HP-25 - what could be wrong, simple question

[albertofenini]

Hi everyone,
we posted few days ago this Restoring HP25 S/N 1702S13138 - Very strange take a look
The aim was however to restore another HP25 RE: HP25 repair - need advice (TESTING POINTS PICTURED)
The symptom was that the unit was working, performing simple operation but we were not able to store data in the registers or store a program.
We thought that was a bad RAM chip 1820-1564
So we recovered this chip from this last unit, note we don't know if it was working or not, but once transplanted it didn't solve the problem.
Are we just out of luck and also this RAM chip was not working ?
The question here is, just to be sure what should be looking for, can we assume that a working unit
where the only not working part are the registers have a defective RAM or it could be a different cause and we should be looking also for something else ?
Thanks anyone for help

[PANAMATIK]

The HP-25 stores its 8 registers, 49 program steps and the LastX register in external RAM 1820-1564
If the registers always show 0.00 values and program steps show 13 00, and the RAM chip is already replaced, there could be a disconnected DATA line pin 11 of the ACT. This serial data line reads and writes from the RAM chip. if there is no interruption, it must be assumed, that the ACT has a defective DATA pin.

Bernhard

[albertofenini]

HI Bernard
First of all thanks for your reply
So this behavior doesn’t necessarily mean that the Ram is defective ?
It could also be the ACT chip ?
Since we were able to do math operations we assumed that the ACT was working
How can we check if the pin 11 is connected with the RAM ?
Could it be a bad trace or you mean is disconnected inside the ACT CHIP ?
This kind of unit could it be restored with a lot of yours ?
Thanks again!!

[PANAMATIK]

Hi Alberto

The HP-25 DATA line ACT pin 11 is connected to RAM pin 4.

I encountered partially defective ACTs in the past where calculations could be done but for example one keyboard row was not working and I tracked down the fault to the pin at the ACT chip. The same could be in your case with the DATA line. If you are sure that the RAM is working in another HP-25, then the DATA line must be under suspicion. But check also Phi1 Phi2 and SYNC and ISA pins if they are connected to the ACT. Any interruption can cause the error.

See HP-25 schematic

You can completely remove the RAM chip and do all calculations with an HP-25. Not so with HP-29C, if the RAM is missing no calculation can be done, because the X-register is stored in RAM and the result is always 0.00.

HP-25A or HP-25E repair kit can repair your calculator.

Bernhard

[albertofenini]

Hi Bernhard
thank you very much for the tips, we will do some more tests over the weekend,
I have also sent you a message at info@panamatik.de, please check it out
take care Alberto

[albertofenini]

Hi Everyone,
these are the waves captured from the unit under repair and a working one,
blue waves are from the working one, red waves from the broken one,
both were powered with an external pack of batteries, in RUN mode and displaying a 0.00
both have a good electrical connection between pin 11 of the ACT and pin 4 of the RAM
we don't know if the ram transplanted was working or not since it came from a broken unit

data wave (pin 11 of the ACT)
   


isa wave (pin 10 of the ACT)
   


FI2 wave (pin 16 of the ACT)
   


FI1 wave (pin 17 of the ACT)
   


SYNC (pin 22 of the ACT)
   


Any idea ??
Thanks for help !!

[albertofenini]

A small update ...
I have sent a message to Bernard because we want to add a couple oh HP-25A to our collection,
however, few days ago we purchased an HP25 that was claimed to be working but with some
keyboard issues.
It turned out that it had some corrosion inside, and keys 4,5, 1,2, 7 and 8 where not working.
Those keys happen to be just behind the battery pack.
We did a complete restoration, and we also have dismantled the keyboard, since the
cleaning obtained so far was too much unreliable.
By doing this, removing all the keys, we have also been able to spray the keyboard with TS-80
from Tamiya.
We don't know if the Woodstock were originally this matte, but we really like the way it came out.
Polishing also the red lens.
Enjoy the photo, this is also an experiment since we shared the photos using iCloud.
Let us know as always your thoughts !
Take care !!!
HP-25 extreme makeover

[teenix]

(06-07-2019 09:45 PM)albertofenini Wrote:  Hi Everyone,
these are the waves captured from the unit under repair and a working one,
blue waves are from the working one, red waves from the broken one,
both were powered with an external pack of batteries, in RUN mode and displaying a 0.00
both have a good electrical connection between pin 11 of the ACT and pin 4 of the RAM
we don't know if the ram transplanted was working or not since it came from a broken unit
Any idea ??
Thanks for help !!

Assuming the same vertical scale was used, it looks like the faulty data line is about half the amplitude and suggests it is being loaded by something.

If replacing the RAM chip didn't change anything, the PCB tracks are clean, and no solder shorts are on the data pins, it may point to the ACT data pin as being the culprit.

cheers

Tony

[albertofenini]

We are happy to share few pictures where we can finally show how well the PANAMATIK chip works.
At first we had some problems but these were due to a bad rom/anode driver and not the PANAMATIK itself.
Once substituted the bad driver we totally recovered these two units that in the past we have posted has having a strange behavior
Thanks to PANAMATIK!!!
One thing that we want to share that is interesting is the type of glue we have used for the upper posts,
the one just behind the on off and run program switches
Overnight it dries very well and looks like it’s keeping its strength
We have not glued all the points because we are waiting for some drilled micro copper rivets that could do the job
The idea is to insert them and grasp the black pins coming from the keyboard
We’ll see ...
Pictures here PANAMATIK and glue for Woodstock keyboards

[AndiGer]

Hi Alberto,
In your opening thread on S/N 1702S13138 ( https://www.hpmuseum.org/forum/thread-13040.html ) you wrote you had VDisp too high. What did you do to repair that? Did you find the appropriate PSU schematics?
You didn't return with some comments ...
Thanx
Andi

[albertofenini]

Hi Andy
We had two hp25 and one Hp25c worth this project
As a start we just ordered two PANAMATIK hp25a replacement

When the two PANAMATIK chips arrived we decided to give up with the board as we had two pcb worth the restoration with no big issues to troubleshoot
Both were working except for the capability to store values and programs
So we didn’t try to repair that unit postponing the troubleshooting as well

However when we started installing the PANAMATIK one of the two good units
was acting weird and we found out the most probably the rom/anode chips was gone

At that point we harvested the rom/anode driver from the unit with Vdisp too high in order to repairs one of these two

At the end we recovered one Hp25 and one Hp25c

Also from that unit we recovered the back of the keyboard
and once the rivets will arrive we will use it to replace a keyboard we repaired some times ago mixing domes from a classic machine on a Woodstock keyboard

About that vdisp our guess is that I t was probably something bad in the cathode driver
I say this because when we tried to repair the one gone bad we harvested both
chips from the one with vdisp too high but we didn’t succeed
When we mixed the rom/anode driver from the donor with the original cathode driver things went well
It may be worth to check now if the PSU is working good on the board
Rainy days are coming so we may as well check it out

As schematics we use those from Tony Dwell

Take care