HP-65 1603S03504 - display not working at all

[teenix]

(11-13-2020 07:30 AM)albertofenini Wrote:  Hi Tony
You are right, the logic board works perfectly when put in another unit
Or to say it right we used a logic board coming from a working unit and we tested that after these measurements it still works when put in it’s own unit

It may be the anode driver but shouldn’t at least work the rcd signal ?
That is coming from the cathode driver right ?

Also I was expecting to see good display waves at the cpu side or the faulty anode driver may silent these ?

Thanks !!!!

I cannot say for sure, but it "seems" like the CPU board fails when connected to the anode driver and (maybe) cathode driver. It is possible over the years of driving the coils, that the chip internals have failed and have shorts inside which load up the CPU board to the point that it stops.

The only way to test is to remove the anode driver and see if the signals reappear from the CPU board.

You might have to be careful with the signals to the anode driver as anything that upsets the relationship between the anode/cathode drivers may damage the chips. This is because the coils need to have a path to discharge which is usually through the LEDs and transistor switches in the cathode driver. If the coils get charged and cannot discharge properly, that voltage may build up and break down the junctions in the driver transistors. This is explained on page 3 of the Classic Notes PDF. In an older engine, an extreme example is where a spark jumps the gap in the spark plug when the coil primary current is disconnected by the points.

If the anode driver is removed, then the coils cannot charge and therefore not damage the cathode driver or the LEDs. The cathode driver cannot step without the anode driver, but should not matter as no LEDs will be lighting.

cheers

Tony