Jazzer said:
Alas I did manage to find a open cap and on replacing the ac appeared
across it but still never got to the fills. Checked for more open caps
in the multipler chain but all seemed ok.
But Arfa did you say Good Luck! In my clumsiness managed to drop a
tool on the main board and blew the secondary fuses in PS board. On
replacing DC ok but power switch not switching unit on. Only lights
show in CD selector and tape arrows. Bad luck and I will shelve this
one for a while and take another look when I feel a little better.
Thanks for your help and patience and hope to contact you again in the
future. Cheers, Jazzer.
That's a shame. You may now have landed yourself with more trouble than you
would care to contemplate ... The power supply circuitry on most models is
pretty complex, employing a dual level rail system to the output stages.
Under low load, low volume conditions, the output stages run from low (ish)
rails, but when the vol is ramped up, and the demands of the output stages
increase, the supply rails are jacked up ' on the fly ' as it were, using a
pair of FETs. These FETs fail and cause all sorts of problems. You only have
to look at them wrongly for them to fail all on their own. Not all models
use this sytem, however, and as I said at the start, the model that you
quote is not one that I remember having specific dealings with. However, on
most models, there is a complex fault monitoring circuit. It checks such
things as presence of AC to the power supply ( via two little surface mount
dual diode packs that look like s.m. transistors ), output DC offset,
heatsink temperature, output stage current, and funnels the whole lot
finally down to a line called " /PROTECT ". This line goes right on back to
the system control micro on the front panel, and if it's low, will cause the
micro to just halt in the non-reactive state that you describe.
The usual way to tackle this problem is to locate one of the jumpers that is
labeled " /PROTECT " ( there are a couple dotted about the main board
usually, one around the middle towards the left, and sometimes another up by
the connector which joins the front board to the main board ) and lift one
end. This should allow the micro side to come back up to +5v via a pull-up
resistor that's on the front panel. Making sure that you have no speakers
connected (!!), you can now power up, and with luck, the unit will come on
and stay on. You can then use normal fault finding to determine which branch
of the fault detector circuit is being activated, and then work out why. For
what it's worth, you might find some additional pico fuses on the main board
near the outputs. Make sure that these have not failed. Check also that all
of the pico fuses on the transformer board are actually ok. I won't say "
good luck " this time ...
I am away for a couple of weeks from Monday. Feel free to post or mail me
direct, but don't expect a reply for a while after Sunday.
Arfa