[Heatercathodeshort]

PYE B16T 9" table model 1946
This PYE B16T was presented to the museum a couple of weeks ago. From what I understand it had remained in the same family since purchased new and had ended it's days as a display piece in a TV showroom.

Recently I have been tackling with my joint curator of the television dept, several interesting receivers, the recent PYE being one of them.

Before we go any further I would like to say a word of warning. These early post war models derived their 6KV EHT from a mains transformer, high voltage rectifier [HVR2] and a .1uf 7KV smoothing capacitor. A very great danger exists if you come into contact with any of the high voltage terminations and that is very easy with the B16T due to the top cap of the HVR2 and the positive terminal of the smoothing capacitor being very much within reach. I have fitted a BULGIN insulated top cap connector to the HVR2 and a short length of plastic pipe over the capacitor connections. The two 27M resistors wired across the EHT supply were O/C creating another very dangerous situation. I switched off and after 30 mins discharged the .1uf to earth. There was quite a meaty flash just waiting for someone to grab hold of it. With the 27M resistors replaced by a chain of 10M, the capacitor was fully discharged in five seconds.

YOU HAVE BEEN WARNED!

The heavy chassis was easily removed. Both control knobs were curled up bits of plastic. This receiver is an early user of plastics and I have come across PYE control knobs in a similar condition on PYE broadcast receivers of a similar vintage. Strange because the decomposition appears to happen regardless of storage conditions, many survive unscathed. I have a few decent PYE knobs in the boxes in the spooky garage loft so it's not a problem.

On first examination the PYE appeared to be very clean and original. Just a thin layer of dust with no signs of any work being carried out. A close examination of the chassis however revealed that the original MW22-7 CRT [This may have first appeared in 1939?] had been replaced by it's official replacement the MW22-14C on the 18th October 1949 the date stamped on the warning label.

The MW22-7 are excellent tubes, I believe produced in the PHILIPS factory at Eindhoven, fitted almost exclusively to post was receivers with mains derived EHT. I can only guess that the tube may have had a mild flash over and with the current available from the EHT supply may have either blown open circuit an electrode connection or blasted the cathode. It is interesting to note that PYE fitted a surge limiting resistor to the input of the HVR2 in later production.

Tipping the heavy chassis on it's side and blocking with 6" stubs of 2X4, a closer examination revealed nothing amiss. The frame blocking oscillator transformer looked quite new compared with the rest of the components and with it's correct part number stamped on the top must have been an official part supplied by the PYE spares dept.

A few checks were made for HT shorts and it was noted that the .1uf EHT smoothing capacitor was the very reliable circular Aluminium can style DUBILIAR with the brown Bakelite insulator on the top. If it had been the TCC Visconol it would have been linked out of circuit no matter how loudly it protested. [Believe me they do protest!] A fellow Forum member discovered one of these in a B16T and to his horror, while he was working on it the brown bomber exploded sending shards of Bakelite, paper and oil all over the room! I hope he was wearing a nappy.. With flyback EHT it simply collapses due to the high impedance low current source but with a mains derived arrangement the power just keeps coming heating the thing up till it pops!

Feeling confident that death was at least a few years away I did what I always do and plug it in. All the valves lit and after a short time had elapsed a very faint wickerwork/partial frame collapse/ lack of width raster if you could call it that, appeared on the tube face. At least the EHT transformer was OK and the tube would produce a picture. Connecting an aerial to the original flying lead produced nothing even with the vision sensitivity and contrast to maximum. With an ear close to the loudspeaker there was a complete absence of sound not even a trace of hum. Thoughts of an O/C primary on the output transformer flashed into view but that's just my pessimistic view having experienced many year of television servicing.

The first to tackle was the HT supply, electrolytics, wire wound supply resistors and other routine checks. A quick meter check confirmed that one of a pair HT distribution wire wound 1k resistors was O/C. A circuit check revealed that this fed the sound output stage and a meter check across the audio o/pTx resulted in slight pops from the speaker confirming it's innocence.

