Help with 9T246 Horizontal Linearity

[dtvmcdonald]

I think I should start a new thread for this problem, here in the right place.
I got an RCA 9T246 at the 2013 ETF conventions. Its recapped and working
quite well except for a very bad horizontal linearity problem. The horizontal area
has been partially re-resistored.

The picture is stretched at the right, which extends off the screen. The
left side is squeezed up and slightly wrapped around (i.e. the image starts
appearing during retrace.) There is a blank space of about 1/2 inch or so
at the left side. The nonlinearity ratio is therefore huge. The width control
was lacking a core so I tried two different ones, both of which work. However,
neither the width nor horizontal linearity controls have a large
effect if adjusted, though they do have some small effect. The previous
owner said that the horizontal linearity coil was getting hot, but I checked it
and at most its getting very slightly warmer than that general area of the set.
When I first turned it on there was a slight "hot coil" smell, but that
went away before I could sniff out the source.

I checked that all capacitor values are per the Sam's schematic and checked
all the resistor values I could around the horizontal area, and they are
correct. I checked the waveform between the oscillator and output tube,
and its as perfect shape as can be. I can't tell if its the right size since
Sams gives only the shape, not the voltage. Finally, I noticed that the
horizontal drive control, a shunt trimmer capacitor from horizontal output
tube grid to ground, is wide open (i.e. adjusting it makes the width smaller
and the wraparound worse.) This latter makes me think that a possible
problem is that the drive for the output tube is too small.

What should I try next? I think I can come up with replacement output
and damper tubes. Would it be worthwhile to try replacement width and
linearity coils if these can be obtained, or even generic replacements if
somebody knows the approximate values? These same coils seem to
be used in lots of RCA sets of that era. Would measuring the waveforms
and/or voltages at different places around the horizontal output be helpful?
Could (heaven forbid!) it be a bad flyback? One more idea ... could some the
values on the Sams schematic for this group of sets be wrong for the
particular 28C chassis that the 9T246 uses? The two capacitors on either
side of the linearity coil have values identical to those on the 630 schematic,
as do the listed width and linearity coil resistances.

Doug McDonald

[dtvmcdonald]

Well, I answered this one easily ... and learned a lesson:

If you can't EASILY read what the type a tube is without taking it out,
TAKE IT OUT! I had looked at the tubes before, but apparently not
carefully enough.

The horizontal output tube had a hard to read type on it. I had
brought home the correct tube (6BG6G) thinking that the tube in
the set might be bad ...
but it was a 6CD6GA! This tube is in no way a proper replacement.

Now it works fine.

Doug McDonald

[Electronic M]

(Slaps forehead) Darn! I should have subbed the HOT(I have a couple of 6BG6G tubes sitting around) before I gave up trying to figure it out.

Nice work. Usually when I hit a wall on something it is me being blind to the obvious just as is the case here.

[dtvmcdonald]

A new problem came up after I put it back in the cabinet: the
picture is rotated. This means the case is magnetized. Luckily I have
a degausser ... if I can find it.

Doug McDonald

[dtvmcdonald]

The saga continues!. I am letting it cook an hour a day. Last night the
picture started shrinking and then jumping back. Today
it started jumoping into severe horizontal ringing, then shrinking horiontally
but expanding vertically, then going dark, and in a minute or two coming back.

This was traced to the 6.8k HOT screen resistor, one of the few
old style 1940's (2 watt or so) power resistors left. When removed
and tested with an ohmmeter it was open, so it must have been arcing
internally. Replacing it fixed the set.

Is this sort of cook-and-fix par for the course?

Doug McDonald

[Kamakiri]

Don't feel too bad. As you can read in my other thread, I ended up replacing the cap on the horiz linearity coil with the wrong value because I ended up having the wrong Sams.....and was scratching my head as to why I was losing sync lock

[Kamakiri]

Was it shrinking horizontally? And was it a total raster shrink, or only on channel?

[dtvmcdonald]

It was shrinking horiontally (losing horizontal size as well as severe horizontal
ringing of the raster, not the image) and expanding vertically and getting dim
(i.e. losing HV and hence blooming). The horizontal drive, on a scope, was
perfectly stable. The waveform at the HOT cathode was showing a current drop
and a phase shift. The waveforms on either side of the horizontal linearity
coil were changing size and phase and showing odd ringing. None of this
depended on presence of signal.

What was really odd is that it was working with an open (to an ohmmeter)
screen resistor on the HOT. The culprit resistor had at some time gotten very
hot, as its markings were obliterated.

Doug McDonald

[Kamakiri]

This is interesting, because the ONLY issue that my set has, I haven't been able to ferret out. I'm not exactly sure if I'd call it an issue, but I guess it is.

My set has a slight horizontal shrinkage, but only on channel....with snow, it's full raster. It's maybe 3/8" on either side, but it bothers me. It's stable as heck though. Can't adjust it out.

[dtvmcdonald]

The only issue left with my set is a small vertical shrinkage on the left side.
that is, at the left the picture is about 1/8 inch less tall than on the right side.
Other than a yoke problem, or (a real long shot) horizontal scanning waveforms making
it into the vertical signal, what could this be?

Doug McDonald

[Electronic M]

Could be a focus coil or yoke position issue.

[dtvmcdonald]

I was just thinking last night .... all old pre-aluminized B&W sets seem so
extremely dim. The ones I saw at the ETF convention were all about the
same brightness, no match for a modern LCD set. OF course even in 1949
a CRT could be very bright (e.g. the ones used in projection sets).

So I got out my camera and measured the brightness of a white area
on my LCD and the 9T246. It was 6:1.

Then I looked at the specs for the LCD set and the 10BP4A.
comparing the peak brightness spec for the 10BP4A to the spec for the
LCD ... they are identical! Is the LCD set being speced for
say an 18% gray? That would make the measurements match well.
Does anybody know?

Doug McDonald

[David Roper]

Holy hat racks, I think the coil that was getting hot must have been the 6.3 volt filament winding!

Because this set has direct-coupled video, the brightness has a "correct" setting, which is that a fade to black should result in virtually no light on the screen. Set that way I can watch mine in essentially broad daylight--as long as the sun isn't falling directly on the screen, of course.