4 Jul
2006
4 Jul
'06
1:30 a.m.
From: der Mouse <mouse at
rodents.montreal.qc.ca>
Reply-To: "General Discussion: On-Topic and Off-Topic
Posts"<cctalk at classiccmp.org>
To: "General Discussion: On-Topic and Off-Topic Posts"
<cctalk at classiccmp.org>
Subject: I am now Annoyed [Re: Old oscilloscope help: ideas sought]
Date: Tue, 4 Jul 2006 03:53:00 -0400 (EDT)
I wrote about an old oscilloscope which was giving me trouble. After
some poking and prodding, I managed to get things tweaked back into
operation. Then I was hacking on some software, pursuant to driving
the 'scope as an X-Y display off a soundcard output, when I became
aware of the 'scope making small popping noises.
Small popping noises are not a normal operating sound for that 'scope.
I used it enough to know _that_. Then, looking over at it, I saw that
(at least some of) the tube filaments were dark, and there was nothing
on the display.
So I turned it off. The popping sounds continued, and I saw a wisp of
smoke. I unplugged it and turned it over in search of the part that
was upset.
Turns out it is the power transformer (not surprising, in view of the
dark filaments). It continued making popping noises well over a minute
after I unplugged it, and for a few seconds I was even mentally making
sure I knew where the nearest fire extinguisher was, since it gave the
impression it might actually catch fire.
It's now been at least ten minutes since I unplugged it, and the
transformer is still too hot to rest my hand on for longer than about a
quarter-second. And, it is giving off odours which I am also quite
certain are not part of normal operation.
Tomorrow, I'm going to try turning it on (very briefly, with a
current-limiter in series with the primary) to see if it's healthy
enough to give me useful voltage measurements. The only winding whose
voltage I am confident of is the one that drives the tube heaters.
Based solely on wire count I am sure there are at least two others -
and based on the application I expect at least one for the final anode
voltage and one for power to most of the circuits.
Anyone know a good supplier for ten-kg power transformers? :-(
/~\ The ASCII der Mouse
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I can't imagine that a transformer makes popping noises *after* the
power supply is turned off. You may have several faults in the scope.
First, a *hot* transformer core usually is an indication that lots of
current was drawn. That could be a shrot circuit between windings
of the transformer coil, but perhaps it is a short circuit external to the
transformer.
Check for fuses ...
The only thing I can think of that makes popping noises are the old
big power supply smoothing electrolytic capacitors, like 100 ?F/450V
or something like that rating. Those old caps are not hermetically
closed like the modern caps. They have a small stop in the bottom
which can open under pressure (from the inside). When that happens,
the cap is dryed and needs to be reformed or replaced. The opening
of that stop can be small (which could make the popping sound), but
it can also be quite spectacular. The stop is blown off, and all the
rubbisch inside the cap squirts out!
The cap would be a plausible explanantion why the popping sound
continues *after* the power is turned off...
Disconnect the secundary coils of the transformer to the circuits
behind it, and turn the scope on. If only the transformer now gets
jiuce (power), but it stays cool, the real problem is in the rectifying
stages. If the transformer still gets hot (without any load), yes, you
do need a new transformer, or remoev the old coil and put on
new windings ...
hope this helps,
- Henk, PA8PDP.
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