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ppppenguin · 12th Sep 2011, 8:48 am

I was out most of yesterday so I'm catchng up with the news on the T311. Sounds like there's quite a bit of the set now potentially working. Dom has covered much of the ground so it'll look as if I'm repeating some of his wisdom.

Just a reminder for those not experienced with TVs. Never connect a meter or scope to the top caps of the LOP valves. This will simply fry your test equipment. Actually there are specialist probes that can be used but they are rare and I don't have one. Everyone I know uses a spark test as described by others in the thread. You can't do much harm to an all valve set, even if you do it wrong.

Need to establish that the line oscillator is working. Line whistle, especially as you get older and your hearing goes at the HF end, is an increasingly unreliable guide. First and foremost look at the 30P4. If anything inside is glowing red hot apart from the heater/cathode then switch off because line drive is almost certainly missing. The standard test is to check for a healthy -ve voltage at the control grid of the LOP. I would have a tendency to get out the isolating transformer and scope. To the TV techs of old this is gross overkill but it's natural for me.

After you've established that line drive is present at the LOP there should be some evidence of sparks at the top caps. Again look at the 30P4 and U191. Any signs of distress? If no distress then the LOP screen resistor and decoupler are possibles. The boost capcaitor (big 0.5uf paper jobbie) and is there HT on the U191 anode. Remember that the U191 top cap is the CATHODE, the anode is pin 5.

Just realised there's a handy 22R resistor in the 30P4 cathode circuit. Not sure why it's there and what good it does but it makes a wonderful test point. Must stick a scope on it one day and see the current waveform in the 30P4.

12th Sep 2011, 8:48 am	#86
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Restorers dream part 2 the chassis. I was out most of yesterday so I'm catchng up with the news on the T311. Sounds like there's quite a bit of the set now potentially working. Dom has covered much of the ground so it'll look as if I'm repeating some of his wisdom. Just a reminder for those not experienced with TVs. Never connect a meter or scope to the top caps of the LOP valves. This will simply fry your test equipment. Actually there are specialist probes that can be used but they are rare and I don't have one. Everyone I know uses a spark test as described by others in the thread. You can't do much harm to an all valve set, even if you do it wrong. Need to establish that the line oscillator is working. Line whistle, especially as you get older and your hearing goes at the HF end, is an increasingly unreliable guide. First and foremost look at the 30P4. If anything inside is glowing red hot apart from the heater/cathode then switch off because line drive is almost certainly missing. The standard test is to check for a healthy -ve voltage at the control grid of the LOP. I would have a tendency to get out the isolating transformer and scope. To the TV techs of old this is gross overkill but it's natural for me. After you've established that line drive is present at the LOP there should be some evidence of sparks at the top caps. Again look at the 30P4 and U191. Any signs of distress? If no distress then the LOP screen resistor and decoupler are possibles. The boost capcaitor (big 0.5uf paper jobbie) and is there HT on the U191 anode. Remember that the U191 top cap is the CATHODE, the anode is pin 5. Just realised there's a handy 22R resistor in the 30P4 cathode circuit. Not sure why it's there and what good it does but it makes a wonderful test point. Must stick a scope on it one day and see the current waveform in the 30P4.