17 Jun
2007
17 Jun
'07
11:11 a.m.
>A quick look at the PCB (and I'll admit I've not removed it, only looked
>for likely signals) suggests the -ve supply comes as follows :
I've now taken my unit apart, and spent the afternoon investigating the
controller board (for my own interest, I hasten to add :-))
I was right aobut the clock, The -ve PSU is driven by the QD output of
the 'a' section of that '393. I assuem this is present and correct (31.25
kHz).
It drives one input of each of sections 'a' and 'b' of the '26. The
other
inputs are tied high (to +5V). This is an open-collector buffer.
The 4 transsitors I thought were the driver are. The otuput of that goes
to a 2-diode, 2-capacitor charge pump circuit.
A very similar circuit was used in some other drive units -- I think the
9133A uses something like it (with '06 buffers, not '26?)> HP did
actually publish that part of the cirucit -- I have a paper copy of the
3.5" disk driv eservice data that includes the PSU schematics, including
this circuit, but not the clock generator/divider (which is, however,
trivial to trace).
I don't know if there's anything applicable on http://www.hpmuseum.net/
but i might be worth taking a look if you get stuck.
There is a very low signal on a output of the 7426: a
very clear digital
signal but oscillating from 0 to 1V only. It's drawing the base of one
2N3904.
May be it's ok because it's open-collector or not, I'm not sure.
That sounds correct. The emitter of that 2N3904 is grounded. so you'll
only see 1 diode-drop of swing there. It's pulled up to +12V by an 8.2k
resistor, that's what provides the base current for the 2N3904 when the
output is turned off.
The 4 transistors in front of the power conenctor
are some kind of
totem-pole output stage producing a 12V p-p square wave with a freqeucny
of 31.25kHz from this signal (and the 12V supply input, of course).
In front of that are a couple of electrolytic caps and some diodes. These
form a charge pump circuit, giving the -ve supply. You should see -9.5V
on the right hand (not earthed) end of the front-most capacitor of the
pair.
Ok, I'll check all components in that part of circuit. I've already replaced all 2N3904 and 2N3906
transistors, but only one
2N2222. May be it's the other that's defective.... ;-)
Also, I'll re-check voltage on capacitor, but as I told I already didn't see
any -V, so probably it's actually at 0 Volt.
Now I've looked at the PCB, I can confirm these 2 points are linked by a
PCB trace. If there's voltage on the capacitor nad not on the LM361,
you've got an opne-circuit there, but I think that's very unlikely.
-tony