[757labs] tube TV's and old radios
kludge at panix.com
Sun Apr 1 21:40:24 EDT 2012
> The TV is a Motorola VT73. I get very light static noise when plugged in
> and from what I can tell all the transistors are lighting up.
The way TV sets work, the filaments are all in series across the AC power
line, so if they light up, you know all the filaments are good but really
nothing else. The horizontal oscillator is a relaxation oscillator and it's
powered directly off the rectified AC line. When you turn the TV on, the
horizontal sweep oscillator starts free-running, and _all the other_
voltages from the TV set are derived off the flyback transformer that is driven
by the sweep.
If the sweep oscillator does not fire up, you will have basically a dead TV
set because there is no HV supply for the CRT anode but also most of the
low voltage supplies will be dead as well.
When the oscillator starts free-running, you'll hear a whistle, and then
when the rest of the set warms up and a video signal is detected, the pitch
of the whistle will change as the oscillator locks onto the synch from the
video. If you don't hear a whistle the sweep is not running, if you don't
hear a pitch change it's not locking.
I don't have a sam's cross for the VT73.... is there a chassis number
stamped on it?
> The radio is a Franklin C-101 It is quite quiet normally but I did manage
> to get an Am station at a really low volume to tune in. With a cheap little
> FM transmitter nearby, nothing was being received by the unit.
FM receivers have a whole hell of a lot of gain in a "limiter" stage
that runs into clipping, and this produces basically a square wave
that goes into the FM demodulator. Because of all this gain, unless
you have a circuit that specifically mutes the signal between stations,
you get a big rush of noise between them.
If you're not experiencing that, your problem is at the limiter stage or
forward. And the fact that there's an issue with the AM as well would
indicate it would be even more forward.
First of all, clean the volume control pot, and clean the AM/FM switch.
That tends to fix a _lot_ of these problems right there. If that doesn't
do it, inject noise from a bench oscillator or from your thumb on a
screwdriver into the output of the pot. If you use your thumb, make
sure the thing is on an isolation transformer since the chassis is
connected to one leg of the AC line.
The audio tube on this one will be a 50C5 or 35C5 and the output of the
pot will go to the control grid on the audio tube. If the voltage on the
plate of the audio tube is 100V or so and there's signal on the control
grid, it should make noise. If it doesn't, check the coupling capacitor
which is apt to be an old paper one failed into a short.
Fixing stuff like this, the first goal is to identify the stage that is
the problem, the second is to identify what part of the stage is bad and
most of that identification has to do with taking voltage readings and
comparing them with the schematic. If you don't have the schematic, you
can look the RCA HB-3 tube handbook (which is online and a google search
will find it) which will indicate what the tube pinout is and what the
normal operating voltages for various kinds of circuits with that tube
Table radios are fun to work on. TV sets are annoying as hell because
all the voltages are sweep-derived. I went to college and got my EE
degree so I'd never, ever have to fix another TV set. For 25 years
I wouldn't even have one in the house I hated working on them so much.
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