4 Jul
2006
4 Jul
'06
3 p.m.
So I turned it off. The popping sounds continued, and
I saw a wisp of
Possible insulation breakdown somewhere, still discharging a charged
capacitor?
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
I would disconnect all the secondary wiring and power it up with a mains
light bulb in series with the primary (a well known way to do this is to
connect the bulb across the fuseholder with the fuse pulled). The bulb
should stay dark (if not, you've got breakdown or shorted turns in the
transformer), you can then measure the output voltages.
I would expect some of the windings to be centre-tapped (it's a fair bet
this thing uses the 'biphase' full wave rectifier circuit). The 2 halves
should be the same voltage, of course.
voltage I am confident of is the one that drives the
tube heaters.
It's not unheard-of for 'scopes to have series-strings of valve heaters
run off fairly high voltage stabilised DC lines. Well, at least Tekky did
this sometimes.
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.
Most 'scopes run the gun electrodes at a high (a few kV) -ve with
respsect to the chassis. That means the deflection plates can be run at
somewhere near chassis potential (a few hundred V above it), so they can
be easily connected to the deflection amplifiers. There may be a
post-deflection-acceleration electrode, which will run at several kV +ve
wrt the chassis, although this may not be present in your 'scope.
It's also not unheard-of for the EHT to come from an
oscillator/transformer circuit run off the normal HT (B+) line, not
straight from the mains transformer.
You said some of the valve heaters were out. If that means some were stil
alight, then there may well be 2 or more heater windings on this
transformer, one of which has failed (or has been shorted out by a fault
elsewhere in the instrument).
Anyone know a good supplier for ten-kg power transformers? :-(
Rewind it yourself?
-tony