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Old 25th May 2015, 6:28 pm   #1
Studio263
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Default Pye PTV patch-up

I picked this Pye PTV up at Mikey’s TV Junk Swapmeet; I understand that before that it had passed through the hands of a number of forum members so I thought that I’d better write up what became of it.

I first saw a Pye PTV about 20 years ago, I went to collect a Beomaster 900M from a chap in Weston Super Mare (I think) and he had quite a few of them. I think he was a paint sprayer by trade and so had re-sprayed the cabinets, some in lurid colours which Pye would never have dared to try when the sets were new! When I got home I went to look up the PTV in the red R+TV S books and found that it wasn’t there, that alone is enough to get my interest going. Funny what you remember…

For those who’ve not seen one, the PTV is a 14” model in a boxy (trans)portable cabinet. The front panel frames the 14” rectangular tube with very little around to spare but this is more than made up by the depth, which is considerable. The casework and chassis are all aluminium (unusual for Pye at this time) for lightness and to make the set safe they fitted a three core mains lead, along with an isolating-type mains transformer. This provides a 200V HT supply via two paralleled contact cooled selenium rectifiers and two 6.3V windings for the valve and tube heaters. The heaters are all in parallel, a practice which had all but disappeared in TV sets by 1956 when these were launched. The bulk of the valves are common “E” series types where this arrangement poses no problems, the only oddballs are the line output valve and boost diode which are an EL81 and an EY81 respectively. The transformer has two heater windings, one for the receiver chassis and one for the timebases and tube. Unlike some portables of this period which were quite basic, the PTV has a full super-fringe specification. The tuner has a cascode ECC88 (gold pins) RF stage and both the vision and the sound IF strips have three EF80 stages each. The sound section has its own mean level AGC system but the vision AGC is line gated (like that of the V4 model), giving constant black level performance over a wide range of signal strengths. The video output stage has a triode cathode follower to reduce its output impedance and the line oscillator is flywheel synchronised – extra contacts in the line hold control break the sync when the knob is pushed in, which also engages the knob shaft onto the control’s rotor. The width is controlled by varying the line output valve’s screen grid current and the sound output stage is stabilised by two negative feedback paths. By contrast the frame circuit is a model of simplicity with a single ECL82 acting as oscillator (triode) and output stage (pentode) – the designers must have exhausted themselves by this stage!

Pye must have had a surplus of ECF80s when the PTV was designed as no fewer than four are used, one in the tuner as the mixer and oscillator, one as the video output stage and cathode follower, one as the sync separator and one as the line oscillator. This unusual valve also appears in Pye’s FenMan II radio (in the FM front end) of the same year. Like the FenMan II, the PTV is a complicated set with a lot of bits and bobs inside. It has 17 valves and 10 signal rectifiers and diodes for a start, not bad for what is supposed to be a lightweight! It probably isn’t a well resolved as a portable as the Ekco TMB272 but the high sensitivity receiver section makes it a better bet than the likes of the Ferguson 454T and the cardboard Murphy sets for internal aerial use. The PTV has so much potential gain that it will show a snowy raster in the absence of a signal, even without turning the brightness control up. Pye’s next portable was the TT1, one of the first transistorised sets available anywhere.

The set looked in fair condition but an internal examination revealed that it had got wet at some stage, the aluminium was speckled in white “fur” and the steel screws were rusty. There was also no mains fuse, which is a bad sign usually. The viability of the set depended on whether the mains transformer was sound so the first thing I did was to fit a fuse and bring up the mains slowly. By 100V the HT was beginning to come up but in a lazy sort of way and the tube heater was already very bright. I was thinking about this when there was a horrible smell of burning selenium, so I switched it off. The transformer proved to be OK but I could see straight away that the rectifiers were shot and that the tube heater was shorted in the usual Mullard way. This is more of an issue in the PTV as the parallel supply tries to “force” 6.3V across the heater whatever its resistance, unlike in a series heater set where things just seem to balance themselves out. Connecting the tube to a bench supply set to 300mA gave just under 3V, oh dear.