At this stage It was decided to replace the main electrolytics very conveniently mounted on a bracket at the bottom of the chassis together with the 1.5k w/w.

Upon removal they were all well down on capacity and a couple had the usual chicken poo seeping from their base seals. The three were neatly replaced and the UU8 rectifier visually checked to see if it had been under stress. I do not test rectifiers on my AVO characteristic tester for fear of blowing up the brand new meter I recently fitted. It's just not worth it. Rectifiers usually blow up, develop O/C heaters or produce reduced emission and you don't need a tester to tell you that! The UU8 appeared to be innocent of any visible destructive qualities. They are very good at covering up their guilt as discovered when I unpacked two brand new UU8s and fitted them in turn to an Ekco TS46. Both produced a shower of sparks and blown fuses. After exhaustive checks it turned out they were simply having a hissy fit. A third one produced 100% results. It is often the rattling old second hand ones that work fine, a bit like the Mullard PZ30 and the U801.

The original rectifier in early versions of the 16T was a Mullard GX32 with a 5V heater. later versions employed the Mazda UU8 requiring a change of base to Mazda Octal and a modified mains transformer to accommodate the 4V heater of the rather dated UU8. No doubt certain valves were difficult to supply in quantity during the austere post war period especially rectifiers.

Powering up produced a better result. With the brightness control fully advanced the scrambled screen mess was a bit brighter, the electromagnetic focus could be sharply set in the center of the focus pot and there was the usual low level hum from the speaker.

Next to tackle was the brightness control circuitry. The tube is cathode modulated with the brightness control simply connected via a potential divider across the H.T rail. The decoupler to the top end of the control [.1uf] read around 100K and was quickly snipped out of circuit. Up flashed a brilliant [search light!] scramble with the brightness control now working correctly.

Next the frame timebase. I won't bore you with this as we have done it many times before. This utilizes a big fat double triode the Mullard ECC34. One half is the frame blocking oscillator with an extra winding on the transformer to allow for injection of the frame synchronizing pulses, the other being the output stage. All very simple and straightforward.

All the usual tired troops were now to be retired from duty including the usual waxies and the incontinent bias electrolytic.
20Powering up again produced a much better result and after smearing the wire wound sliders with MS4 silicon grease easily managed to obtain a steady raster. It was lacking in width and the line linearity was a bit off but hey! It was waking up!

We now needed a signal to see what was happening. Application of full blast from my RF system produced nothing, not even a peep of sound. The PYE 41.5/45mc/s TRF strip is a very well constructed unit. How those ladies on the production line produced such magic in such quantities never fails to amaze me. The RF interstage coils are tiny, enclosed in copper cans. EF50 valves are employed as they were in similar Radar receiver strips during WW2. These excellent valves were originally introduced in 1939 and were very welcomed by the industry capable as they were of dealing with 50mc/s signals with ease compared with the large B7 based types such as the TSP4 and the Mazda AC/SP3. Mazda were producing the well known SP41[4v] and later the SP61[6v] but it had a very greedy heater current, a top cap and was fitted on that lossy Mazda Octal base. The EF50 was a revelation with it's direct glass button base.

Saying that the EF50 did have a few problems mainly due to quality control and inconsistency but considering they were made in many countries, they did a good job. I think the American versions, Sylvania/RCA etc were the best quality and were often marked up as Mullard valves with the best quality ones used in sensitive positions. These were marked with two black lines on the top. The pins also suffered from tarnishing and could have been a tiny bit longer to make better contact with the holder itself. The holders were of mixed quality. The Paxolin type were nasty but the ceramic types are much improved but of course more costly. Various types of securing rings and springs were employed to keep them in place.

Removing the screening plate from the receiver unit was a mixed experience. Nothing had been disturbed but I was presented with a sea of tiny .001uf 400v wax decoupling capacitors. Upon assembly these had obviously been laid in a special sequence as they were tucked in everywhere and there exhumation was not going to be easy..

