HP-19c continuing

[Jonas Sandstedt]

Hello,
I have previously asked about tips about HP-19c disassembly and soldering problems, and I think I now have solved those problems. I have also acquired one more HP-19c that also required a vinegar bath and some touch-up, and everything looks fine. The transistors also measures fine. Unfortunately, neither of them works, and I would ask for some tips.
Number one first worked erratically and printed numbers (and sometimes gibberish) with different widths. After a new light cleaning and re-assembly it displays either nothing or zeroes. I have measured the voltages at the “golden” connector pins at the side that connect the power PCB and the logic PCB when the calculator is turned on. The voltages are, from right:
1: GND
2. 4,95V
3. 0V
4. -12,1V
5. 0V
6. 6,2V
7. 6,2 V
8-12. 4,9-5,0V
Number two I have disassembled and re-assembled several times, and it is normally dead when turned on, but suddenly works, very seldom, when turned on. It the also prints except for one printhead that has the resistance 35 ohms (almost not broken!). When it runs, it runs. When it does not run, no manoeuvres (shaking it, gently twisting etc.) does not light the LED. The voltages are the same as for number one, except connector 6 from the left, it is 4,2V instead of 6,2V.
I am open for any kind of suggestion, and for info about correct working voltages on these pins.
best regards
Jonas

[teenix]

I can't help much because I don't own one, but 6.2V on pin#6 would suggest that the printer strobe circuit is not working. The 4.2V on the other 19C would be more likely as the voltage here is a square wave to control the print intensity of the print heads.

If the vinegar bath was the entire circuit board, then perhaps the vinegar got inside the trimmer resistor that adjusts the print intensity and has corrupted its resistance.

I would check the circuit board tracks and connections closely to see that any corrosion and the vinegar has not degraded them.

From what I can see of the circuit, the other voltages appear to be ok.

cheers

Tony

[Jonas Sandstedt]

Thanks,

I will of course look further into this, although it may take its time. I just remembered that some valuable information that is applicable for the HP-19c is available in the HP-97 service manual, will take a look there as well.

best regards
Jonas