10 Mar
2007
10 Mar
'07
10:04 p.m.
I was looking at the power control board for the H742A (5409730) that is
supposed to be putting out my +15v. I couldn't find any schematics on this
board but the system engineering prints appear to indicate they should be
They're in my PDP11/45 printset.
present. In the absence of schematics (and any obvious
scorching) I figured
the likely first thing to check is the sole TO-3 power transistor on the
board. Sure enough, the C-E junction is shorted so at the least I'll need
one of those.
Did you test it out-of-circuit? It's a power darlington (read : high
current gain) and there's a fairly low resistor (100 ohms -- 2 off 200
ohm in parallel) between C and B. It's quite possible an in-circuit test
will show it as shorted.
1) The critter is Motorola "DEC3000" which
I'm betting is a DEC house part
number (since ECG xref shows nothing && it starts with DEC). Can anyone clue
me in to what part that really is?
The pritn I have shows it (Q1) as an MJ1000
2) Are the schematics for 5409730 around so I can try to understand it a
little better?
I can describe the circuit for you..
Firstly a sort-of layout ;
With the PCB so that Q1 is at the bottom right, and the fat electrolytic
capacitor is on the far left, then :
That capacitor is C1. Alongside it, towareds the bottom of the board are
2 small transsistors. Q2 is near the bottom edge, Q3 jsut above it. Then
thers's Q1, the To3 can on the heatsink
Above Q3, roughtlu on a line with the top of that capacitor C1 is a
bridge rectifier, D1. And alongisde that is a fuse, F1
The rest of the compoonents on the board are part of the ACLO and DCLO
circuits.
Now, for a circuit description of the +15V supply.
30V AC comes in on pins 3 and 4 of J1 (the 4 pin connector on the board).
It's rectified by D1, and smoothed by C1. The +ve side of that capacitor
then gores to the fuse F1.
Nwo, the regualators is quite conventional. It consists of a 16V zener
with that 100 Ohm resistor arrangement I mentioned feeding it from the
outpt of C1. Q1 acts as an emttier-follower, and buffers the output of
the zener circuit to provide the +15V line to the rest of the machine.
Now for the unconventional part. Q3 and Q2 form a circuit that disables
the +15V supply (shorts out the zener, effectively) if the DCLO line --
the 'spaer one' on pin 9 of J4 (the conencotr wired to the PCB) -- is
aserted.
So, what I'd do is ; Check F1. If it's OK. power up and check the voltage
on the +ve side of C1 wrt the system ground rail. You should have 30-40V
here. If you do, check the base voltage of Q1. If that's 16V, and you
still ahve no output, suspect Q1. But if that's essentially 0V, check Q2
and Q3, and if they're OK, it's time to check back into the DCLO circuit.
-tony