-----Original Message-----
From: cctalk-bounces at
classiccmp.org [mailto:cctalk-
bounces at
classiccmp.org] On Behalf Of ard at p850ug1.demon.co.uk
Sent: 01 May 2011 11:01
To: cctalk at
classiccmp.org
Subject: RE: H7140 PDP11 PSU
If I am not mistaken, the chopper transistor you are referring to is
Q1 (p88). I checked the resistance with the component still soldered
onto the
Correct.
board. Looking at it from the front (with a plate
behind connected to
what looks like a diode), the resistance between the middle and the
left pin
was
170Kohm (or 1Mohm, the multimeter scaling
confuses me somewhat, I
get
different numbers depending on whether I put the
dial on 200K or 2M,
either
Well, it's not totally shorted, then, which is the common failure mode of
chopper transistors.
Does your meter have a 'diode check' range? If so, use that, and check the
base-emitter and base-collector junctions with the probes both ways round.
This will at least tell you if the junctions are good. You really need to
remove
the transistor from the PCB to test it, though.
But perhaps before that we should check a few more things. The chopper
circuit is driven by E2 (p88), a good ol' 555 timer wired as an astable.
For
this to work, it needs to be gettign power. This power
comes from one of
two places
:
When the supply is running, it comes from T1 on the PSU motherboard, via
pins
7 and H of the connector and diode D7 (p88).
At start-up it comes from the mid-point of the mains smoothing capacitors
via pins 2 and B and Q6 (p88). The base of Q6 is driven from the network
R2
and D2 (p82). You should check the power resistors on
the motherboard,
actually, R1, R2 and R3 (p82).
Now obviously the first can't be doing anything since the supply is not
running. But the second source should be there. You should measure the
supply voltage at pin 8 of E2 _with respect to pin 1 of E2_. That is, the
black
meter probe is connected to pin 1 of E2, the red one
to pin 8 of the same
IC.
Be warned that this circuitry is directly connected to
the mains, and will
have _lethal_ voltages on it with repect to ground. So connect the wires,
make sure they can't touch anything, then plug the machine in and see what
voltage you get. Unplug the mains and wait for things to discharge before
touching anything.
I measured 8.6V across pin 1 and 8 of E2.
Regards
Rob