Yes, I hit one of those. I forget the frequency other than around 1 MHz, but it was pretty warm when it shouldn't have been, and it was several volts amplitude.

This thing I'm chasing is much more subtle.

-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@febo.com] On Behalf Of Chuck Harris
Sent: Wednesday, December 7, 2016 1:57 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Totally unrelated, but..

I don't think so.  I first ran into a batch of LM340-5's that were excellent oscillators back in the 1970's... long before counterfeiting was even remotely possible.

The symptom is the regulator puts out only 4.5 out of 5V.

LM309's were, however, totally immune.

Usually, I had to be really bad to make it happen, things like using clip leads between the power supply and load with the LM340-5 dangling in between.

The answer is as simple as a couple of 0.1uf ceramic caps soldered right at the input and ground, and the output and ground pins.

LDO (low dropout) regulators are very susceptible to oscillation.  They need to have a couple of hundred uf of good quality capacitance right on the input and output leads.  Where people usually get in trouble, is in not knowing that electrolytic capacitors lose most of their capacitance as the temperature starts hovering around 0C.

The circuit works great on the bench, but fails when out hanging on a light pole...

-Chuck Harris

Joe Leikhim wrote:

time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.

One has to be very careful with LDOs.  As we say in applications,
RTFDS.  :-)  The output capacitor often needs a finite ESR (effective
series resistance).  A too-large ceramic cap WILL make them oscillate.
I have noticed that they are used in some GPSDOs.

David

On 12/8/16 11:39 AM, Van Horn, David wrote:

In message <CY1PR04MB2347AE1EF637565B5B4D0C2FE0840@CY1PR04MB2347.namprd04.prod.
outlook.com>, "Van Horn, David" writes:

How long are the wires supplying the regulator ?

The LC of the wires inductance and the shunt capacitor over the
regulators input has a resonance frequency...

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Never attribute to malice what can adequately be explained by incompetence.

HI

Ok, so to fully evaluate the problem, you will need to de-encapsulate the parts. That is the next step
in this adventure. First pass would be to pull the plastic off of a couple of bad ones and get some pictures
of the die. After that, trace them back and see if they are Brand A or Brand B. Next, do the same for a good
one (or two) and see if the die trace to the same manufacturer.

Next after the die are traced down would be probing with something like a Cascade probe system. You can
get them that are alleged to be good into the > 20 GHz range. Team that up with some low noise test gear
and you will be able to rule in / rule out the regulator it’s self. Yes this is a bit expensive. If you want to troubleshoot
the issue, it is the next step.

It’s a whole lot easier to swap out the regulator for a different one and just move on.

Bob

I am pretty sure that I ran across this once as well with the
7800/LM340 series in the early 1980s with parts from a major
manufacturer like Motorola, National, or Texas Instruments; the
regulators ran hot and the output voltages were slightly low no matter
what decoupling arrangements were made at the input and output.  The
stability was poor as if the parts were oscillating but no oscillation
was apparent.

There is some discussion here about different designs for these
regulators although the photographs are broken:

http://www.righto.com/2014/09/reverse-engineering-counterfeit-7805.html

On Wed, 7 Dec 2016 15:56:55 -0500, you wrote: