73 I want to plug it in. What do I look for?
Description
This article is from the Antique Radios And Phonographs FAQ, by Hank van Cleef vancleef@netcom with numerous contributions by others.
73 I want to plug it in. What do I look for?
OK, I've checked that the tube heaters are continuous, that the
filters are OK, and generally walked through and done the visual and
ohmmeter inspection. I want to plug it in. What do I look for?
This is the moment of truth, even for an old grey-hairs. Fortunately,
tubes will take abuse that transistors won't tolerate. But you want to
have your eyes and ears wide open, and be prepared to shut the thing
back off instantly. Some people like bringing them up on a Variac,
which is an expensive piece of equipment unless you are in the
restoration business. So I'll assume you are going to plug the thing
into the 100 volt line, turn it on, and see what happens. Make sure you
have some sort of antenna connected on sets without a built-in loop.
a. On AC-DC sets, turn it on. The tubes should light up, and in 10-15
seconds (when the rectifier and power tube heaters warm up) you should
hear 60 cycle hum in the loudspeaker. Indeed, hum is a built-in feature
of these sets. If it is overwhelming, you've got a bad filter cap.
Check for smoke signals and signs of overheating. If you can tune in a
station, you are probably in business. On 35Z5/35W4-type radios, if the
pilot lamp burns out after the set warms up, you've got a short in B+
somewhere---probably a shorted filter cap. Turn the set off and find
the problem---if you've got a short, the rectifier heater will take the
load and burn out after a while.
b. On transformer sets, I like to connected a 600 volt DC meter across
B+, preferably in the supply to the IF screen grid or plate. If the
rectifier is a filament type (80, 5Y3, 5U4, etc.) you'll see full B+ a
couple or three seconds after turning the set on, and it should drop to
about 100 volts on the IF screen when the cathode tubes warm up (around
10 seconds). Check for smoke signals, burning, and that all the heaters
glow. A low level of 120 cycle hum is to be expected, though a really
fancy set will give almost no hum at all. Once again, if you can tune in
a station, you are probably in business.
Watch in particular for a violent purple glow in tubes, particularly the
power output and rectifier, plates beginning to glow red, and other
signs that there is a short circuit. If the radio doesn't play, keep a
close watch on things, although if you have good B+, no gassy tubes, and
no red plates, and things are OK after five or ten minutes, you are
probably safe in continuing on to do trouble-shooting. A few sets use
tubes with mercury vapor in them, which normally glow purple between
the elements. Typical are the 83 (not 83-V) and 0Z4 rectifiers, and
the gas-discharge VR tubes (0A2,0C3, etc.).
Trouble-shooting. If all the tubes light up, you've got B+, and no
smoke signals, you can begin your walk through the radio. If the radio
is completely dead---no stations, no static---try rocking the bandswitch
if the radio has one. Also, the volume control, any tone controls, etc.
I've found that on ancient sets, it's a good idea to walk right through
and do voltage checks everywhere, no matter how well the radio seems to
play. If you have a schematic with voltages marked on it, so much the
better, although some of the voltages given by manufacturers can
disagree rather markedly from actuals that can be figured by
reverse-engineering the design.
a. Power output stage: Check screen and plate voltages. These should
be close to B+ at the rectifier. Check for positive bias voltage at the
cathode on self-bias circuits or negative voltage at the grids if
separate bias.
b. Audio amplifier. Usually a triode. If the 6SQ7 diode-amplifier
type, the only thing to check is plate voltage, which should show a drop
across the plate resistor. On resistance-coupled output circuits, make
sure the coupling cap is not leaking current to the output tube grid
circuit, which will pull up the grid voltage and make the output tube
plate(s) glow red. Probing the AF amplifier grid generally shouldn't
show any voltage, but should make plenty of noise in the speaker.
c. IF amplifier. Check for screen voltage. If you don't have any,
you've got a shorted bypass cap and a dead radio. Plate voltage should
be near the supply voltage (generally fed by a blue wire to the 2nd IF
transformer). Cathode should show some bias being developed (i.e.,
plate current through the tube). The grid will generally show the AVC
voltage, though your meter will shunt a lot of it, unless it is a
high-impedance type, such as a VTVM.
d. Mixer. If the pentagrid type, tetrode, or pentode, check screen
voltage. Check for proper bias voltage on the cathode.
On voltage checks: if you have a schematic and voltages, these can be a
general guide to the voltages you should see on the tube elements. If
you don't have voltage measurement data, most of the tube manuals give
standard values for circuit DC levels as "typical operation." Most
designers used these "typical operation" values in circuit design.
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