Hello and asking for help with HP 67 power unit

[AndiGer]

I'm no expert in electronics. As Jacques Laporte explains Vgg is derived thru C1/CR6 and C2/CR7 interaction. C1 and C2 are 2.2µF tantalums. I think those are the small black ones in the picture shown at http://home.citycable.ch/pierrefleur/Jac...ceDeck.htm
(also seen as black ones in pictures from Alberto).
Correct me if I'm wrong. Colour code is not always on the parts I found out.

[Tapti]

Yes, I you're right. On my good board they are also black as on the photo, on the damaged board, they are red ...
But ! You wan't believe it, my board seems to be self repairing ! Since this afternoon the maschine is running fine, although Vgg is still only -10.1 V. Only thing I did was removing it once again from its place inside the calculator and doing some measurements with a DVM, nothing else. I really hate these situations, because it's now even harder to find the fault, which must be somewhere and which certainly will come up sometimes later on. I'm still concerned about the low Vgg. Perhaps I will exchange those two tantalums anyway. But in the meanwhile I think the problem is somewhere else on the board since the calculator seems to work well with a voltage that low. Perhaps it was a contact problem (some oxydization) somewhere at the long chain of contacts along the lower side oft the board. Due to the frequent removement and reinstallation perhaps the contact has cleaned up ?
Keep you informed when I have news
Stefan

[AndiGer]

Stefan, that's good news . Yes, maybe there is some dirty connection in the holes to or at the golden forks. Maybe you clean them thoroughly with IPA and the problem is gone permanently.
Wish you success

[Tapti]

Andi,
thank you for your kind words. The problem is, I'm a physicist by profession and these physicists think, they have to understand everything. That's their profession and their problem ...
Let's see what I can still get out of this antique piece of electronics. Perhaps I'll give it to the electronics workshop in our institute, if nothing else helps. Nevertheless I'm really thinking about purchasing one of these teenix boards. They have obviously a very good reputation and it will certainly be nice to play around with.
Greetings
Stefan

[Tapti]

Dear all,
there are good news and I would like to share them with you just by interest. I canged three of the five capacitors of the logic board, being the two responsible for producing Vgg, and the main capacitor in the output of Vss. Now I have -12,18V on Vgg ! At least one of the two 2.2uF tantalums has had a problem, most probably its ESR value. I measured the caps for capacity and internal short and both were ok.
Just a question belonging to the tanatlum caps on the logic board: There is one cap, 47uF, running from terminal 2 of the inductance to ground. On the board it looks like a tantalum just as the others. But I think at this point a bipolar cap is needed as there is AC coming from the coil. Are there bipolar tantalum caps at all ? I bought a unipolar replacement for this, but didn't touch it in the end. I'm just not sure, whether and how to use an unipolar tantalum at this point.

[teenix]

Just a normal tantalum will do, the connection is DC 3.75V.

Pin 2 of the coil and +ve of the cap connects directly to the battery +ve terminal when switched on.

cheers

Tony

[Tapti]

Tony, ok I overlooked the positiv voltage from the battery. Than it's clear how the polarity has to be.
Many thanks,
Stefan

[Dreato]

That frozen display sounds like a bad ACT chip. Bernhard at Panamatik may be able to help. I *THINK* the HP67 uses the same ACT as the Woodstock series.

Regards,
Dave

[GreyUser]

I have seen many bad black resin tantalums in 67’s and Topcats. Most were thermally sensitive, that is working fine until the unit warms up, some needed 30 minutes or more to fail. Never bothered to determine what was changing, most likely ESR but going open or up value is always possible.

The black ones are one of the few parts I replace without question as they have cost me too much time and aggravation for a few dollars worth of parts.

[Tapti]

Hi GreyUser, hi Dave, thanks for your help.
@Dreato: I got the unit running again by exchanging two specific tantalums.
@GreyUser: I'm just thinking whether it makes sense to exchange all of the tantalums on the board. I think, they are four. One cap, the one in the psu oscillator, seems to be a ceramic one. And since the psu is working fine, I tend to leave this cap alone. the others seems to be tantalums, but not black. They wear some strange color coding, with I can't translate so far. I tried two different color schemes, but couldn't reproduce the capacity values from the widely accepted psu schematic made by Laporte.
Greetings
Stefan

[Tapti]

Dear all,
there is some continuation in the story so far. Sure enough, the spontaneous healing of which I reported last time was not for long. After some days of working the calculator once again showed the symptoms described. And this even more seriously, there is now no time period of operation after switching on as it was in the beginning of my work on the unit. Directly from the start it showes just some nonsense / flickering / fading seen from the display. So I decided to buy a new mainboard from Tonys workshop. After about two month it finally made its way from Australia to Germany. I just plugged it in before some minutes and what should I tell you ? It's working absolutely flawless from what a can say so far. Many many thanks to you, Tony, it' really a great work !
As soon as I get the taxes back from 2020, I will order another unit just as a spare for the original board, which is still working in my other 67.
Now I will lean towards the magnetic card reader. The gummi wheel
have been changed by somebody in advance. Nevertheless there is some work left: The card reader motor is running continuously and do not stop unless the calculator is switched off ! Perhaps some dirty micro switches ? Or can it be an electronic problem situated on the long narrow board connecting the main board to the card reader unit ? Should I exchange the rest of the tantalums ? Did somebody of you observed this behaviour, too ?
Anyway I'm just very happy to see my 67 operating my 'Hello World' program beeing counting up to infinity or better until the batteries will be empty ;-)
Best regards,
Stefan