How Can We Help?
When the set is first switched on a thick black band appears in the middle of the screen, from top to bottom. If the channel selector button is pressed the picture will appear normal, except for shimmering from left to right at the top of the picture. After about ten minutes the black band reappears again. This sequence can be repeated several times, then the picture breaks up into four pictures all mixed up. All line timebase valves have been replaced without success. There is also a fault in the sound section. When the set is first switched on the sound is normal, but after ten minutes a loud screeching noise starts. This can be stopped by turning the volume down or fully up. The black bar and line break up are likely to be due to the flywheel line sync discriminator diodes. These are 3MR1/2 in the five -leg block on the right-hand side of the timebase panel. We suggest you replace them with 1N4148 or similar silicon diodes. For the sound fault, check the PCL82’s triode grid leak resistor R50 (10M) then suspect that the printed board is conductive around the base
of the PCL82. This can be the case without being visibly obvious and can be cured by drilling holes between the pins.
We assume that the PCL82 itself has been checked, also the audio screened lead earth bonding at P/S3-2.
Update from an ex tv engineer:
I have just read the TV 148 KB about flywheel sync diodes, suggestion to replace with 1N4148 or similar silicon diodes. The silicon diodes have much less forward resistance compared with the original. RBM service instructions were to put a 15k-22kohm resistor in series with the diodes, only require one for the flywheel diodes and one for the interlace diode. The line lock seems excellent without the resistor but can cause some peculiar problems under some reception conditions.. Without the resistor the frame interlace is poorer.