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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