Help on HP 82901 drive
ard at p850ug1.demon.co.uk
Sun Jun 17 13:11:31 CDT 2007
> >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
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
> 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
> Ok, I'll check all components in that part of circuit.
Do you have a 'scope? If so, look at the collecttors, coveniently on the
metal cans, of the 2N2222 or 2N2907 (they;re linked together). You should
see a 12V p-p square wave here (switching between ground and +12V
essentially. If not, suspect the transistors, If that signal is prensent,
check the charge pump compoentns.
> 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.
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