"There's an old telly in the loft. Do you want it?"
Yeah, why not....
I take the set home, much to the delight of Mrs. Doz
The cabinet has seen much better days, and it looks like it's been damp at some stage in it's life...
No model number, as the sticker's fallen off, but some research shows it to be a 2103, dating from 1971. It's a single standard (625 line PAL) colour set.
... and un-molested...
Removal of the line output stage screening can, shows the anode lead to the line output valve's insulation has failed, leading to a bit of a burn-up
... and there's a resistor on the side of the line output transformer which falls into pieces as soon as I touch it. This is replaced, and the wiring made good.
The CRT is tested...
... and doesn't look good initially, but after a couple of hours wait, the meters rise into the green as the tube wakes from it's long sleep!
Some mains is gently applied to the set via the variac... nothing. The mains fuse is open circuit, so a new one is fitted... and as the valves warm up....
... a blurry green raster appears ...
The tuner is seized solid as always, so it's removed...
... and cleaned up...Being careful not to stress those rusted in plastic nuts!
The tuner is refitted once it's all moving freely, and I start fault finding on the lack of colour.
I prevent the colour from being removed from the picture (for when black and white programmes were broadcast) by disabling a circuit called the colour killer. There's still no colour, so it's likely the reference oscillator isn't running... Just as I'm about to start looking at that fault, the sound and picture disappear! The set is still operating, but there's nothing coming out of the IF (Intermediate frequency) stages after the tuner. Damn.
There are 3 stages of combined IF amplifier. These amplify and filter both the sound and vision the signals together, coming from the tuner. The signals are then split off, and amplified further, the audio being separated and decoded from the video. As both sound and picture went off together, I suspect the fault lies in the combined amplifiers.... I check all three stages, all appears well. Perhaps the tuner has failed? A super chap called Gary has a spare, but it makes no difference....
After some further checks, largely out of desperation, I check through the vision amplfier, and find there's an emitter follower amplifier, and the transistor has failed. It's a BF194 HF bi-polar transistor. A replacement is fitted, and pictures are restored! Still no sound. I look at the first sound IF, and , sure enough there's another failed transistor. A BF194... still no sound!... I check the ratio detector, which is used to demodulate the FM sound, and.... it's another failed BF194! Sound a pictures restored, I look back to the colour decoder.
I check the reference oscillator, and it's running fine, producing a locked 4.43MHz signal. Good. I check through the stages, and find 3 faults. Two of the faults are transistors in the R-Y (Red minus lumenance) and B-Y (blue minus lumenance) ... guess what .... BF194's. Also the colour bust transistor is open circuit... you've guessed this next bit ... it's a BF194...
There have been questions raised on various forums in the past about the reliability of BFxxx and BCxxx lockfit type transistors. It's quite possible that that un-reliability (they are 40+ years old!) is what's been at work here. It's possible that flash over when the insulation failed on the anode lead in the line stage caused a spike which saw off the transistors, but the BF197's and BF196's in the IF stage survived, as did other transistors in the decoder...
I replace the line output valve and boost rectifier (PL519 and PY500) and the width is better, set up the convergence on the tube, so the three colour guns are lining up correctly, and I'm rewarded with reasonable results.
Note quite sure what I'm going to do about the tatty cabinet though...
The guilty parties....
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