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Full Version: HP-25 - what could be wrong, simple question
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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
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.

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!!
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.

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
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 !!
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