27-12-2016, 01:03 PM
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.
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.
