Frame linearity on Pye PTV - UK Vintage Radio Repair and Restoration Discussion Forum

brianc · 4th Mar 2006, 12:57 pm

Hi all
I have been working on the Pye "PTV" which I purchased a short while ago. It has cleaned up extremely well and the performance is excellent. However, there is a problem with foldover on frame flyback. The castellations at the top of Testcard C start before the spot has reached the top of the raster and appear as 3 rows of white lines (not unlike broad pulses) acoss the top of the picture. I have checked all the components in the FTB - including changing all the paper caps - but cannot cure the problem. The oscillator is of the blocking type using an auto-transformer.
Can anyone suggest where the problem may lie?
I have not replaced the metal rectifiers and the HT is about 20V low but the line scan & EHT are fine!

As a matter of interest, the construction of the PTV is very similar to the Pye 405 line colour TV featured in a current thread - remarkably good for a commercial set!

Cheers

Brian

Ed_Dinning · 4th Mar 2006, 10:05 pm

Hi Brian have a look at the resistor in the circuit there are some high values that may have gon a lot higher. That also includes the pots.

Ed

brianc · 8th Mar 2006, 11:51 am

Hi all
I took Ed's advice and checked the high-value Rs in the FTB and replaced a couple (50% high) but that didn't cure the fault.
I eventually traced the problem to the field flyback starting much too late - it seemed to be initiated by the vertical interval test signals. This, of course, meant that the scan proper had not started by the time that the video had, hence the castellations during flyback. There were two faulty components: a 2000pf integrating cap (O/C) and the interlace diode (O/C). How the timebase was locking I don't know but after replacing the two components, the hold was much more solid and, more importantly, the field flyback started on the third broad pulse.

It just shows that it is easy to jump to the wrong conclusions if the whole problem is not carefully analysed at the beginning!

Cheers

brian

4th Mar 2006, 12:57 pm	#1
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Frame linearity on Pye PTV Hi all I have been working on the Pye "PTV" which I purchased a short while ago. It has cleaned up extremely well and the performance is excellent. However, there is a problem with foldover on frame flyback. The castellations at the top of Testcard C start before the spot has reached the top of the raster and appear as 3 rows of white lines (not unlike broad pulses) acoss the top of the picture. I have checked all the components in the FTB - including changing all the paper caps - but cannot cure the problem. The oscillator is of the blocking type using an auto-transformer. Can anyone suggest where the problem may lie? I have not replaced the metal rectifiers and the HT is about 20V low but the line scan & EHT are fine! As a matter of interest, the construction of the PTV is very similar to the Pye 405 line colour TV featured in a current thread - remarkably good for a commercial set! Cheers Brian

4th Mar 2006, 10:05 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Frame linearity on Pye PTV Hi Brian have a look at the resistor in the circuit there are some high values that may have gon a lot higher. That also includes the pots. Ed

8th Mar 2006, 11:51 am	#3
brianc Rest in Peace Join Date: Jun 2004 Location: Burghfield, Reading, Berkshire, UK, Posts: 1,055	Re: Frame linearity on Pye PTV Hi all I took Ed's advice and checked the high-value Rs in the FTB and replaced a couple (50% high) but that didn't cure the fault. I eventually traced the problem to the field flyback starting much too late - it seemed to be initiated by the vertical interval test signals. This, of course, meant that the scan proper had not started by the time that the video had, hence the castellations during flyback. There were two faulty components: a 2000pf integrating cap (O/C) and the interlace diode (O/C). How the timebase was locking I don't know but after replacing the two components, the hold was much more solid and, more importantly, the field flyback started on the third broad pulse. It just shows that it is easy to jump to the wrong conclusions if the whole problem is not carefully analysed at the beginning! Cheers brian