17 Jan
2010
17 Jan
'10
2:32 p.m.
Tony Duell wrote:
<snip>
Do you mean AC, or DC with excessive ripple. Did any
of the lines go the
wrong side of ground? (my guess is they didn't, but it's best to be sure).
DC with excessive ripple. Nothing on the wrong side of ground but 60 and
120 Hz waveforms almost down to ground.
replaced all
of the electrolytic caps and that got rid of the AC
but some of the voltage levels are very wrong, at least as compared
to what is written on the schematics. The worst offender is just
off the "plus" side of the full wave bridge (BR-1). It should be
nine volts but is actually at 11.5. This makes what should be the
I wouldn't worry about that one. It's an unregulated supply, and will
drop on load. Assuming it's the input to a regulator (like that 7805 you
mention), it'll be fine.
main five volt rail a little more than 9 volts...
the far side of
VR-1 (a 7805). The wave form coming off the transformer is really
Now that does worry me, for obvious reasons. You need to correct that
before you put the ICs back (but you knew that ;-)). 3) There is resistor in parallel with the regulator.
Some manufacturers
connected a high-wattage resistor between the input and output pins of
the regulator, of a suitable value to carry most of the load current at
the expected voltage drop, thus leaving the reguator to carry (and
control) a much smaller current. This is an evil trick because the output
voltage will rise (and the rewgualtor can't do anything about it) if the
load is too small. Again, I've come across this in an HP device, it was
fitted with a crowbar circuit, and I wondered why it tripped with the
supply on dummy load. The reason was my load was not large enough. Do you
have scehamtics? If so, that'll show if the resistor is present or not.
Yes, there is an 18 Ohm 2 Watt resistor between the +9 and +5 (the input
and output pins of the 7805). I was wondering what that is supposed to do.
Is this resistor causing the +5 side to "look higher" than it really is?
Is everything actually OK, even though my VOM and scope both agree the +5
rail is at +9.2 _WITH_ automobile light dummy loads?
A PDF of the schematics is here:
http://www.altair680kit.com/manuals/Altair_680-Schematics-03-Beta_1-Searcha…
The power supply is on page 2.
Thanks,
Bill