HP-67 - strange behavior - any advice appreciated
06-15-2019, 04:53 PM (This post was last modified: 06-15-2019 05:01 PM by albertofenini.)
Post: #1
 albertofenini Member Posts: 262 Joined: Mar 2014
HP-67 - strange behavior - any advice appreciated
Hello to everyone,
we have just received an HP67, we posted in the Classified section a WTB request for a logic board
but with no success, so we purchased a unit from TAS.
It was corroded, but pictured ON so we went for it.
As a matter of fact, the logic board was working, so we began our restoration procedure, both aesthetically and on the circuits.
We cleaned everything with a warm mixture of 50% water and 50% vinegar, and when we put all together it was working pretty well.
We have also rebuild the card reader but it was giving a "Error" message with any card.
However, that's the reason why we are writing it stopped to work, quite suddenly.
Or better, when we move the switch from OFF to ON, the display flash for a second a 0.00 or a string of 0.0000000000 then goes off.
You can see a short clip here

It must be also said that there is still some oxidation, very little on the board.

Also, just before to behave so we had this problem : inserting a card in the card reader
didn't turn on the motor, as opposite to when just reassembled that everything
was working correctly (gave Error message but the cards were going thru nicely)
So we checked the switches but nothing, now as a precaution the card reader has been inhibited by putting some tape
between the card reader switches and the keyboard plate, but even with the card reader pcb removed from the logic pcb nothing change

Has it ever happened to someone before ?
What could we check ?
Thanks in advance for any advice you would like to share with us

P.S. we have already check the obvious, switches, cleaned the holes in the logic PCB, battery was fully charged, and it behave equally with the card reader disconnected.

Edoardo & Alberto
06-15-2019, 11:42 PM
Post: #2
 teenix Senior Member Posts: 629 Joined: May 2016
RE: HP-67 - strange behavior - any advice appreciated
Hi Alberto,

Seeing that [0.00] was displayed proves that the CPU worked properly, albeit briefly. Does [000] appear if in W/PRGM mode?

Swapping the CPU board if you have a spare one, would verify the rest of the calculator operation.

Because it was corroded and cleaning seemed to have stopped it working, as a start I would look closely at the circuit board traces, then the battery/board wiring terminations, and the brass link at the charge socket.

All that corrosion probably came from the pcb traces and washing the board may have degraded one (or some) a little more than the circuit can handle. Maybe some "gunk" ended up (or dislodged) under a chip, or a weakened component connection has now become a dry solder joint.

Probably a rarity, but board components can be damaged by static electricity during handling.

cheers

Tony
09-16-2019, 06:53 PM (This post was last modified: 09-16-2019 06:55 PM by albertofenini.)
Post: #3
 albertofenini Member Posts: 262 Joined: Mar 2014
RE: HP-67 - strange behavior - any advice appreciated
Hi Tony
thanks for your reply, sorry it took so long to get back but we were out for vacation,
we started to work again on this unit.
We spotted two points on the pcb that were not clean and we cleaned them up,
in one case removing the circular orange capacitor and between to ICs (those with 8 pins)
What happens now is the following :

the display briefly shows a 8.00 and then it shuts off, the position of the switch W/PRGM - RUN
it does not affect the behavior (to be continued since only one video can be attached per message) ....

Attached File(s) Thumbnail(s)

Edoardo & Alberto
09-16-2019, 06:54 PM
Post: #4
 albertofenini Member Posts: 262 Joined: Mar 2014
RE: HP-67 - strange behavior - any advice appreciated
or, and this is even stranger , if we press ANY key prior to turn it on and we keep it pressed we get :

a steady 0.00 but with the key pressed the rest of the keyboard is non functional,
if we release the key than everything shuts off again.

Thanks again to everyone for helping us out !

Edoardo & Alberto
09-16-2019, 07:56 PM
Post: #5
 jonese Junior Member Posts: 20 Joined: Jun 2018
RE: HP-67 - strange behavior - any advice appreciated
Shot in the dark...

