In article , David Looser
wrote:
"Jim Lesurf" wrote in

During the first years of manufacture the ampliers/recievers in the
Armstrong 600 range used a thermal delay relay to avoid a surge when
the unit was switched on.


I remember it well as I had a 600 with one in. To me it appeared to be a
cheap & nasty little thing that failed very early on. I simply shorted
it out and the receiver continued to work for many years without it.

In that case you were fortunate to get away with not realising what other
changes needed to be made! Sometimes the Ghods forgive those who make
changes in ignorance of the likely consequences. The main point of the
delay relay was that the bridge diodes that had been used had too low a
surge current rating. So bypassing the relay without also changing to new
diodes was increasing the chance of PSU failure.

These were expensive.

Maybe, but it was still cheap & nasty.

Odd that you haven't noticed that 'expensive' and 'cheap' clash here. :-)

However I tested a number of them over a period of time, and compared them
with alternatives. I note your opinion based - presumably - on appearance,
though. :-)

And they were one of the main failure modes which brought sets back
for repair. One of the reasons I avoid physical relays.

But a thermal relay is a totally different animal from a conventional
electromagnetic relay. They have very slow make & break of the contacts,
and the characteristics of the bi-metal strip changes with age.

You may need to think more carefully about the application in the example
before assuming that is relevant. You could also check to see if the relay
was such that it flipped state with some hysteresis.

Your original PSU didn't need it either IME. :-)

As I said, you were lucky. I probably saw more of the sets than you did.
:-) Without the relay the surge peak applied was well above the rating of
the original bridge diodes. ...And the orginal PSU wasn't actually mine. It
was designed by Ted Rule. I just changed it a few years later to be cheaper
and far more reliable.

The particular problem was easily solved simply by using high surge rated
diodes. So far as I was concerned any mechanical relay at all was expensive
and nasty in such a situation, and easily avoided with far more reliable
solid state devices. Hence my mentioning it in this thread.

Slainte,

Jim

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