1950 12" Defiant TV set.

[Geordie McBoyne]

Just acquired this 12" Defiant TV set.   Collected it today from an excellent chap in South Sheilds.

The set was made in 1950 and employs the Plessey Mark one chassis. In fact you could call it the Mark 2 version because this chassis has the electrical picture shift controls instead of the usual magnetic shuffle plate for picture centering.  All the valves are missing but that's no big deal really except that the pentodes in the IF amplifier deck will have to be identified, it's most likely these will be the Mazda 6F1 rather than the earlier 6F13.
Line generator valve is a Mullard EL38, no separate oscillator. The frame output valve is a Mazda 6L18 triode and the oscillator can be either a 6SN7GT or as this is a late production chassis, a Brimar 12AU7 double-triode.
The finned metal HT rectifier is a bit bashed up and will be replaced, the replacement rectifier will be a silicon diode along with the usual 25ohm series resistor.
The CRT is a Mazda CRM121B, this could be the critical part because this is a tube not known for good emission. But you never know I have a similar Defiant TV, the only difference it is the version with radio and this set displays an excellent picture.
Considering the horrible sets I've trying to service recently this Defiant set could prove to be an easy fix, but again you never know.

Geordie McBoyne.

[Geordie McBoyne]

Photie of the internals of the set.  That metal rectifier will have to be replaced, there's no chance anything can be done to save that part. There is an octal valve socket present on the chassis so that determines that the frame oscillator valve is a Brimar 6SN7GT.  The loudspeaker is missing, now this might present a problem because some sets employed an energised speaker magnet, this device also functioned as the HT smoothing choke. If it is proven that it is that type of loudspeaker there is no point hoping one will ever turn up and the simple solution is to fit a conventional smoothing choke and a PM 'speaker.
Later today I'll remove the chassis an find out which type of RF pentodes were employed, 6F1 or 6F13.
The CRT heater checks OK, 0.4ohms. The Plessey Mark 1 chassis was designed to accept the various CRTs available at the time of manufacture, although Defiant used exclusively Mazda tubes other set makers also used Mullard and Ferranti CRTs.

Geordie McBoyne.

[Geordie McBoyne]

The RF pentodes are the Mazda 6F1, this B8A base valve later morphed into the 30F5. Main difference between the 6F13 and 6F1 is that the latter has two cathode connections, pins 5 and 7.
The set has an HT smoothing choke so a PM loudspeaker can be fitted.
The underside of the chassis is in excellent condition.

[Murphyv310]

Hi David.
That looks an interesting set to restore. I'm also surprised how clean the under chassis is in comparison to the upper chassis.
I like the fact it has parallel fed heaters. Does it use an auto-transformer or is it fully isolated?
Looking forward to the progress.

[Geordie McBoyne]

Hi Trevor,
               All the valves in this set have 6.3volt heaters and are fed from a winding on the mains transformer. The primary functions as an auto transformer with the 250 volt upper tapping supplying the HT rectifier. There is an additional heater winding with taps for 2, 4 and 6.3 volts for the CRT.
The attachment shows the circuit diagram of the similar Regentone BIG12 Mark 2. Like the Defiant 1250 under discussion the BIG12mk2 has an HT smoothing choke and also the electric picture shifet controls.

Geordie McBoyne.  

[Geordie McBoyne]

The chassis has cleaned up nicely, that layer of gunge has preserved the metalwork from corrosion.
New main lead fitted, the original had been cut off. The wrecked rectifier removed in readiness to fit a silicon diode and associated components. Just one more 6F1 pentode needed and then there will be a full set of valves fitted. Chassis has been connected up to the mains and so at least the valve heaters are glowing, proves the mains transformer is OK.  Next step, HT.

Geordie McBoyne.

