79 RCA model 630 ten inch TV set
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
79 RCA model 630 ten inch TV set
I found an RCA model 630 ten inch TV set at a flea market. The
power cord is shot, and when I pulled the chassis out, I found the wires
to the switch appeared to have had the insulation burned off. I found
that the 5U4 plates were melted together. I put in a new 5U4 and
plugged the set in, but it doesn't do anything---no picture, no sound.
What should I do now?
First of all, a TV set draws substantially more power than a
radio. Do yourself a favor and install a fuse in the primary power
wiring to the switch. Use a slow-blow fuse rated at about 150-200% of
the set's power consumption. For a set drawing 250 watts, a 4 amp
should give reasonable protection.
On a 630, there is a black box mounted on the left rear of the
set, with some power resistors inside. Open the box and check the
resistors. These are back-bias resistors, in the B- circuit. If they
are open, check all the filter caps. Replace the resistors, if
necessary.
Bringing up an old TV takes some care, and the order in which
you check things out is important. As with all old electronics, assume
that it has several things wrong with it. Check that the CRT heater is
continuous (ohmmeter)---you should be able to see it glow when you turn
the set on. The first thing to fix is the power supply. Once you have
good B+, and all the tubes are lit up, do you have a raster? If not,
check the horizontal oscillator and amplifier. Note that the horizontal
amplifier has very high voltages in it, and that some faults may cause
these high voltages to appear where they shouldn't be. Don't go probing
around in the horizontal circuit with the set turned on. Horizontal
amplifiers on magnetic deflection sets ran with voltage and current
levels appropriate for a transmitter, and several postwar sets continued
to use the 807 beam tetrode as a horizontal amplifier tube, rather than
one of the purpose-built tubes. Shut the set off, connect your probes,
then turn the set on, take your readings, then shut the set back off again.
Don't touch anything in the set without first assuring that it is shut
off, then touch an insulated probe connected through a 1K resistor to
ground to all of the terminals in the circuit to assure that there isn't
a high voltage charge somewhere. If the horizontal circuits are OK, the
1B3 high voltage rectifier filament will glow. Make sure that the high
voltage cable isn't shorted somewhere, and that there isn't a lot of
dust or crud to bleed off the high voltage---problems here are usually
pretty obvious in the dark, where you can see corona discharges, arcing,
and other leakage problems. Unless you have equipment of measuring
10KV, you can't measure the high voltage directly, but if the 1B3
filament is lighting, and the flyback plate winding to the 1B3 is not
open, you probably have high voltage. If you have high voltage, and the
tube does not show any light (this may be a spot or a line, rather than
a raster), check the CRT grid-cathode bias voltage---once again, keeping
hands completely away from the CRT socket unless the set is turned off
and you've grounded terminals through 1K. The brightness control should
be able to swing the voltage back and forth from about -20 to -60 volts.
Check grid 2 voltage---should be around 250.
If you have a horizontal line on the CRT, you are not getting
vertical deflection. Check that the oscillator is oscillating, that the
output stage is operating.
Once you have a raster, then you can start debugging any
problems in the video and audio circuits. Prewar and early postwar TV
sets trapped the audio right behind the tuner and used separate IF
strips for video and audio. Later sets use "intercarrier" IF's, with
one IF strip and a sound trap at the end of the IF chain. In either
case, "raster, no picture, no sound" means that the problem is between
the tuner and the sound trap. "Picture, no sound," or "sound, no
picture" means the problem is after the sound trap. Don't fuss with the
tweaks on the IF strip (strips) unless you have the proper equipment and
instructions for doing an alignment. Unlike most radios, these are
stagger-tuned, and you don't just "tweak them up" for best performance.
The video comes from a conventional AM detector and a "high fidelity"
voltage amplifier, connected to the CRT cathode. Note that the bandpass
of the video amplifier is very wide, and the term "video amplifier" has
become a generic term from a wideband untuned amplifier. The audio is
through a conventional ratio detector and single-ended audio amplifier
to a (incredibly cheap setup for something that cost $400 in '46) small
speaker.
One fairly standard complaint is loss of raster sync. If the
tubes are OK, this is generally the paper capacitor bugaboo at work.
Loss of both horizontal and vertical means that the coupling out of the
video amp has a problem. Horizontal sync comes from differentiating the
video signal, and vertical sync from integrating the double-speed
interlace "trick" pulses that ride on the "pedestal" portion of the
video signal (the vertical sync portion).
These are some basic things about forties TV sets. Note that
the CRT's on early magnetic deflection sets had offset guns and "ion
trap" magnets. This was to prevent burning a spot in the center of the
CRT. Around 1948, the aluminized phosphor coating, which was impervious
to ion burns, went into production, eliminating the need for offset
guns. If the ion trap is misadjusted, the electron beam won't be aimed
at the phosphor screen properly, so the raster will be dim or
nonexistent, or have "neck shadows" at the edges. This, like the IF
tweaks, another "if it's working, don't fix it." Electrostatic
deflection sets that used tubes like the 7JP4, did not have ion burn
problems, so are mounted with nothing on their necks. These sets also
did not require transmitter-like power for horizontal deflection, so did
not have high voltage derived from the horizontal circuit. Instead, a
separate RF oscillator was used. CRT circuits in electrostatic
deflection sets are quite similar to oscilloscope CRT circuits.
There are several books on servicing television sets that
generally apply to forties sets, although they are generally oriented
toward later sets. Compared to later sets, most forties TV sets were
powered through transformer supplies, did not have any tricks like B+
boost.
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