"robb" wrote in message
...

wrote in message
oups.com...
Ignore all that.

1. As Arfa suggested, isolate the L387 output from the 5V rail.
(Probably doesn't matter if you lift Pin 5 or disconnect some
other way.)
2. Replace the 5V fuse (and only that fuse? or do you also need
the 30V fuse to feed the L387?).
3. Power up.
4. If the fuse didn't blow, measure the voltage from Pin 5 to
Pin 3
of the L387 (5V OUT to GND).
5. If you got ~5V, reconnect the 5V rail and test again. If
not,
connect a 470-1k ohm resistor from Pin 5 to Pin 3 and test
again.
6. If still no 5V, measure the voltage from Pin 1 to Pin 3 (Vin
to
GND)
- it should be ~30V.
7. If ~30V was present on Pin 1, the L387 is bad. If not, the
30V
bridge is probably bad.
8. If the fuse(s) blew, test the resistance of the bridges.

TM


hi TM,
I tested the L387 and i think it must be bad as i get more than
5v from the Vout ?

i had already desoldered it from some previous suggestions about
isolating the different supply rails

so i tested by connecting 5V/1A DC source between Vin and
GND-Pin3 and i checked the voltage with DVM between Vout and
GND-Pin3 and i was getting 3.47 V

i changed to a 18 v/3.5A DC source and i was measuring 8.67 V
between Vout and GND-Pin3

L387 is cool
and when i connected an LED load to Vout the L387 got very hot
very quickly
so i suppose i at least need to get an L387 ?

thanks again for help,
robb


From that, it sounds as though the '387 is not doing its job correctly, so
replacing it would probably be a good move.

Arfa