Banishing such grim thoughts, I stripped the chassis to its component parts: the outer panels, the two fold-out chassis, the tube, the scan coils and their support and the tray at the bottom where the power supply components are. Since the set won’t do anything without a power supply I started there. The tray was filthy so I stripped everything off and washed it in the bath. The water turned black but the aluminium came out clean, ready for the bits to go back on. I left the original rectifiers in place but added a modern silicon bridge on top of them and transferred the connections over to it (see picture). 1nF 1000V capacitors were then added from the negative pin to all the others and a 15R resistor was connected from this point to the chassis to keep the HT line at the correct level. The mains switch is located under the screen, it has a clear Perspex button which lights up when the set is on. This uses a 12V bulb connected across both heater windings (which are connected in series for this purpose). All these parts were cleaned and repaired where necessary, switching on afterwards gave plenty of HT volts and the correct glow from the bulb. After an hour or so neither the transformer or the smoothing block was warm and the HT decayed slowly when the power was removed so the supply part at least seemed to now be in good health.

I dislike doing anything the “normal” way if there’s an alternative so I went for “first sound” instead of “first light” - the tube was ruined anyway so why not? Looking at the circuit it seemed that the tuner and the sound IF section should work (after a fashion) in isolation so the receiver chassis was brought onto the bench and all the gooey wax and crumbling Hunts capacitors were replaced. This is a complicated set so there are loads to do, although luckily Pye made good use of the small red dog-bone shaped ceramics which I’ve never had any problems with so it could have been worse. Also on the list of things to go were the four selenium signal rectifiers (they are always faulty by now), replaced with IN4002s (just what I had, practically any silicon diode will do). The three germanium detector diodes were checked but measured OK so they were left alone.

Applying the power saw the valve heaters light but as the voltage approached operating level the sound stage started to motorboat like mad. This is a common problem whenever an ECL80 is used (as here) and is the result of poor decoupling of the common cathode. The capacitor must be open circuit but where is it? Unbelievably, this one component lives on the timebase chassis and shares a common can with its counterpart in the frame output stage. All that wire and all those tagstrips to save one capacitor! A 220uF 25V replacement was tacked in to get it going and once the tuner valves were re-seated (they are easily disturbed due to the angle at which the tuner is fitted) the Aurora’s drone could be clearly heard. Switching to programme material gave strong but distorted sound, the wetness had got to the loudspeaker and jammed up the coil in the gap. Luckily this type of loudspeaker is assembled with nuts and bolts so I was able to remove enough of the magnet from the back to reach the centre polepiece, which was very rusty. Cleaning it up returned it to its original size though; on reassembly the loudspeaker cone moved easily and “tuning” the magnet position with the sound on resulted in good quality audio without having to replace anything.

Sound is one thing, but what everyone wants to see is a picture. Before reconnecting the tube heater I added a 15R resistor in series with it to protect the remaining section, a flat tube is after all better than one with an open circuit heater… As with the receiver chassis, the timebase chassis needed many capacitors replacing, not least in the complicated sync separator and line oscillator stages. The line output transformer’s pitch-covered overwind looked a bit dodgy but there’s no point panicking at this stage, the first thing to do was to get it going. Once everything was back together (and much to my surprise) I heard the line oscillator start up and lock and saw the EY86 EHT rectifier light, raising my spirits more than somewhat. Fiddling about with the brightness control and the ion trap resolved a dim and fuzzy line across the screen although it did get brighter the longer the set was left on. With the line squared and centred I looked into the operation of the frame timebase. More to “tick a box” than out of any real conviction I tried another ECL82: no change. Connecting a frequency counter to the triode anode showed that there was a 50Hz signal there of a good few volts whose frequency could be varied with the setting of the frame hold control. A good trick when dealing with a frame collapse in a set like this is to turn the height control and see what happens; if the white line moves around then the fault is probably in the oscillator, if not the output stage is the place to look. Everything seemed OK and the voltages were plausible, however nothing would make the line move – even pulling the valve out which was my last desperate act! I’d already dismissed open circuit scan coils as the white line was straight (stray coupling from the line coils tends to add a “wiggle” if the coils are open) so what’s left? Just one thing, the frame output transformer primary was open circuit – rats. The resistors across it which damp the inductance ensured that the pentode anode still had volts on it, so this simple fault took a lot longer to find than it should have done.

