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Old 19th May 2020, 10:19 pm   #64
PortugalTV
Tetrode
 
Join Date: Nov 2018
Location: Serzedo, Portugal.
Posts: 83
Default Re: B&O Capri 606 TV (17") restoration

Hi
tube pins are always counted clockwise viewed from the bottom and anti clockwise from the top. counting starts at the place were there is a gap between pins,on some bases there is a notch etc, this is valid for crts also and any pin base you can imagine
i have some tube socket test adapters i made to test tubes from the upper side with the tv turned on,they are very helpfull you could make or buy one, i can post a picture if you want to make one

the agc "comes" from the video amp, simplifying it , it takes black level and the more black the pictue has the more negative it goes, and as you know putting negative voltage on g1 of tubes lowers the gain. they inject that negative voltage on the IF and tuner, sometimes on the video amp. then you have a reservoir cap that provides a "time buffer"so the agc does not jump around fast on different scenes in a movie for example. on your set it does not seem to act on the video amp itself but i might be wrong. you can always disconnect the agc for maximum gain on any set but it often overloads and you loose sync etc

on some sets the reason the agc circuit is tied to the flyback is to obtain some ac, then its chopped to get a negative voltage, that voltage is then regulated by a tube depending on black level and there is your agc
i have a set were they get that negative voltage from the ac on the filament string at a certain point, every maker and model has a different setup


about the tuner, if you have zero ohms from that point to chassis DEFINETELY you have a short there. maybe a coil is shorted against a the tuner chassis or inside 971h96 metal can maybe? maybe
also shielded cables often short on old tv's, had that problem many times but never in the IF/tuner but always on the horizontal stage because the high pulses heat up the wire and it gets brittle, anyway the short has to be checked and cleared

with that short you will definetely loose any signal but you should be able to inject a signal at the video amp and get it dispplayed.i would try removing the detector diode and inject a signal at the grid of the tube via a coupling capacitor. use composite video form a dvd.

maybe the short creates a black picture and the agc blocks the gain of the video amp? have to study better the schematic, i only glanced at it. with that short i would expect a blank raster but that depends how they designed the circuitry. it would be great if other people also gave their insight, they might more skilled than i.

if your tuner uses a rotating drum they dont be afraid its very easy to take apart and service. just dont move any components around or you will mess up the alignment just by changing the component position. the shortshould be easy to find. often there are some ceramic tube insulators and a wire is run on the inside between different sections on the tuner. its not uncommon for the ceramic to break and thewire to short to gnd when wiggling the tubes around too hard

Last edited by PortugalTV; 19th May 2020 at 10:46 pm.
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