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VINTAGE TELEVISION
By RODNEY CHAMPNESS, VK3UG
The AWA P1 portable 11-inch
B/W television set
This month we have changed the heading on
this article to reflect the change in subject
material to television as we deal with one of
the early portable TV sets from the valve era.
Portable valve TVs are good collectables
because they don’t take up a lot of space.
Back in the valve black and white
TV days, most sets were great lumbering beasts which often needed
two hefty chaps to lift and shift them
from place to place. Some were just
moveable with one person providing
they didn’t mind the risk of getting a
hernia. As time went by a few small
portable sets made their appearance
in Australia.
One was a General Electric – a set
modified slightly from American
standards and powered by an external 240V to 110V transformer. It was
a beast to get at and as a result I took
quite a dislike to servicing them. On
the other hand, AWA produced a similar-sized set in the P1 series of the late
60s. It was easy to disassemble and get
at, and with practice it was possible
to replace a picture tube in just 15
minutes. It was a nice set to work on
and it worked well too.
I had one of these for quite a few
years – in fact it was our one and
only set. The drawback was that you
needed binoculars to see the screen
if you sat too far away! I sold it and
a few years later bought it back again
when I got interested in vintage radio
and television. I also managed to get
four more defunct units, so restoring
at least one to working order was a
good bet.
As I suggested back in the November 1999 issue, if you intend to restore
valve black and white television sets
to work
ing order it is desirable to
have more than one available. The
reason for this is that picture tubes,
horizontal output transformers and
deflection yokes in particular are
virtually unobtainable.
Restoration of television sets to
working order is a rather different task
to restoring valve radios but with care,
there is no reason why this should not
be just as successful.
Restoring the P1
The AWA P1 before restoration. It is a good candidate for restoration since it
does not take up a lot of space.
82 Silicon Chip
Removing the cabinet back is a
snap. The set was tipped onto its
front with a blanket on the workbench protecting the front. The four
Nylon screws securing the back were
removed and the cabinet lifted upwards. The aerial terminal board was
This rear view of set shows how the PC board swings out, allowing easy access
to most items for service.
unclipped from the turret tuner and
the whole of the set was exposed; five
minutes work at most.
The set was sat upright and three
screws around the edge of the printed
circuit board were removed. The coaxial cable was unclipped from near the
picture tube and the board swung out
to gain access to most of the set, which
can be seen in one of the photographs.
A general clean-up and dusting out
were in order. TV sets attract lots of
dust due to the high voltage used on
the picture tube; 11kV in this case.
A number of grubby sections responded well to washing with a kerosene-soaked rag and now look good.
I removed the valves and washed
them in soapy water, then rinsed
them in plain water. They come up
well. I keep the bases of octal valves
out of the water as it will get into the
base and take some time to dry or
may make a conductive path inside
the base between pins; a good way to
ruin a valve.
The turret tuner was likely to need
some attention on the channel change
contacts, just as wave-change contacts
in dual-wave radios often do. Two
self-tapping screws hold the side
panel/shield of the tuner in place
and these were removed. With a rag
soaked in contact cleaner I rubbed
each of the exposed coil terminations
along each channel biscuit to clean
any muck off. All 13 biscuits were
cleaned.
Then CRC (or similar) lubricant/
cleaner was used to free up the small
screw at the righthand end of each
tuner biscuit. This screw is adjusted
to set the fine tuning on each channel, so it needs to be free – they get
rather tight when the grease congeals
on them.
Having cleaned up the turret tuner
biscuit contacts, the shield was put
back on the side of the tuner. I’ve
never really found out why the coil
assemblies used in the tuners for each
channel are called biscuits. However,
the early ones do look like small rectangular brown biscuits. (That’s why
. . . Ed).
