HP-80 with bugs ... ?

[ThomasF]

(02-13-2024 07:56 AM)teenix Wrote:  You could probably try briefly either shorting the 2.2uF capacitor or use a test lead from the base of the left transistor in the circuit diagram to the test point ground on the circuit board to initiate a reset.

Hi Tony!

I followed your tip and measured around the reset/init circuit, and I saw that some signals were a bit "bad".
I replaced the 2.2uF cap as a first step, but that made no difference, so I continued.
Finally after some fault tracing (only had a multimeter at hands) I realized that one of the transistors must be bad, most probably the one connected to the Init signal.

Replaced both transistors, and voila! Another Classic brought back from a faulty state!

Also realized that the default state is DSP 2 ("0.00"). Whenever I turned it on before, it always ended up in DSP 0 state ("0."), so even if it seemed to work, probably some internal states was wacko.

I also found out that the picture you sent didn't correspond with the schematics.
The reset/init circuit is the one on the right hand side of the board (not left as you indicated - that was the PSU circuit), the init circuit is indicated on the included picture below.

So now I have to search for another Classic to repair so I can make use of the spare board I bought from Harald

But big thanks for your explanation that sent me in the right direction ..

Cheers,
Thomas