10 Oct
2015
10 Oct
'15
8:46 p.m.
On Sat, Oct 10, 2015 at 8:44 PM, Josh Dersch <derschjo at gmail.com> wrote:
On Sat, Oct 10, 2015 at 2:38 PM, tony duell <ard at
p850ug1.demon.co.uk>
wrote:
Thanks for the explanation. I tacked on some wires to the bias board (to
bring them out for safe probing); the wires are connected to the 5V logic
supply and the +12V bias supply and I get nothing at all out of them when
the H7140 is turned on.
I'm slowly coming to terms with the schematic but I'm not quite clear
where these two voltages are being generated. I note from the block
diagram that the +15 and +12 are rectified on the motherboard (backplane)
-- which should make it GREAT fun to probe. So it looks like some careful
disassembly is now required (unless there's a trick to probing things on
the backplane...)
Thanks,
Josh
And a quick correction, it's -15V, not +15; I need to proofread more :)
- Josh
From a quick glance at the schematic I think that this part is related
to
the fan driving circuitry. The diodes look like
some kind of protection
Yes.
diodes which is good to have when switching
inductive loads. I have
really
no idea why they have been removed. But I
don't think it is related to
the
Remember that said diodes do slow down the decay of the current (and thus
the magnetic field) when the transistor turns off (this can be a problem
with
solenoid drivers for paper tape punches, etc). It is possible that with
the diodes
in-circuit the fan drive waveforms were not right.
PSU not working at all.
Correct. The fan driver is powered by the 36V output of the 'memory'
SMPSU,
Even if it is defective, the bias supply should run, the logic supplies
should come
up in the ON position of the key, and so on.
That 'bias/interface' PCB contains (at least) the bias supply chopper and
control
circuit; the fan driver; and the control logic to start the 2 main PSUs
in the correct
posiitions of the keyswitch,
-tony
/Mattis
- Josh
-tony