28-05-2017, 06:13 PM
(This post was last modified: 28-05-2017, 06:16 PM by Geordie McBoyne.)
Readjusting the aerial input coil increases the vision gain. With the Aurora set to channel 2 the video developed across the vision demodulator load resistor is 15 volts P - P, and that's with the contrast control set to minimum.
So the performance of the vision stages is improving but it's not so with the sound. The performance of the sound channel is absolutely hopeless! The attachment shows the sound take off from the anode of the mixer valve.
The set came with a Mazda SP61 as the sound IF amplifier, this was subsequently replaced by an MO W61 vari-mu pentode. I've now replaced this valve with a Marconi Z66, a valve that resembles the SP61, so in a sense the sound IF amplifier is now back to the original design.
Because the timebase is inoperative, without any special measures the HT voltage will rise to a dangerous level, as high as 400 volts. To reduce the HT voltage to a safer level a 15Kohm resistor is inserted between the negative tag of the HT reservoir capacitor and chassis. This was in fact done in early production 1804 and VT50 sets which had the facility to operate sound only, the resistor is bypassed when the set is switched to normal TV operation. The vision off switch was ganged to the brightness control. The Ekco TSC30 has the same control arrangement.
Geordie McBoyne.
So the performance of the vision stages is improving but it's not so with the sound. The performance of the sound channel is absolutely hopeless! The attachment shows the sound take off from the anode of the mixer valve.
The set came with a Mazda SP61 as the sound IF amplifier, this was subsequently replaced by an MO W61 vari-mu pentode. I've now replaced this valve with a Marconi Z66, a valve that resembles the SP61, so in a sense the sound IF amplifier is now back to the original design.
Because the timebase is inoperative, without any special measures the HT voltage will rise to a dangerous level, as high as 400 volts. To reduce the HT voltage to a safer level a 15Kohm resistor is inserted between the negative tag of the HT reservoir capacitor and chassis. This was in fact done in early production 1804 and VT50 sets which had the facility to operate sound only, the resistor is bypassed when the set is switched to normal TV operation. The vision off switch was ganged to the brightness control. The Ekco TSC30 has the same control arrangement.
Geordie McBoyne.
