It's time to restore the 11/45 - progress!
ard at p850ug1.demon.co.uk
Fri Feb 6 00:42:27 CST 2015
> OK, please forgive my ignorance, here. To give you some idea of my
> electronics expertise level, I'd like you to know that I just googled
> crowbar :D
I (hopefully) never have and never will flame somebody who is trying to
learn. Sharing repair information (at all levels) is one thing this list is for.
> From the wikipedia article I read, I guess this crowbar thing tripping,
> is just its way of trying to protect the rest of the system from
> overcurrent, right?
Overvoltage, not overcurrent. The idea of the crowbar circuit is to short
circuit the output of the power supply if the voltage rises too high, thus
protecting the rest of the machine. This, will, of course, cause a high current
to flow in the PSU circuit and the overcurrent circuit should then operate 
but that's a result, not a cause.
 I once had an H754 (yes) where the overcurrent circuit had failed and the
crowbar operated. The result was a number of transistors literally blown off
the PCB. I think I had to replace 3 or 4 of them, the 723 chip, the fuse, etc.
> And the dried up cap is one thing what could make it think there was an
> overcurrent situation, correct?
This is as switching regulator. Basically there is a power transistor that is either
turned hard on (saturated) or cut off. When it is on, energy flows from the
transformer and is stored in the magnetic field of the inductor in the module.
When it is off, tht stored energy supplies the load. The capacitor is another
energy store, effectively to smooth out the voltage changes from this switching.
If it fails (the electrolyte dries up and it seems to have a large-ish resistor in series
with it), then you get high voltage spikes on the output. These then trip the
> I'v had a little luck in the past with such things, but I don't know that
>I'm going to understand how to read the schematic. I'm going to try and
> find one now. Also going to try to find a manual that describes the power
I've not seen what the 11/45 manuals cover, but somebody else mentioned the
11/34. The 11/34 PSU, for all it is different, uses the same regulator bricks, so I
assume that bit of the manual is applicable here.
> supply. But how do I know which one is the output cap? What is a valid
When I have unpacked all my printsets I can give you a circuit reference.
But just look for an electrolytic capacitor between the -15V output and
ground (I think it is the second largest can on the board, the really
big one is the input smoothing capacitor).
> test once I find it? I guess I could charge it up on a car battery and see
> how long it holds or something like that.
No, you are looking for series resistance, not leakage. The only real way to
measure it is with an ESR meter (a device for checking such capacitors, the
name means 'Effective Series Resistance' and it's a very useful tool when
working on PSUs.
> I don't have much electronics gear here. Borrowed a Craftsman digital
> VOM from my electrician friend (although it's telling me I have 140v AC on
> the mains!) and I have a soldering iron, heated desoldering bulb thing and
> some desoldering braid, flux and lead solder.
You need the flux-cored 'electronic' solder (resin cored), the real lead/tin stuff
is what was originally used and should be used for repairs. Do NOT use any
other fluxes, certainly not the acid flux used for plumbing/metalwork (OK, there
are times when you add some more resin flux, particularly when doing SMD work,
but not here)
I think a desoldering pump is useful too, much better than the bulb.
You need your own DMM. A 'scope is very useful too. And when you start working on
the CPU itself, a logic analyser is handy
> BTW, according to this VOM, there's 24.5 AC and 14.2 volts DC being input
> into the H745 regulator. Is this too low? Maybe this has something to do
> with the H742 adjustment that I didn't find? Or maybe this VOM is
Those voltages are fine.
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