HP-67 1805S02334 [RA and RB noisy, doesn't read but writes]

[albertofenini]

Hi Everyone,

this is about asp HP-67 with what looks like a noisy card reader
While CA and CB looks fine, both when reading and both when the calculator displays a 0.00
RA and RB show a noise immediately after power on
When reading a card you can still see the the reading among the noise
We have already replaced the sense amplifier with no improvements
We have already swapped the CPU board with a known working one with no improvements
What could it possibly be ?
Any suggestion will be greatly appreciated !
Take care !

[EDIT]
I have also uploaded some WA and WB captures
As a matter of fact the captures show no noise and a good wave on WA and WB
both when reading and when writing (switch W/PRGM in Write position)
The so written card can be successfully read on a different working HP-67

Pictures here : HP-67 1805S02334

[teenix]

Maybe change the capacitors on the sense board. The 2 channels of read data have to pass through the ones connected to the CA and CB pins on the sense chip.

If the connections from the head to the sense board are the plug in type, check for corrosion on the wire ends.

cheers

Tony

[albertofenini]

Hi Tony, Hi everyone,

I did change the two 22microF capacitors on the card reader PCB
I have also cleaned the contacts as this is a plug versions
but the behavior it's exactly the same

I have also uploaded more pictures and besides the missing battery connectors
both the keyboard PCB and the card reader PCB had been reworked because of
the corrosion

I have soldered very thin wires to replace the corroded traces, so may be while I
had been quite careful in doing this, this wires and soldering could be the origin
of the noise

Would have been better to use a silver conductive pen rather than soldering wires ?

The next steps could be opening a known working (plug version) calculator and try this whole
card reader assembly in that unit, and eventually do the same with the keyboard assembly

take care !!

Updated album : HP-67 1705A01234

[albertofenini]

Did the swap ...

When the 02334's card reader has been put in a working unit it has proven
to be working, although reworked I would exclude the card reader for the fault

When I have put into the working unit the 02334's CPU board
I have had the same faulty behavior than before

Finally, I have put the working CPU board and the 02334's card reader
in the 02334's chassis so that I have used the 02334's reworked keyboard pcb
and I have been able to read and write

Possibly the 02334's CPU board is the responsible for the noisy RA and RB,
would it be the 1820-1751 CRC chip ?
How can I check it ?

[EDIT] I have put the 02334's CPU board back in its original 02334's chassis,
with a 0.00 on the display and the card reader disconnected, RA and RB measured at the CRC chip
are clean, but once the card reader is connected they immediately get noisy
This, does not happen if I use a different CPU board

any help is appreciated !
take care !

[teenix]

It could be faulty CRC chips.

To be fair, they are getting on the high side of forty years old so will be more prone to failure as time marches on. Who knows, corrosion may have managed to get inside the chip, the calculator might have been dropped a few times or handled roughly over its life and caused micro fractures.

cheers

Tony