28 Feb
2007
28 Feb
'07
3:36 p.m.
Some of you may reacll that, back in July, I wrote about an
oscilloscope whose power transformer failed in service. Well, today I
was going through my tuit collection and found some round ones. I've
now clipped the transformer free of the circuit and removed it.
I then applied an ohmmeter to the transformer, now that it's free of
the machine, which gave me good guesses at what winding is what. Then
I got out the current-limiting light bulb rig and hooked the primary up
(well, half the primary; it has a split primary, for 115/230 mains
switching). Then I measured the primary voltage and the voltages on
various interesting windings. Based on all of this, I think I have a
good guess what's what:
- Input mains: four wires, two windings, which are connected in
parallel for 115V operation and series for 230V operation.
- 5VAC filament winding for B+ rectifier tube.
- ~800VAC CT (400-0-400) winding which is rectified for B+. (With 26V
on the 115V-nominal mains primary, this measured 176V, or ~778V when
running normally.)
- Filament winding for one HV rectifier tube.
- Filament winding for the other HV rectifier tube.
The type of those valves will give you a good idea as to the voltage
required of course.
Almost certainly the EHT rectifier vavles are directly-heated, so the
filament will be at some high voltage wrt earth. These windings will need
good insulation.
- HV supply winding.
- 6.3VAC CT winding to run the heaters for most tubes. (There are some
12V-heater tubes, but they all have centre-tapped heaters.)
I assume things like 12AX7s...
It's intereting. for vavles with centre-tapped heaters, the US code gives
the voltage for the 2 parts in series (as 12AX7), whereas thte European
code gicves the voltage for parallel connections (ECC83 -- E = 6.3V heater)
- There is one more wire unaccounted for. On opening
the transformer
case, I find it is a case ground.
It may also go to an inter-winding shield.
The HV supply winding appears to be fried. I didn't bother with all
the filament windings (though I did measure the heater winding, which
appeared to be intact); the B+ supply winding seemed intact, but the HV
supply winding showed ~2V instead of the 200-300V I would expect with
~25V on the primary.
My first concern is that the primary voltage is very low -- in other
words this transformer draws excessive input current even when unloaded.
This suggests to me shorted turns, probably in the HV winding (which
would alos explain the very low outptu voltage there.
So now I'm thinking of finding a half-dozen different transformers to
run the six different pieces off of....
A couple of ohter thoughts. You might consider rewinding the original
transformer, although getting the insulation to withstanda a few kV would
be 'interesting.
I don't know how easy it would be to remove the shorted HV winding (is it
near the outside of the windings?). If you can get it off and put the
rest of the windings back with the right insulation, you could keep the
exisitng transformer fo the heaters, HV rectifiers filaments, B+ line,
etc and only have to provide the HV input. Maybe a seprate transformer
(or a transformer + voltage multiplier) for that.
-tony