30 Apr
2011
30 Apr
'11
1:23 a.m.
I will add some diagnostic wires to the board as you suggest, I don't
remember a lot about TTL chips, is there a convention about which are the
power pins?
In the meantime someone else told me that the click I would hear when
connecting to the mains (before switching it on with the key) was some
kind
of relay. I don't hear that anymore, would that
help to isolate the
problem
at all?
OK, I am looking at the schemaitcs... I will take the referenes from this
manual on bitsavers, since I guess we all have access to it.
http://www.bitsavers.org/pdf/dec/pdp11/1124/MP01018_1124schem_Aug80.pdf
This is a complex PSU, and it is not obvious at first sight what is going
on,
but I will try... I will put page references (to that PDF) in brackets.
The relay is K1 (p70). It shorts out a soft-start resistor in series with
the
input to the mains bridge rectifier. This produces 350V across C1/C2 in
series.
Now much of the PSU circuitry runs off a little SMPSU. The chopper
transformer is T1 (p82). It provides about 12V across C3 (p82).
Now we need to look at the 'Bias and Interface Board'. The relay is driven
by
E4 (p86). It's essentially a 'mains OK' circuit, the relay is energised when
the votlage across the mains smoothing capacitors is high enough to trigger
E3a. E3a is on the mains side of the isolation barrier, of course. But the
relay driver (E4) is powered from a signal called '+5V'. This is not the +5V
that you're expecting. It comes from the 7805 regulator E1 (p87). The input
to that comes from that little SMPSU I've been talking about. This is
isolated
from the mains.
The control circuitry for that is on page 88. Be warned that this circuitry
is
NOT isolated from the mains. It's a relatively conventional SMPSU with the
control circuitry powered straight from the mains. Q1 (p88) is the chopper
transistor.
So, if everything's working right, this SMPSU starts up when you apply
mains
to the machine, the 'mains OK' circuit triggers and the relay pulls in.
I would start by checking that '5V' supply, remember it's not the one to
the
backplane. Check it at the output of E1, for example. If it's missing, as I
think it will be, you need to sort out the SMPSU I've been talking about.
But
let's find out if you need to do that.
-tony
Regards
Rob
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