06-01-2017, 11:02 AM
(This post was last modified: 06-01-2017, 11:07 AM by 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.
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.
