5 Jul
2006
5 Jul
'06
1:10 a.m.
For those of you following my Dumont oscilloscope saga (based on an
offlist email, that set of people, while perhaps small, is definitely
nonempty :-)....
I just now tried hooking power up to it with all the tubes pulled
(including removing the CRT's socket from the tube) and a (40W) light
bulb in series with the mains.
The light bulb came on, to the eye about as bright as full power. A
voltmeter across the transformer primary showed about 30V (vs about
110-115 if I have the 'scope's power switch off). The clip-on ammeter
shows nothing at all until I wind the wire around to give it a x5
multiplier, at which point it shows 1.2A, meaning actually about 1/4A.
So either the primary is very sick, or some secondary winding is
shorted and is loading the primary.
I then tried something else. I have a filament transformer which
produces (nominal) 12.6V centre-tapped at 1.2A (according to the
markings on it). So I hooked half its secondary across the heater
terminals of a convenient tube (of the 6J6s) and applied power (with
that light bulb in series with the mains, to be sure).
The bulb burnt dimly. (It is completely invisible if I leave the
secondary open; this filament transformer itself seems to be OK.)
Measuring the voltage across the primary, it had sunk to about half
mains voltage; a clip-on ammeter on the secondary showed it pulling
about 3.5A. (This is with all the tubes completely removed from their
sockets, CRT included.) A dead short across the secondary carries
about 4A. The voltage across the secondary - nominally 6.3V, and that
is what I get with the oscilloscope disconnected - is about 1V; a lot
is being lost in the transformer (not surprising, as it's carrying some
two and a half times its rated current).
The winding that provides the plate supply for most of the 'scope
measures about 12V. Since it is rectified by a 5Y3GT and then fed to a
0B2, it must be designed to produce at least 75V or so, probably more
like 90 or 100. If we assume the fried transformer is acting normally
as a transformer for the two windings in question (admittedly a large
assumption), this is right at the margin of reasonable: it's getting 1V
instead of 6.3V on the heater winding and is producing 12V on the
medium-voltage winding, for 12V*6.3V/1V=75.6V nominal. This is not
quite enough to make the 0B2 regulate, even ignoring the drop in the
5Y3GT, but is in the right ballpark. However, with the same setup I am
unable to measure any voltage whatsoever across what I think must be
the high-voltage winding, the one that's rectified with two 2X2As to
produce the HV for the CRT. Perhaps I'm wrong about which wires are
the ends of that winding, or perhaps it's failed open....
I shall be investigating more, but it's now late enough I need to call
it quits for the night.
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