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Old 27th May 2020, 8:53 pm   #73
staticmind
Tetrode
 
Join Date: Sep 2019
Location: Aalborg, Denmark
Posts: 97
Default Re: B&O Capri 606 TV (17") restoration

You're of course right about 971H97 ang g1 of PCL84, don't know how to actually interpret my last post to be honest.

So going by calculations, the DC at g1 is given by the voltage division by the contrast pot + the 1.8M resistor and the 4.7k to ground. With +a being 200V, this should be around 5.9V max at g1, when the contrast pot is at max. As my 1.8M resistor measures about 2 Mohm, the voltage is closer to 5V.

I thus don't see how they're ever able to get 15.5V at g1 of PCL84. It shouldn't be possible? Or does white noise from the tuner create a DC offset as well due to the video detector diode circuit?

I lifted one leg of the RL41 and measured again. A forward drop of 0.293V and nothing in reverse. Perhaps one of the 6pF caps are not as good as they should be, as this is the only way I can see a "leak". But I don't think this can be the cause of my troubles.

I'll try to find an old DVD player or similar with composite output - I think I'll have to ask around. I use the TV with a variac, which is also an isolation transformer type.
But firstly I might want to check the rest of the sections and check the tube voltages (of course not the top caps of PY88, PL36, DY87....). G1 of PL36 is stated to measure -49Vdc, so let's see.

I measured the voltages at the CRT socket as well. G2 measured nearly 500V, but drops quickly as I was measuring. So that's terrible. I will try to draw up a schematic of how the modifications were made with the AT2021 LOPTx.

By the way, I collected a Capri 606, but in 21" as well. Components look much less tired so that set was retired relatively early. This is so that I have a backup source of special components and the tuner unit. The LOPTx is not the AT2016 either, but an AT2018. The durability of that AT2016 must have been downright awful.
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