It was a labour of love. First was to deal with the black valve pins. These were carefully cleaned, including the spigot that locates the valve and earths it's Aluminium can, finishing with a tiny spot of MS4 grease on each pin. There are as many as five caps per valve each clutch soldered to a common earthing point. I used micro pliars and a selection of sharp 'pickers' to deal with each stage. A test after dealing with the first common RF stage produced an encouraging weak signal. It was a very tedious job replacing these capacitors attempting to keep them in the same position as the originals. Failure to do this can result in miss alignment and instability.

With all the caps replaced, strong audio and video signals were at last coming through and it was necessary to reduce the signal input considerably. With some adjustment a locked picture could be obtained with good contrast and brightness but a bit smeary.

Two electrolytics required replacement C10A 16uf the video amps HT supply anode decoupler cleaned up the picture. At the same time the C11A 2uf connected in parallel with R13A 47k the video feed to the tube cathode was replaced. It is mainly present to reduce aircraft flutter. It was completely O/C reducing the tube drive. The same network was used in the later PYE B18T and most other makers receivers, with varying component values.

Replacement of these two caps made a dramatic improvement in picture quality producing a very high brightness [thanks to the mains EHT] stable and high contrast picture.

The results were good but not up to the standard there receivers are capable of.

The next to examine was the line output stage. The picture was lacking about an inch either side with poor linearity. The line output valve is the familiar Mullard EL38 in a typical class A configuration. It is not required to generate flyback EHT resulting in the line output transformer being very similar to an audio output transformer. The difference being of course it is designed to operate efficiently at 10kc/s and enjoys very good insulation.

The transformer is enclosed in a Aluminium can with a Bakelite top that contains the line linearity correction/damping components namely R45A 2k 6W C30A .007uf 700v and the wire wound adjuster slider R46A 1k 6W. You guessed it. Operating the slider massively increased the picture width at one end then decreased it abruptly mid way. It was O/C. The resistive element was replaced with some considerable difficulty by removing the guts of a Radiospares 11 watt ceramic wire wound resistor and fitting it into the Bakelite top with adjustment to the slider contacts. It now works correctly and presented a correctly presented line scan.

From cold switch on the width took longer to fill the screen that it should have done. The EL38 was the original fat version with internal glass sprayed grey/blue. A replacement soon sorted that out and a full picture appeared after a the normal warm up.

The EHT rectifier HVR2 is designed to heat slowly allowing the timebase valves to come to operating temperature before the EHT. The Mazda U22 has similar cathode characteristics but does not have anything approaching the life of an HVR2!

The finished result is outstanding. The vision and sound gain presets are set a third advanced together with the contrast control. Definition is incredible complete with plenty of good quality sound. [EL33] output. A few valves were replaced due to microphonic effects and the HVR2 glowed a bit odd. All the usual valve holder cleaning and tickling of course was carried out.

These were ground breaking receivers that soon gained a reputation for reliability and capable of receiving a good signal from distant transmitters. I believe they were first released in early 1946. The ERT states the table model B16T was released in May 1946 and the console model in July. I had a customer, a very grand and influential lady that purchased her D16T console and had it installed the day before the Victory Parade in London. [June 6th ?] I have that very receiver in good working order together with the PYE V14C she purchased 10 years later! It must be one of the very early models.

It's surprising how many of these have survived. The cabinet was bone dry and faded. Some Beeswax thinned with White Spirit has given it a better appearance and I'm going to retain the original finish. The cabinet back is like new complete with it's original PYE retaining screws.

Pictures show :1 Internal. 2 Chassis on bench. 3 Lethal EHT power unit before fitting protectors. 4 Another internal shot. 5 UU8 rectifier and ECC34 valves.

[This PYE D16T can be seen at the museum when conditions permit.]
John and Peter. Curators, TV Dept. Dulwich museum.