Another HP-67 repair query

[coitboy2000]

(10-14-2020 10:40 AM)teenix Wrote:  

(10-14-2020 10:11 AM)coitboy2000 Wrote:  I decided to give it another good clean, in particular the processor board board. It is behaving as before but the sense amplifier does not seem to get as hot.

There is an adjustable potentiometer (10k) on the card reader board that currently reads around 1.4k, at least in circuit anyway. What should this be set to ? The 3x capacitors on the card reader board still seem ok.

Regards

Richard

If the sense chip is still getting warm, it suggests a problem remains. If it was allowed to get "hot" then it may be damaged, but hopefully not.

Switch mode power supply output should be around -12V and +6.25V.

The resistor sets the motor speed. It can be any value within the pots range to set the correct speed, although I doubt it needs to be full scale one way or another. I wouldn't touch it until you know the card reader is working.

Setting the correct speed practically speaking would be to use an oscilloscope to measure the data stream bits sent to the CPU board and they should be around 1mS Hi to Lo or Lo to Hi, or you can see the entire data stream fits on the card just after being sensed in and before sensed out. See Page 7, Fig 8 of HP Journal 1976-11.

cheers

Tony

Thanks Tony

With just 4V supply should I be seeing -12V somewhere on the cpu board ? The highest voltage I see is -11.33V on pin 2 of 1820-1596 and 6.3V on the majority of pins on 1820-1751 ?