On Thu, 14 Jul 2016 18:53:25 -0700, Jeff Liebermann wrote:

I can't argue with that, except that several people and the data sheet
have mentioned that the LM317T doesn't like light loads and has a
minimum load current specification. Since this is your problem, not
mine, I can only offer suggestions and things to try. Since your
regulator doesn't like a light load, kindly put a load resistor across
the 12V line and see if the situation improves. (Hint: When
troubleshooting, I like to do easy things first. A resistor is easy).

I did try that resistor idea before, but it didn't make any difference.
But it was 1500 ohms, so maybe too light to produce the desired effect.

I've now installed a 100uF electroylic in parallel with the existing
100nF ceramic on the output, but it's made no difference I'm sorry to say.

Incidentally, it is possible to damage an IC to the point where it still
functions, but no in the manner commonly expected. I've done this
numerous times with RF power transistors, where there are a bunch of
transistor in parallel inside the packages. The LM317T probably doesn't
have this exact problem, but might not appreciate having some holes
blown in the IC's insulating layers. It's not easy to test for this, so
just replace the LM317T with one that has a good chance of working.

OK, your 'partially-blown theory' is noted. I'll try replacing it if all
else fails as it's a bit tricky to get at.


I'm also wondering if there's something odd about your construction
methods.

Yeah, I'm aware of the importance of good layout so we can't attribute
the issue to that in this instance.

As for the 555, it allegedly draws very little current, but it doesn't
hurt to check. Insert a 1 ohm resistor in the PS line and measure the
drain with a differential input scope (i.e. using 2 scope probes). Look
for giant current spikes and high freq oscillations, that don't belong.

I do actually have a current probe somewhere. Anyway, before I try any of
that, I'm going to give these earlier suggestions from you and others a
chance:

a) Load it down with a lower-value resistor in parallel with the
electrolytic.

b)Insert bypass cap to the 'adjustment' pin

There is also the more remote poss that the regs I have a fakes. There
are fake regs out there and I've had some myself off Ebay, but I'd be
surprised if that were the case here since all the fakes I've heard about
can barely provide a few tens of miliamps. I shall order some fresh ones
from Farnell in a moment or two, though, just so I have *known* good
reference regs for comparison purposes.