[Geordie McBoyne]

Borrowed a 6F1 pentode from the Ekco TSC1113 so now the Defiant has the full set of valves. The Mazda 6F1 is becoming a very difficult valve to find.
The metal rectifier has been replaced by a BY127 silicon diode. Because the forward voltage of a silicon rectifier is considerably lower than the old metal rectifier a 22ohm resistor is connected in series with the diode. Also two additional capacitors fitted to protect the diode from high voltage spikes.
The set is now connected up to the Variac and the supply voltage will be brought up slowly in order to reform the electrolytic capacitors.  
Attachment shows the slightly over engineered replacement HT rectifier assembly.

Geordie McBoyne.

[Geordie McBoyne]

Report about yesterdays work carried out on the Defiant TV.
The full mains voltage is now powering the set, no bangs or flashes.
The HT voltage was 300V, that's a bit high. Scope probe connected to the control grid of the EL38 line output valve. Negative pulses of 200 volts P - P at line rate so there is some activity in the self oscillating timebase, but no spark at the anode of the EY51. The screen grid volts was found to be absent and the 6.8Kohm feed resistor was found to be OC. After replacing that component more activity in the line TB. a weak spark apparent at the anode of the EY51, things are getting better, however, it was observed on the 'scope that the line hold had had little effect on the line TB frequency. This is not good, a faulty line output transformer? Before the transformer is condemned try another EL38 first, now that's more like it, a nice flame like spark at the anode of the EY51.
Also, the line hold control alters the frequency over a wide range, easily adjusted for the correct repetition rate of 100microseconds. Worth noting that the negative going pulse at the grid of the EL38 is now 600 volts!
The main HT line is now reading 260 volts.
Turning my attention to the frame timebase. This is a part of the set known to be tricky and this set is not an exception. After replacing a few capacitors the oscillator can be shocked into some sort of activity by shorting the anode of one of the 6SN7 triodes to chassis, this not satisfactory so more work is to be done in this part of the set.

Geordie McBoyne.

[Murphyv310]

Hi David.
Great progress in such a short time. The 600v on the EL38 grid seems rather high would you not think? If you have normal scan then it must be correct though.
Finding old TV's up here in Scotland has always been poor but it's a long time since I found one, not so good for the hobby as most sets on E-bay are collect only. The Defiant was a lucky find and hopefully an easy restoration for you.

[Geordie McBoyne]

In the late forties the timebase oscillator of choice of designers was the crude thyraron gas discharge valve type or the much better blocking oscillator, a few set makers opted for the Miller transitron oscillator. Plessey was one of the few to employ the multivibrator using a double-triode valve.           The frame oscillator is now free running and requires no stimulus to get it started. Another 6SN7GT double triode solved the oscillator problem. Out of curiosity the original multivibrator 500pF feedback capacitor was refitted and has been proven to be faulty. The capacitor is C46 in the Regentone T15L circuit diagram which can be found on page 89 of the Molloy and Poole Television Servicing book, volume II.
But there was no sawtooth waveform reaching the grid of the 6L18 frame output valve. The slider of the height control VR9 is not making any contact with the track, the part was taken out and checked with an ohms meter to confirm that it is faulty, and sure enough it was. A replacement 1Mohm potentiometer was found and fitted but there is still not sawtooth drive at the 6L18 grid, just what is going on? Also the 6L18 anode is glowing red hot. Then I realised what was causing the fault condition. The frame linearity control is a variable resistance in series with the cathode to chassis return of the 6L18 and it was set to minimum resistance, thus removing any grid bias for the valve. The circuit reference is VR7. In the Plessey Mk1 circuit there is no coupling capacitor between the frame oscillator and output valve, there is always a positive voltage on the grid of the 6L18 output valve, this voltage is countered by the higher voltage developed across the bias resistor, VR7. Of course if the linearity control is set to a low resistance the valve will go into grid current and this will result in the loss of the sawtooth drive. An additional series resistor will be fitted to prevent this fault condition occurring again.
I'll check out the frame timebase performance of my other two sets equipped with Plessey Mk1 chassis to find out if this linearity control fault exists in those sets.

Geordie McBoyne.