PC-jr PSU (again!)

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Tue Jul 15 16:32:49 1997

I dug out the PC-jr Techref (in order to look up details on the joystick), and
I also looked at the PSU diagrams (page B-23 in my edition). We had a thread on
this about a month ago, mainly about the external transformer unit which I
don't have.

Now, the input connector is a 3-pin thing. The centre pin is connected to
shield ground, and then via L3 and L4 on the system board to logic ground. It's
thus a DC connection to the system ground.

The outside 2 pins go through a filter to the AC terminals of a bridge
rectifier (CR7 - CR10) and the output of this goes (via the power switch) to a
1500uF 25V capacitor (C13). The -ve side of this capacitor is also connected
to system ground.

Thus, IMHO it makes no sense at all for the transformer to be centre-tapped
with the tap going to pin 2 on the connector. If it was, the winding would
be paritially shorted out by the diodes in the bridge rectifier, which would
cause a lot of damage. Also, since C13 is only rated at 25V, the maximum input
rms voltage is 25/sqrt(2) volts, which is a lot less than 34V. I therefore
still believe that it's a plain 17V transformer between the 2 outer pins on
the connector.

Incidentally, the +12V output comes from a standard linear regulator, the
+5V output from a switching regulator (Z2 is the control IC, Q2 the chopper,
and L1 the switching inductor). The -12V output is supplied by a secondary
winding on L1, which is then rectifier and smoothed.

It's always possible that IBM used several versions of the PSU card, but the
schematic in my Techref seems to agree with the one in my machine.

Received on Tue Jul 15 1997 - 16:32:49 BST

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:30:27 BST