I found that the speaker cloth had
The AWA P1 rear view with the
cabinet removed and the track side
of the PC board exposed.
been pushed back from the grille so
I had to take the speaker out to gain
access to it. This wasn’t the easiest job
as the tuner had to be taken out too. It
is attached to the set frame with three
self-tappers, of which only two were
easily accessible. The chassis/frame
or whatever you call the metalwork
supporting the set had to be separated
from the front escutcheon and picture
JULY 2000 83
This side view shows the turret tuner with its metal shield removed to give
access to tuner channel change contacts and the biscuits.
tube. Only two self-tappers hold the
two sections together and these were
removed.
The channel-change knob was removed and the set moved back away
from the picture tube a few centimetres – as far as the EHT lead would
allow. The tuner screws were now
all exposed and were undone and the
tuner removed, to give access to the
speaker mounting screws. They were
removed and the speaker pulled back.
The cloth was pushed back into place;
just a few seconds work. I decided that
the control potentiometers which are
mounted in front of the speaker could
do with a squirt of contact cleaner
while I had all that section apart.
Reassembling it all was the reverse
of the disassembly procedure. However, before doing that I cleaned around
the sections of the set now exposed, so
that all sections ended up being clean.
Circuit diagram
It is most desirable to have a circuit
diagram and any other information
pertaining to the set. I am fortunate to
have a reasonable supply of black and
white TV circuits and information
so I was able to look up and find all
the necessary information on the P1
which covered four foolscap pages.
This included the circuit with oscilloscope waveforms, a circuit of the
84 Silicon Chip
tuner, parts list, a PC board layout
and a description of how to go about
various servicing activities.
J & R Publications put out a series
of loose leaf manuals over a period
of several years from around 1960.
These cover television sets, valve
radios and Australian and overseas
origin transistor receivers, in several
different volumes.
Careful inspection of the set did not
reveal anything looking out of place
or overheated. Most of the set uses
polyester capacitors and knowing
their reliability, it was not thought
any problems should arise through
them being faulty.
Power was applied to the set with
the cabinet back still removed. A raster came up on the screen, and then a
weak picture appeared on the screen
when I held one of the aerial/antenna
terminals. But there was no sound
except a low level buzzing noise.
These are VHF-only sets so if there
are no VHF TV stations nearby, a
down converter or a VCR with a VHF
output channel will be necessary if
any TV signals are to be seen.
Troubleshooting
Now where do you start to look for
troubles in a TV set with 13 valves?
One of the good things about TV sets
is that they are easier to sectionalise
than a radio, just by observing what
the set does or does not do. The fact
that I could get a picture on the screen
indicated that the power supply, tuner, video IF, video amplifier, picture
tube, and the vertical and horizontal
deflection circuits and the extra high
voltage (EHT) were all working. What
was left?
The AGC circuit could have been
faulty but the signal level I had was
too weak to actuate it anyway. However, the likely culprits were the sound
IF which comes off the output of the
video amplifier and possibly the audio amplifier.
I was able to inject some audio into
the grid of the audio stage and got
output, so it seemed likely the sound
IF was faulty.
I tried a fresh 6CS6 but with no
improvement. Ah well, I’d better get
serious about this. I looked at the
circuit around the sound IF stage and
noted the voltages I could expect to
find. I looked at the PC board pattern
in the information I had and started
tracing where the various pins of the
valve appear on the track side of the
board.
Most voltages appeared within the
normal expected range but the plate
showed a big fat zero. Testing the
other side of the plate feed resistor
revealed 180V, which is normal. The
470kΩ plate resistor had gone open
circuit. This was replaced and the
set tried again. Off-station we had
glorious hiss and on-station, once a
good antenna was connected and the
fine tuning adjusted, we had good
clean sound.
In reality this was the only electronic fault in the set. Lucky me.
I adjusted the fine tuning on the
only VHF channel available and the
video recorder output channel, check
ed the vertical hold and the other
controls, and the set worked just as
it should. I have a suspicion that the
6GK5 triode RF amplifier may be a
little weak, and when I get hold of a
spare I’ll try it, but at the moment it
is quite satisfactory.
