6 Aug
2014
6 Aug
'14
3:39 p.m.
Once the shroud was out of the way, I found a small
circuit board with
two vacuum tubes plugged into it, and a number of other components, as
Do these valves [1] have top cap conectors, or jsut base pins?
[1] I am much more used to usign the UK term so I'll stick to it.
well as a transformer of some type, but definitely not
a flyback. I
suspect that the tubes and circuitry on the small circuit board are an
oscillator that is used to pump the transformer to generate the high
voltage that goes to the CRT.
Sounds very likely. A high-ish frequency oscillator (typically 10's of
kHz) dring the trasnformer (a sort-of SMPSU, but free-running) means you
can get way with a smaller transformer and smaller smmothing caps (maye
the capacitance between inner and outer coatings on the CRT) than one
runing at mains frequency. As a result it's also safer. The stroed energy
in such a capactior will make you jump, but noting more. The
mains-frequency sumpplies could kill...
Given this discovery, it added more confidence that it will be OK to
power the thing up without causing any damage.
I pulled out one of the tubes. It is a mini-style tube made by RCA.
However, it has no part numbering on it at all. It has 8 pins, and
8 pin miniatures are uncommoon. The common bases are 7 pin (we call it a
B7G) and 9 pin (B9A). The 8 pin one I cna think of has 8 equally spaced
pins, locaiton is done by a bump in the glasss o nthe side of the valve
which fits into a groove on the skirt of the socket.Nasty little thinbs,
it'sfar too easy to break that bump off, letting the vacuum out. We call
that a B8A ('Rimlock'), but I didn;t think it was used in the States.
Those valves (B8A, B9A) are about 3/4" diameter (and about 5/8" for the
B7G). Is that what you meena by 'miniature').
given its application, it's probably a simple
triode. The other tube is
Unlikely. Single triodes are not hat common. DOuble triodes are _very_
common.
difficult to get out without disassembling more, so
I'm not going to
mess with it. I do hope, though, that these tubes are good, as
getting replacements could be difficult if either or both are not good.
If you knew what they were, it wouldn't be too hard to get them in most
cases, but without a type number it is goign to be interesting...
[...]
When I went to 100%, I could clearly see the heaters
glowing (with the
room darkened) in the two tubes in the HV oscillator. I don't have a
high voltage probe, so I didn't try testing the HV supply to see what it
was running at. I definitely don't want to get zapped by trying to use
normal probes to check this.
Try fixing an NE2 neon bulb to an insulating rod and waving it around the
valves and HV transfoemer (not connected to anything). There's of ten enough
stray electirc field to light it.
-tony