At this stage I asked forum member Heater Cathode Short for some help. We tried a few transformers in the PTV; the original idea was to use one from a VT4 but this is an autotransformer (who knew?) that would require quite a few circuit modifications to get it to work. The best answer turned out to be the frame transformer from a scrap Ferguson chassis that was once fitted with a 12” round tube. Surprisingly, this worked really well with plenty of height and an adequate range of adjustment for the linearity. A locked, full width picture appeared and the AGC system appeared to be working but the tube did look very sad. HKS suggested that I remove the resistor that was protecting the tube heater and despite being a bit dubious about this I gave it a go. The heater lit up with a fierce glow (rivalling that of the bulb for the switch!) but the picture was greatly improved – not to “good tube” standards but it certainly made the set usable for a bit. After this there were a few “tidy up” jobs to do and the set was pronounced fit.

After a few days of use the picture went negative and the contrast control stopped working, what could cause a fault like that? I thought that maybe standing the set up on its back (which is the easiest way to work on the fold-out chassis) had busted the contrast control but this turned out not to be the case – in fact the EB91 heater was out – this valve forms the interference limiter and the part of the vision AGC circuit. Wiggling the valve around in its holder brought the heater back on and after a few seconds the picture corrected itself. Cleaning the valve pins carefully seems to have made a permanent repair.

A few faults remain. The tube obviously, but the frame hold control seems to have an odd action. Turning it even slightly stops the oscillator and causes a brief frame collapse, which then slowly recovers after a few moments. Currently I can’t see why that should be, the circuit is simple with a blocking transformer with a single winding; I’ll have to spend some more time on that. Finally the width is a bit low; it needs to be on the top setting to avoid a gap at the sides. No doubt the EY81 and EL81 are tired but they are a bit unusual so I think it will take a while to find replacements, I’m sure something will turn up though.

The final picture shows the set working, the viewer is suitably impressed (until he gets square eyes that is!)
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Last edited by Studio263; 25th May 2015 at 6:38 pm.
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Old 25th May 2015, 8:06 pm   #2
Tazman1966
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Default Re: Pye PTV patch-up

A very interesting write up and the type of set that I once had given to me as a teenager. Mine had been spray painted glossy white (not by me) and worked without having to do anything to it.

Glad to see your son enjoying a bit of vintage kit too

Good luck sourcing a replacement CRT.
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Old 25th May 2015, 9:04 pm   #3
Restoration73
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Default Re: Pye PTV patch-up

I have got a couple of these to do - yours has the later case styling.
I've seen the EL81 & EY81 (Australian Philips) new at radio rallies in the last year.
I'll report my progress on these.
The design and earthed chassis suggest the same chassis is also used in a professional
studio monitor (I have seen a 625 line conversion with video input on these)
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Old 25th May 2015, 10:28 pm   #4
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Default Re: Pye PTV patch-up

Good work Tim (& John). Really enjoyed the write-up. Despite the crt problem the results are obviously good enough to hold the attention of the audience. What an excellent photo that is!

As mentioned, the design of these sets did lend itself to monitor conversion, probably most famously on this footage of the first images received from Telstar in 1962:

https://www.youtube.com/watch?v=PVXwQVks0Lk



Steve
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Old 3rd Jul 2015, 7:48 pm   #5
Restoration73
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Default Re: Pye PTV patch-up

My research is that the Pye TVT monitor version is known as Pye 2822.
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