Note that did say a triode RF amplifier. These caused many problems
for early radio experimenters and
here they are being used successfully
at VHF from 45-222MHz. Triodes are
low noise so work much better as an
RF amplifier at VHF but they do have
to be neutralised.
Luckily, the picture tube was in
good order with no sign of flaring
and overall the set worked like it had
years ago
The rabbit ears telescopic antenna
was broken in the set, so the easiest
method of replacing it was to swap
the back off another set which had an
intact antenna. This certainly was an
easy job. The cabinet was a bit grubby
too so a dose of detergent, warm water,
a small scrubbing brush, a cloth and
some elbow grease had the cabinet
looking quite good.
Scuff marks and other marks in the
plastic are not easily removed regrettably, but it can be clean. (Editor’s
hint: try car polish). Once scrubbed
up, a wipe of with a clean wet rag
will leave the cabinet free of detergent
and dirt.
Set summary
The AWA P1 series of small portable valve receivers were well thought
out, relatively easy to service (picture
tube change in 15 minutes), gave good
pictures and were reliable.
The only criticism of them is
the sound IF system used – ie, the
quad-rature detector. The limiter type
After restoration, the set produced a good black and white picture.
IF amplifier followed by the FM ratio
detector was a far superior IF system
in my opinion but a bit more expensive to put into the set.
That said, the quality of the audio
with the quadrature detector was
quite OK as long as the signal coming
SC
in was of good quality.
Restoration Tips For B&W TV Sets
These tips are just a few to help you
get into black and white TV restoration. There are not many of these
sets left, so now is the time to start
collecting even if restoration is not
contemplated for a while. By taking
it carefully and asking for help if
needed, the restoration of our early
TV history can be just as much fun
as our radio restorations.
(1) Try to obtain more than one example of the same chassis set.
(2) Obtain a circuit diagram and any
other information available on the
receiver.
(3) If this is your first go at restoring
a TV set, enlist the aid of a fellow
restorer or perhaps a retired TV serviceman for advice.
(4) Providing the picture tube, deflection yoke and horizontal output
transformer (and maybe the vertical
output transformer too) are OK, the
set should be quite restorable. If any
of these three are faulty, another old
set with these as good components
will be necessary – otherwise it will
be suitable as a static restoration
(cabinet and general clean inside).
(5) Replace all paper capacitors,
taking particular note of the voltage
ratings. Some values are critical, others non-critical as in radio receivers.
(6) Check all resistor values (within
reason) as many of them can be
expected to go high or open-circuit.
(7) When replacing components,
particularly in the video IF and video amplifier sections, pay particular
attention to maintaining the same
lead dress, as this may affect picture
quality.
(8) Have a reasonable supply of
valves on hand. Sometimes a valve
won’t work in one circuit position but
will work in another position.
(9) Mark the chassis with the valve
type alongside each socket, so that
valves are not plugged into the wrong
sockets.
(10) If you are confident all has been
done that can be done in the way of
replacing faulty components, the set
can be turned on. It is necessary to
observe the HT line and the horizontal output valve. The rectifiers plates
can glow red if they are thermionic
types as can the horizontal output
valve if things are not right.
MOST IMPORTANT
(11) DO NOT adjust IF and tuner
tuning slugs unless you know exactly
what you are doing and have suitable
test equipment. TV receiver IF and
tuner sections can tolerate quite a
bit of drift without giving a bad picture
due to the broadband nature of the
tuned circuits. The tuner and video IF
stages have a bandwidth of at least
7MHz, not 10kHz like the AM radio
IF bandwidth.
(12) Treat old picture tubes like un
exploded bombs, particularly the
larger ones. If handling a picture tube,
wear a leather apron, gloves and
safety glasses for complete safety.
(13) NEVER pick up a picture tube by
the neck; this is a weak area where
the glass is thinnest. Tubes can implode, doing enormous damage. It is
rare but it does happen.
JULY 2000 85
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