ACT chip, but the display didn’t function.  I swapped out the ACT from the 2nd HP29C into the 1st HP29C, and voila!, I now have a fully functional HP29C!  I was very excited!!!   I had socketed the IC’s in the 1st HP29C so I have a test bed for testing future ACT chips.

I still need to diagnose why the 2nd HP29C wouldn’t function (no display) even though the ACT chip was fully functional.  I swapped out the display IC’s (cathode and anode drivers) between HP29C calculators but it didn’t make any difference in the display functionality.  The power supply was fine in the 2nd HP29C.  So now I’m down to trying to diagnose the other IC’s or the electrolytic caps, or perhaps a bad diode, however I need another functioning ACT IC.  I’m not certain if I can swap out one of my HP25 ACT IC’s into the HP29C.  I’ve read posts where others have said they will work even if the part number is different.

I noticed that the HHC 2012 conference location has been announced for Nashville, TN USA.  That’s a good day’s drive for me, but one I may carefully consider so I can attend HHC 2012 this year.  I’ve enjoyed following the discussions in the MoHPC forums.  Since I’ve become an HP calculator collector/enthusiast, it would be a great activity to actually meet some of the people that I’ve corresponded with.   I’m finishing up a diagram of the HP29C.   Hopefully I will have that completed by the HHC 2012 conference.

Thanks again for your correspondence.

Cheers!
Jim J.

Re: Resurrecting my HP-29C

Hi Jim,

I'm very happy you could resurrect your HP-29C.

I'll read the follow up carefully.

The thread is very interesting (for me and others).
Would you care if I publish it on my forum?

Another thing, when you're done with your diagram drawing, I'd be glad to publish it on the main part of my site under your name.


> I’m not certain if I can swap out one of my HP25 ACT IC’s into the HP29C.  I’ve read posts where others have said they will work even if the part number is different.

As long as I know, they came at least in 2 kinds :

1820-1596 for 21, 25 29C 67 and 97 Mostek MK6216
1820- 1741 for 21, 22, 29C, Mostek MK6217N

but I don't know the differences and regarding swaping them out the only way to know is try.

Expecting to read from you,
All the best,
Jacques.