hot resistor 97/97S

[Duane Hess]

Finally getting back to overdue things. Have a 97S-I/O where a resistor gets hot and the battery discharges fairly rapidly. My unit matches the picture (didn't notice differences):
-- https://www.hpmuseum.org/97sint.jpg
Used a number of fully charged batteries which work without problems in other 97, 92 or 91 units.

There are various service manuals available, the one which most closely matches my unit is at (currently unavailable):
-- http://www.teenix.org/97ServMan.pdf
from thread:
-- https://www.hpmuseum.org/forum/thread-16655.html
The service manuals here at the museum and hpcalc show a Logic PCA have varying levels of not matching my unit.

However all the manuals, in section 2.37 or 2.39 (depending on manual) have the same power supply/inverter circuit (which some component name typo differences). But they all agree transistor Q2 seem to be an oscillator transistor responsible for producing Vss +6.25VDC. They all agree in the over-voltage protection circuit function and schematic.

Please take note of the archived thread from the museum:
-- https://www.hpmuseum.org/cgi-sys/cgiwrap...read=17322

Tony Duell appears to be describing the same issue and indicates an oscillator transistor failure which causes the over-voltage protection to kick in. Haven't checked in a while, but recall my unit having Vss being a bit lower than battery voltage at the time and Vgg was screwed up (don't recall how). Further, he seems to suggest this failure is not uncommon and suggests a replacement transistor type.

Is anyone familiar with this? Tony's discussion seems to point to transistor Q2. The only tools I have is a multimeter. So scope traces here.

Any comments/familiarity from anyone?

thanks

[teenix]

The 2N3904 transistor is fairly common and cheap so couldn't hurt to replace.

cheers

Tony

[AndiGer]

(05-23-2022 10:02 AM)Duane Hess Wrote:  The service manuals here at the museum and hpcalc show a Logic PCA have varying levels of not matching my unit.

I think first at all you should get the service manual from 1982 @
https://literature.hpcalc.org/community/hp97-sm-en.pdf

The newer main PCB layout is on page 78

Andi