17 Jun
2007
17 Jun
'07
4:08 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 :
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).
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.
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.
Thank you very much: you are of very great help.
Roberto