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Old 28th Feb 2018, 11:53 pm   #24
1100 man
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Join Date: Apr 2016
Location: Ventnor, Isle of Wight, & Great Dunmow, Essex, UK.
Posts: 1,377
Default Re: Marconiphone VT161 17" TV

During the last week, I've been gradually sorting the remaining issues such as getting the tuner to work on all channels, attempting to improve the line linearity and dismantling the concentric contrast/ hold controls so that both spindles don't turn together!

It now produces a very good, watchable picture. However, one niggly fault remains which I have not been able to get to the bottom of:- the top 1/2" of the scan is only composed of about 10 lines, widely spaced.
By careful adjustment of the linearity controls, the effect can be reduced but is still just visible on a picture and it annoys me!
Connecting a 'scope to the grid of the PCL82 output valve, clearly shows the problem. Immediately after the flyback, the waveform rises rapidly before the linear part of the scan starts, hence, the first 1/2" of scan happens much too fast so the lines are spaced too far apart.
The top linearity control affects this bit of the waveform.

The frame oscillator is a multivibrator comprising 1/2 of an ECC82 and 1/2 of a PCL82. What is most significant is that the anode of the ECC82 is 75V which is correct, but the anode of the PCL82 (triode section) is only 20V! It should be 55V. It's fed from the height control so is obviously variable. If the height is increased to 55V, the waveform on the 'scope looses that initial peak and the scan looks linear (on the scope).
The picture is then extremely tall!! So I suspect some form of feedback issue but I have not been able to find the cause. All the caps have been replaced in the frame stage, the actual circuit has been traced out on the chassis and agrees with the book. All resistors are in spec and apart from the PCL triode anode, all other voltages are OK. A different PCL82 has been tried.
So I'm rather stuck for ideas
I'll try and post some pictures of the problem a bit later on tonight.
All the best
Nick
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