On the Woodstock's (21/25) wasn't there a ground circuit path carried over the daughter keyboard board between both connectors, such that you couldn't power it on without it fully assembled?? Is it possible that when you press a key while powering up that you somehow complete the circuit?
09-16-2019, 08:00 PM (This post was last modified: 09-16-2019 08:01 PM by albertofenini.)
Post: #6
 albertofenini Member Posts: 262 Joined: Mar 2014
RE: HP-67 - strange behavior - any advice appreciated
Hi Jonese
It’s true an HP67 belongs to the Woodstock family more than to the Classic family,
however I can’t figure out what the difference could be between powering up the unit with a key pressed or without ...
And it makes this with any key ...
We’ll check it out ground connections that’s a good idea, thx !!!

Edoardo & Alberto
09-17-2019, 03:00 PM (This post was last modified: 09-17-2019 03:01 PM by teenix.)
Post: #7
 teenix Senior Member Posts: 629 Joined: May 2016
RE: HP-67 - strange behavior - any advice appreciated
(09-16-2019 06:54 PM)albertofenini Wrote:  or, and this is even stranger , if we press ANY key prior to turn it on and we keep it pressed we get :
a steady 0.00 but with the key pressed the rest of the keyboard is non functional,
if we release the key than everything shuts off again.

Thanks again to everyone for helping us out !

The microcode executes exactly the same up until the point where the keyboard is tested, whether a key is pressed or not.

If a key is down at this point, the code will stay in a small loop until the key is released, then it will sit in the main loop waiting for a key press, card insertion, or PGM/RUN switch change.

If no key is pressed at the mentioned point, the main loop executes.

During the main loop, I tried to force the code to branch to other code sections but it did not result in a blank display.

Hard to say but sounds like a ROM issue which may cause a code failure after the mentioned point. The ROM addresses for the main loop are below 256, so the first ROM may have a failure.

cheers

Tony
09-17-2019, 03:11 PM
Post: #8
 albertofenini Member Posts: 262 Joined: Mar 2014
RE: HP-67 - strange behavior - any advice appreciated
Thanks Tony,
that's a good hint, now tell me if'm wrong:
Can I say that from an electrical standpoint the PSU works fine ?
If PSU was not to be working I shouldn't be able to get the display lit up, right ?
Since the unit was working before we cleaned it with water and vinegar, can I say that
because of that degraded around the ROM chip ?
Which chip is ROM0 and what can we measure to check what's wrong ?
Would you suggest to remove it and re-solder it ?
Thanks for help Alberto

Edoardo & Alberto
09-17-2019, 07:29 PM
Post: #9
 albertofenini Member Posts: 262 Joined: Mar 2014
RE: HP-67 - strange behavior - any advice appreciated
Tony, when you say "the first ROM" do you mean the 1818-0268 so called ROM0
or the four little chips 1818-0231, 1818-0232, 1818-0550, 1818-0551 ?
Thanks for help, Alberto

Edoardo & Alberto
09-17-2019, 09:35 PM
Post: #10
 albertofenini Member Posts: 262 Joined: Mar 2014
RE: HP-67 - strange behavior - any advice appreciated
Tony,
we re-worked all the logic board,
de-soldered all of the ICs, found little oxydation left from the previous cleaning procedure,
removed that as well, but once we have re-soldered all of the components the behaviour
remains exactly the same.
We have few working units, how can we compare a working unit with this - step by step -
to find out what's wrong with this logic board ?
Remember that when we receid the unit it was working (basic operation)
Thanks again !!!

Edoardo & Alberto
09-18-2019, 12:16 AM
Post: #11
 teenix Senior Member Posts: 629 Joined: May 2016
RE: HP-67 - strange behavior - any advice appreciated
(09-17-2019 07:29 PM)albertofenini Wrote:  Tony, when you say "the first ROM" do you mean the 1818-0268 so called ROM0
or the four little chips 1818-0231, 1818-0232, 1818-0550, 1818-0551 ?
Thanks for help, Alberto

Hi Alberto,

I'm not sure which ROM is which on the PCB, except for the larger ROM 0/display decoder.

