HP-67 stuck in scientific (DSP 0) mode

[Roland57]

(11-02-2023 03:10 PM)Hiwi Wrote:  Hi,
i have another display problem, see picture.
My HP-67 is in very good condition and working well, however there is one part of the segment in the exponent sign permanently turned on.

Is this a problem of the display or driver IC? (Cathode or anode driver)
Has anyone seen this before?

The exponent and the normal sign are working as expected.

Ralf

Hello Ralf,
I had a hp67 showing this kind of "ghost segment". The reason was corrosion (coming from the battery compartment) on the main pcb. There was visible green dirt covering several traces of the pcb. This green stuff is conductive. After cleaning with IPA the ghost segment vanished.

Roland

[Hiwi]

Hi Roland,
i will try to clean the PCB, thanks for your advice.
Ralf

[teenix]

Does the lit segment change when you key in a 1 or 2 or 3 etc?

[Hiwi]

(11-02-2023 07:26 PM)teenix Wrote:  Does the lit segment change when you key in a 1 or 2 or 3 etc?

No it is still there. Also when using EEX. But it is only visible in RUN Mode not in PGM Mode.

I cleaned everything, this was necessary!
But cleaning did not solve this display problem.
I changed the cpu PCB with no sucess, must be a driver or led problem.

Ralf

[teenix]

I'm trying to think how this can be in a multiplexed display. The segment can only come on if powered by the anode driver when the cathode driver is active low on that digit. The faulty digit is in a separate LED module to the first digits so unlikely to be an internal short.

If the cathode driver had a faulty output that segment may light, but only if the same segment was lit on another digit, which is why I suggested entering a number that changed the lighting of that segment on other displays, as it might have change the faulty one.

It appears to be linked to a decimal point because the W/PGM display doesn't use one.

What happens if you key in [f] [-x-] in RUN mode?

This flashes the decimal point and might flash the faulty digit.

cheers

Tony

[Hiwi]

(11-03-2023 12:04 PM)teenix Wrote:  What happens if you key in [f] [-x-] in RUN mode?

There is only a flashing decimal point as expected.

But the problem seems to be solved. One or two hours after cleaning and reassembling the calc it looks like the phantom segment was much darker and a day later it completly disappeared!

So there must be conducting dust corrosion or something else.

Ralf