4 Jul
2006
4 Jul
'06
5:06 p.m.
On Tuesday 04 July 2006 04:46 pm, Brent Hilpert wrote:
Welcome to the world of old tube equipment. :/
Yeah...
That popping sound is usually the sound of boiling tar
in the transformer
(and of course the latent heat keeps it boiling after power is turned off).
The symptoms you describe are not unusual and it's most likely a short in
the transformer. If, as you say, only some of the tube filaments were dark,
it may be that there are multiple filament windings and one of them
developed a short.
In equipment of that sort, shorted caps don't usually present such a degree
of problem to the transformer. The vacuum-tube rectifiers have some
hundreds-of-ohms of plate resistance, as well as maximum emission (current)
limits, and act as a limiting impedance for shorted/leaky caps.
If it's any consolation, there probably wasn't anything you could do to
avoid the failure.
I pulled out the schematic for a Stark OSK-4, a very basic vacuum-tube
'scope, probably similar in category to the DuMont. Here are some
observations which may (or may not) be of assistance in your situation, if
you are RE'ing the power supply:
- The CRT accelerating potential does not come from a high positive
supply for the anode, rather it is a high negative supply for the cathode.
It is obtained through a half-wave rectifier, supplied by a transformer
winding internally stacked on one side of the main B+ winding. The filament
for the half-wave rectifier is obtained from an additional tap on the
high-voltage winding.
- The CRT has it's own filament winding, as the cathode is operating at a
high potential and the cathode-filament insulation potential is limited.
(In other words, the transformer is providing the high-potential
insulation.)
This much agrees with much of my experience as well.
- The B- (center-tap) does not go directly to
ground, rather it goes
through some resistance to ground so as to develop a negative supply (for
cathodes of the V input amp.)
I don't particularly remember anything like that, but...
If there's no rush and you're willing to wait
on a possibility I can poke
around the radio museum here in the next couple of weeks to see if there's
a junkered/junkable DuMont (let me know the model number). We have piles of
old vac-tube scopes that we should get rid of and I know I've seen a DuMont
or two. Shipping (cost) would be from Vancouver.
I know that of the three units I have in storage that don't work, one is an
Eico 425, one is a Paco SS-50, and one is a Precision S100 (?). One of
those needs a transformer, and one needs a CRT. Unfortunately I don't have
access to the storage unit until I pay them off significantly more money, so
I'm not in any particular hurry either.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin