Another HP-67 repair query

[coitboy2000]

(10-18-2020 11:08 AM)teenix Wrote:  

(10-18-2020 10:10 AM)coitboy2000 Wrote:  I am out of depth on this I think. I have captured some scope images for the cathode, anode & led pins when the calulator is powered up, If I twist the the keyboard near the connector with the cpu board I get some random elements light up on some of the leds'.

I would expect fairly uniform waveforms being displayed on the scope. Some seem to be changing amplitude a bit which may be a symptom of the pin connections you mention.

Figure 4.7 on Pg 4-12 of the HP-97 service manual shows proper STB and RCD waveforms.

http://www.decadecounter.com/vta/pdf/HP-...Manual.pdf

Can you notice changes in the partially working display at all if any number keys are pressed? If so then maybe the CPU components are working.

Maybe the connector pins and/or holes are corroded or not quite making proper contact. Deoxit spray to clean things up??

If you suspect some are a loose fit in the holes, then I did manage to successfully widen some a bit. A word of caution though, these pins are brittle so it is not really a good idea to try and bend them, they can snap quite easily.


With the CPU board removed, and observing static handling procedures, I gently inserted a metal scriber (which has a long tapered shaft and a pointed end) into the gap between the two prongs just enough to expand them "a little bit". Once the scriber is withdrawn, the prongs spring back towards each other, but they may have opened out a bit which provides some more tension for better contact in the hole. This method fixed a similar problem, but if the pins have lost most of their springiness by being inserted into the circuit board holes over the years, then this fix probably won't last long.


cheers

Tony

Still trying to get the calculator going.

I did find that the transistors that supply the display current through pins 3 & 4 of J1 did not appear to be working correctly. My calculator actually had an aluminium capsule that contained both transistors.

I am a little confused about the type of transistor to replace it with as this schematic:

https://www.hpmuseum.org/forum/thread-83...+schematic

shows them as NPN 2N3904 but the schematic by Tony Duell (I think) has them shown as PNP, at least the symbol anyway ?