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
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
- 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.
Yes. All agreed. This is basically the first set of tests that I was
suggesting, which Tony has usefully now converted to specific measurement
points for you.
Just to try to help you understand the points about the regulators that we
were making. Without being too 'blanket', there are two types of linear
regulator chips in common use. These are fixed, and variable. The fixed ones
are typically three pin, IN - GND -OUT, and come in many different voltages
eg 5v 8v 9v 10v 12v 15v and so on. They come in both positive and negative
flavours. In general, they don't care too much whether their output pin is
loaded or not, but they do have a minimum load figure specified to
*guarantee* regulation. Also, disconnecting the output pin from the board,
will likely isolate it from a small decoupling cap which will usually be
present, and this might, under some circumstances, lead to the regulator
The variable ones also often have just three pins, such as the LM317, but
sometimes have more. The main pins that are generally of any concern, are
IN - SET - OUT. The output voltage is determined by a potential divider
comprising two resistors, one strung between OUT and SET and the other
between SET and the circuit common ground. By altering the ratio of the two
resistors, any voltage to within a couple of volts of the input, can be set
as the output. Clearly, for this setup to work, the 'top' resistor of the
voltage-set divider, *must* remain connected to the output pin. What Tony
was saying, is that if you just disconnect the output pin from the board,
you will also be disconnecting it from the v-set resistors, which could lead
to all manner of nasties ...
The good thing about these regulators is that they are pretty bomb-proof.
They are safe area protected, short circuit protected and thermally
protected, and will go into a voltage foldback condition if any anomalies in
their operating conditions are detected. That is not to say that they don't
fail themselves. They do. But often, the measured conditions, particularly
the parameters measured by the Mk 1 finger-thermometer, are a big clue to
what's going on. Example. 5v fixed regulator - say a 7805 1 amp version.
Measured output voltage 1.8v. Measured device temperature, using Mk 1 finger
= Ouch. Conclusion ? More than an amp is being drawn, the device has
overheated, and gone into thermal voltage foldback, to keep within its SOA.
Example 2. Same regulator, same set up, same measured ouput voltage.
Measured device temperature = cool. Conclusion ? The regulator device itself
is faulty. Before there are lots of cries of anguish, a bit simplistic, I
know, and yes, there could be other reasons for the measured conditions, but
those two examples cover probably 90% of problems around those regulator