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-----Original Message-----
From: [email protected] <[email protected]> On Behalf Of Chuck Harris
Sent: Tuesday, 3 September 2024 1:41 PM
To: [email protected]
Subject: Re: [HP-Agilent-Keysight-equipment] HP 141T Power Supply Help Needed

To amplify your comment, many times older linear supplies, when they are brute force recapped, with the best, long life, low esr caps you can find, will become beacons of chaotic oscillation.

I put a low ESR ceramic capacitor in the feedback loop of a unity gain OPAMP circuit the other day, and was amazed that it went totally unstable. It was an older LM301 variety, where the response characteristics of the opamp are a blank slate until the designer adds in the stability network. The designer's efforts were wiped out by my ceramic capacitor replacement for his 1uf back-to-back emulation of a non-polar electrolytic.

Sometimes the modern best replacement is not suitable for use in an older instrument.

-Chuck Harris


On Mon, 2 Sep 2024 22:40:20 +0100 "Dave_G0WBX via groups.io"
<g8kbvdave@...> wrote:

Hi.

It's just a generic term (damping is another) to slow down the
response of a loop, to prevent it going unstable.

Not an uncommon issue in some linear PSU's like those -12.6V or -100V
regulator topologies that are not "conventionally" laid out.

Unless you 'scope the thing, you often don't know it's gone unstable,
other than some passive parts sometimes cook for no obvious reason.
(That R49 for example! See Mike's comment in another mail.)

It is also not uncommon, when modern transistors are used to replace
older failed parts (that are not available any more) and the new
device has "a lot" more gain at HF than the originals, that regulator
or other control loops suddenly become "unruly" under some operating
conditions.

Sometimes a low value "Base Stopper" resistor (a few Ohms, or a
ferrite bead) in series with the new device's Base connection can
restore sanity!

Take care.

Dave B.