16 Jun
2007
16 Jun
'07
3:56 p.m.
So there is a -ve supply (I'd half-suspected there
was, knowing that
other HP drive units which use a load of transsitors + an LM361 as the
VCO/PLL also have a -ve supply). I've not yet discovered where it comes
from, it must be generated on the controller board (my guess is that
those 2 metal-can transsitors at the front left corner have somethign to
do with it).
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 :
The master clock (8 MHz oscillator package) is divided down to 500kHz by
the 'b' side of the '393 next to it, then divided down further to
31.25kHz by tghe 'a; side of that chip.
The 'a' and 'b' sections of the '26 next to that are the drivers for
the
-ve supply. Yo ucan't see any obvious signal at the output of the 'a'
section. it may well drive the base of a transsitor or something (it is
an open-collector output, so this would be OK).
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.
I've not traced any more....
-tony