[Heatercathodeshort]

Part Two. The timebase chassis.

With a working power supply the time had arrived to sort out the timebase chassis. I decided to carry out some quick essential tests before continuing further.

Continuity tests were carried out on both the line and frame blocking oscillator transformers, audio, line and frame output transformers and the scanning coils. The focus coil had already been checked while servicing the power supply. To my relief all gave typical readings and were assumed serviceable.

Starting with the line output stage. This is entirely conventional with an EF50 valve in a blocking oscillator circuit driving the well known and almost indestructible EL38 octal based line output valve. Due to the EHT being developed via a mains transformer the circuit is very simple, similar in fact to a class A audio output stage.

Width adjustment is provided by adjusting the bias of the EL38 and a useful degree of linearity is obtained by applying variable damping to the primary of the line output transformer.

I have a love hate relationship with the EF50. Due to war time conditions and the fact that EF50's [service number VR91] were produced in a number of countries, the characteristics of these valves tended to vary. You may have to try a few in positions that they were never designed to be used in but with patience you will usually find one that works as intended.

In the early post war years EF50 valves and their equivalents were widely and cheaply available on the surplus market. It was soon realized that the American produced valves had the edge on valves produced elsewhere.Valves were graded and the very best had two black stripes painted across the top of the aluminium can and demanded a slightly higher price. As you can imagine the surplus dealers soon worked out that adding the stripes produced a higher profit! To be honest, in those austere post war years, who could blame them? You have to laugh!

PYE were very careful in their use of EF50 valves in the first AC/DC receiver, the famous B18T receiver of 1948. They used the 'double stripe' [genuine] versions in sensitive parts of the chassis.

The usual culprits were then pounced upon. Any capacitor with a wax overcoat was condemned the first being the line coupling .01uf. The resistors as always were very close to their design spec and were left alone. The white ceramic types with bright colours are incredibly reliable, made by the million I believe by The Radio Resistor Company [?].

All the rear presets were checked for continuity and found to be satisfactory.

With the rest of the waxies replaced in the line oscillator I moved over to the frame stage. This comprises a larger than life Mullard ECC34 medium power octal based double triode. One triode generates the frame scanning pulses feeding the other half as the triode output valve, a similar arrangement to the earlier PYE D16T of 1946. The Pye version always struggled for height but the Peto Scott version has no problems in this respect probably due to a better quality output transformer.

Once again I had to enter anti waxie mode and replace all the messy sticky capacitors that were sixty years beyond their design life. The cathode by pass capacitor to the output triode was replaced at this stage as it is always a cause of frame cramp when it dries up. It did check OK on the tester but testers lie...

Now I had carried out all this work without powering the chassis, a sure recipe for trouble. It's takes only a second to make a wiring or component value mistake, no matter how vigilant you are, resulting in at least a frustrating hour to dismantle everything and recheck your errors. Don't feel too bad about it. [Think of the early experimental space rockets that have exploded on take off because somebody failed to clean the EF50 pins..]

The connecting leads were far too short from the original power chassis to reach to the the timebase octal plug. This is where my old Griffin power supply was woken up and and brought back into service. I pulled it off the tip years ago. Like a lot of test equipment, it is not used a lot but invaluable when you need it. The Griffin will supply up to 400v Dc fully smoothed and fully variable and 6.3v for heaters at 4 amps.

I connected it up to the timebase chassis picking up the HT on the tag strip connected to the octal plug. I wired the heater connections directly to the heater pins of the EL38 removed for the initial test.

After double checking everything I connected the scope to the anode of the output section of the ECC34 and with the H.T. pot at minimum I switched on. With the H.T. adjusted to around 250v It didn't take long for a sizable waveform to appear on the scope that could be varied in frequency and amplitude with the vertical hold and height controls. Things were looking good.

Well the frame time base was certainly producing something that would show a scan. so attention was paId towards the line output stage.

As mentioned this comprises an EF50 valve in a blocking oscillator circuit feeding the EL38. The line output transformer is a circular aluminium can with the windings encased in sorbo rubber presumably to reduce the 10kc/s line whistle. The top cap connection to the anode of the EL38 exits via a hole in the top. This transformer appears in many receivers of this vintage and I reckon it is another Plessey product. Vidor and Regentone come to mind.

Yet another waxie extermination expedition and a check of all the resistors in the circuit. Not a lot to go wrong really other than pulse voltage flash over in the LOPT can but I was saved that misery.

Once again with HT and heaters applied a good waveform appeared on pin 5, the control grid of the EL38.[The numbering on the base of these truly excellent McMurdo valve holders always catches me out, yer after 60 odd years!] I plugged the monster valve in and heard a low level line whistle as expected from such a well insulated transformer not bothered with the complication of supplying flyback EHT. A small spark could be seen when tickling the top cap anode of the PL38 with an insulated screwdriver. Things were looking good.

The sound I.F. and detector stages occupy a small section of the timebase panel. Two B8A based EF42s and a single EB91 provide a high level of audio feeding the familiar high gain EL33 output valve. Four waxie decoupling capacitors were replaced. They have a value of .01uf much higher than usually employed in this position. Feeding the input of the sound I.F. amplifier with a modulated 9.5mc/s signal [the sound I.F. frequency] resulted in a good signal at the detector output. Things were looking good.

All presets, valve holders, plugs and sockets and valve pins were carefully cleaned paying particular attention to the EF50 with it's short tarnished pins. These I finish off with a dipping in MS4 grease used sparingly. [I have had the same tube since 1964]

Wow! This takes a lot of writing up.

Part 3 will attempt to explain the restoration of the RF and I.F. Panel including the sync separator. Please stay awake for the next installment. Thanks for your interest.
Pictures show The timebase chassis complete with it's pillion passenger the I.F. board mounted on the top. The underside of the chassis with the I.F. board unplugged and the scope showing the line timebase waveform powered from the Griffin power supply. John.