Initially, the code executes through ROM 0, ROM 3, ROM 1 and ROM 0. Then it waits for a key press in ROM 0. This code must be functional or the [0.00] won't be displayed when you hold a key down on power on.

If you press a key, say [5], different code executes through ROM 0 then ROM 3 and back to ROM 0.

As the ROMs appear to be selecting properly, it may be bad data in a ROM address in these ROMs.

1 page in ROM has 256 words of ROM code.
ROM 0 has pages 0 - 3 ($0000 -$03FF)
ROM 1 has pages 4 - 7 ($0400 -$07FF)
ROM 2 has pages 8 - 11 ($0800 -$0BFF)
ROM 3 has pages 12 - 15 ($0C00 -$0FFF)
ROM 4 has pages 20 - 23 ($1400 -$17FF)

cheers

Tony
09-18-2019, 09:34 PM (This post was last modified: 09-18-2019 09:41 PM by albertofenini.)
Post: #12
 albertofenini Member Posts: 262 Joined: Mar 2014
RE: HP-67 - strange behavior - any advice appreciated
ciao Tony,
How can we identify the fault then ?
Would you suggest to replace all of them ?
Would an HP97 be a possible donor ?
Last, sorry for all these questions, what could we compare between a working unit and this one in
order to find out what could be possible wrongs (voltages, waves ?)
Thanks again to everyone !

Edoardo & Alberto
09-18-2019, 10:52 PM
Post: #13
 teenix Senior Member Posts: 629 Joined: May 2016
RE: HP-67 - strange behavior - any advice appreciated
(09-18-2019 09:34 PM)albertofenini Wrote:  ciao Tony,
How can we identify the fault then ?
Would you suggest to replace all of them ?
Would an HP97 be a possible donor ?
Last, sorry for all these questions, what could we compare between a working unit and this one in
order to find out what could be possible wrongs (voltages, waves ?)
Thanks again to everyone !

It could be a difficult one, I wish I could pull a rabbit out of the hat :-)

If it is a ROM problem it would try replacing ROM 0 or ROM 3. My first try would be ROM 3 (smaller - easier to replace).

However, it may not be a ROM issue as such. It could be a problem with a component (cap, resistor, diode etc) that "changed" after being cleaned and has upset the works.

Firstly, I would go back to things you have tried in the past and run your CRO over the board and see that clock signals appear to work ok, and power supply voltages are good. It might be something simple like a poorly operating capacitor.

The HP-97 ROMs are a different kettle of fish. They have the printer code embedded as well as other changes, different keyboard etc.

cheers

Tony
09-18-2019, 10:57 PM
Post: #14
 albertofenini Member Posts: 262 Joined: Mar 2014
RE: HP-67 - strange behavior - any advice appreciated
Thank you very much!
What’s a CRO ?
Do you know where we can find some good schematics about the 67 ?
Looks like there is a loro less documentation compared to what’s available for the HP65
We’ll keep posting our results !

Edoardo & Alberto
09-18-2019, 11:39 PM (This post was last modified: 09-18-2019 11:41 PM by teenix.)
Post: #15
 teenix Senior Member Posts: 629 Joined: May 2016
RE: HP-67 - strange behavior - any advice appreciated
(09-18-2019 10:57 PM)albertofenini Wrote:  Thank you very much!
What’s a CRO ?
Do you know where we can find some good schematics about the 67 ?
Looks like there is a loro less documentation compared to what’s available for the HP65
We’ll keep posting our results !

Sorry, a habit - Cathode Ray Oscilloscope - or these days just an oscilloscope. I thought you had a pico-scope which will help. Some of the wave forms shown in the 97 service document may serve as a guide.

My web site has some schematics that I put together in pdf format...

http://www.teenix.org/classSchm.zip

or try...

http://www.hpcc.org/cdroms/schematics5.0/index.html

cheers

Tony
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