HP9915B with PSU problems

Vassilis Prevelakis vp at cs.drexel.edu
Sat Mar 19 13:05:01 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.

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.

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

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
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
look at some part on the underside, I know which components are on the
other side. Like an adventure game that you have played many times and
have the layout in your mind.

Real fun.


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