HP9915B with PSU problems

Tony Duell ard at p850ug1.demon.co.uk
Sat Mar 19 18:24:35 CST 2005

> Yipppeeee! The 9915B now works. My remaining worry is that I do not


> know why! After removing things to allow me to check them in isolation
> and then putting them back, I started providing power to the different
> parts of the PSU. First the +12V supply worked, then the -5V and -12V
> supplies worked, then the +6V, and finally the +5V. Every time I expected
> the fuse to blow, but it didn't. So now the whole thing works fine.

I _hate_ faults like that. There almost certainly is something wrong, and 
it'll come back at the wrong moment. However, it's difficult, if not 
impossible, to find a fault that's not there at the time, so all you can 
do is wait...

> The 9915A still does not work, but I have found that C45 (the thin tall
> 390uF capacitor connected to pin 4 of transformer T2 via diode CR12) is
> shorted.  That removes power to the V+ supply taking out the +6V and
> the +5V supplies.

RIght. And doubtless really loads T2, either triggering the current limit 
circuit, or at the very least pulling other supplies down.

> Now all I need is a replacement for C45 and I am in business.

HP really loved to use silly value capacitors in their PSUs for no good 
reasons as dar as I can see. The tolerance on an electrolytic is 
typically something like +/-20%, or even +50%/-20%. Which makes me wonder 
why I have a PSU board with an assortment of 5600uF and 6000uF caps on 
it. As another example, the 9845 PSU contains som 810uF caps -- 820uF is 
a prefered value. Hmmm...

I, therefore, wouldn't worry too much about getting an exact replacement 
for C45. I would think a 470uF would be fine.

> Many many thanks Tony.
> BTW the first time I looked at the 9915 motherboard I freaked out,
> because the PSU is all over the place. In some cases analog components

At least HP did provide some links to disconnect the 3 main PSU outputs 
from the rest of the machine..

Incidetnally, the 9915 PSU is by no means the most complex HP computer 
supply I've seen. That honour goes to the 9845.....

> are located right next to the digital logic. But with Tony's help and
> his schematics, I managed to map my way around the board. Now when I

If anyone else needs them, they're now on a CD-ROM with many other 
reverse-enginnered HP calculator scheamtics. You get it from Dave Colver 
(secretary+hpcc.org), IIRC it costs \pounds 15.00, and I think he can 
take Paypal (unlike me, he deosn't mind new ideas :-)).

Here's a list of what you get : 

chargers/eramco_pp                     Portable Plus car charger (Eramco) 
chargers/hp82054                       Car charger
chargers/hp82055                       Woodstock car charger
desktops/hp110                         `Portable'
desktops/hp86b                         desktop computer, external video monitor
desktops/hp87                          HP87 and HP87XM desktop computers
desktops/hp9100b                       First generation RPN desktop
desktops/hp9810                        Second generation RPN desktop
desktops/hp9815                        RPN desktop calculator
desktops/hp9825                        First HPL desktop (also covers 9831 BASIC desktop)
desktops/hp9830                        First BASIC desktop
desktops/hp9915                        Industrial control computer (based on HP85)
desktops/integral                      Portable unix machine
handhelds/classic/hp35                 First handheld scientific
handhelds/classic/hp45                 Second handheld scientific
handhelds/classic/hp55                 Scientific with timer
handhelds/classic/hp65                 First handheld programmable
handhelds/classic/hp67                 Handheld programmable
handhelds/classic/hp70                 Simple financial handheld
handhelds/classic/hp80                 Financial handheld
handhelds/hp48sxKeyboard               HP48SX keyboard matrix
handhelds/printing_classic/hp46        Printing 45
handhelds/printing_classic/hp81        Printing 80 (with many more functions)
handhelds/spice/hp32e                  Non-continuous memory Spice (other 3xE are similar)     
handhelds/spice/hp33c                  Continuous memory Spice (other 3xC are similar)
handhelds/sting/hp10a                  Printing adding machine
handhelds/sting/hp19c                  Programming printing calculator 
handhelds/topcat/hp91                  Printing scientific calculator
handhelds/topcat/hp92                  Printing financial calculator
handhelds/topcat/hp95c                 Never-released printing programmable calculator
handhelds/topcat/hp97sio               I/O module part of HP97S
handhelds/voyager/hp16c                Programmer's calculator
handhelds/woodstock/hp21               Scientific calculator
handhelds/woodstock/hp22               Financial calculator
handhelds/woodstock/hp25               Programmable calculator
handhelds/woodstock/hp25c              Continuous memory programmable calculator
handhelds/woodstock/hp27               Financial and scientific calculator
handhelds/woodstock/hp29c              Continuous memory programmable calculator 
peripherals/hhp_eprom                  EPROM box for HP41
peripherals/hp2671g                    HPIB Thermal printer
peripherals/hp7440hpib                 HPIB ColourPro plotter
peripherals/hp7470opt003               HPIL Plotter
peripherals/hp7475                     HPIB Plotter
peripherals/hp82104                    Card reader for HP41
peripherals/hp82143                    Dedicated thermal printer for HP41
peripherals/hp82162                    HPIL thermal printer
peripherals/hp82168                    HPIL modem
peripherals/hp82440a                   IR thermal printer
peripherals/hp82440b                   IR thermal printer 
peripherals/hp82985                    Portable Plus video interface
peripherals/hp9114a                    HPIL disk drivc
peripherals/hp9114b                    HPIL disk drive
peripherals/hp98034                    HPIB interface for HP9825
peripherals/hp98035                    Real time clock module for HP9825 
peripherals/hp98036                    RS232 interface for HP9825
peripherals/hp9866                     Thermal printer for 9800-series
peripherals/hp9877                     External tape drive for HP9825 
peripherals/hp9878                     Expansion chassis for HP9825
peripherals/rsu                        Eramco MLDL unit for HP41
peripherals/rsu2                       Eramco MLDL for HP41 (card reader case)
peripherals/wand75d                    Wand for the HP75D


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