TMB272 - contrast "crushing". - UK Vintage Radio Repair and Restoration Discussion Forum

TNC · 26th Aug 2008, 10:04 am

I found this comment from HeaterCathodeShort in an ealier posting...

"Interesting point. I had this many times with the EKCO series of television receivers with the diode mounted on the top of the final vision i.f. transformers under a metal screen. The T330 comes instantly to mind. Similar flat picture with a dramatic improvement when the diode was changed."

and having just about completed another TMB272 restoration was interested to see that the contrast on this set also moves the picture negative when advanced...although a mid-setting provides quite sufficient contrast for an excellent picture?

My question is therefore: is it the case that "over-advancing" a contrast past its optimal setting will always result in a negative picture?

Grateful for any thoughts/explanations.

Trevor.

murphyv310 · 26th Aug 2008, 10:17 am

Hi Trevor.
Yes indeed nearly all 405 line sets will do this and more especially those without any form of AGC, basically you are overdriving IF and the video stages, into distortion. A look on the scope on the anode of the video o/p valve will show the waveform gradually crushing, and it can even flip over this is the negative picture you will see. The Diode you refer to though gives a milky picture quite different to the negative effect.
Trevor

TNC · 26th Aug 2008, 10:26 am

Goodness me - 13 minutes for a reply is pretty amazing!!
Thanks for the info Trevor - that makes a lot of sense - I tried substituting
the diode after V5 (ERT) and it does not affect the crushing but now I realise that it also proves the existing diode is OK as there is absolutely no change in picture content - milkyness (or anything else)...so I can now put on the covers!

Thanks,

Trevor.

Mike Phelan · 26th Aug 2008, 3:49 pm

Another reason for this effect on some sets is the "black spotter" or "interference limiter" control being turned too far, or a fault therein.

It's supposed to limit peak whites like the brilliant white spots due to car ignition or motor brushes. Turn it too far and all the whites become grey, but not the sort of defocused, silvery grey of a flat CRT.

26th Aug 2008, 10:04 am	#1
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	TMB272 - contrast "crushing". I found this comment from HeaterCathodeShort in an ealier posting... "Interesting point. I had this many times with the EKCO series of television receivers with the diode mounted on the top of the final vision i.f. transformers under a metal screen. The T330 comes instantly to mind. Similar flat picture with a dramatic improvement when the diode was changed." and having just about completed another TMB272 restoration was interested to see that the contrast on this set also moves the picture negative when advanced...although a mid-setting provides quite sufficient contrast for an excellent picture? My question is therefore: is it the case that "over-advancing" a contrast past its optimal setting will always result in a negative picture? Grateful for any thoughts/explanations. Trevor.

26th Aug 2008, 10:17 am	#2
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,422	Re: TMB272 - contrast "crushing". Hi Trevor. Yes indeed nearly all 405 line sets will do this and more especially those without any form of AGC, basically you are overdriving IF and the video stages, into distortion. A look on the scope on the anode of the video o/p valve will show the waveform gradually crushing, and it can even flip over this is the negative picture you will see. The Diode you refer to though gives a milky picture quite different to the negative effect. Trevor __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

26th Aug 2008, 10:26 am	#3
TNC Hexode Join Date: Apr 2003 Location: Westcliff-on-Sea, Essex, UK. Posts: 386	Re: TMB272 - contrast "crushing". Goodness me - 13 minutes for a reply is pretty amazing!! Thanks for the info Trevor - that makes a lot of sense - I tried substituting the diode after V5 (ERT) and it does not affect the crushing but now I realise that it also proves the existing diode is OK as there is absolutely no change in picture content - milkyness (or anything else)...so I can now put on the covers! Thanks, Trevor.

26th Aug 2008, 3:49 pm	#4
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: TMB272 - contrast "crushing". Another reason for this effect on some sets is the "black spotter" or "interference limiter" control being turned too far, or a fault therein. It's supposed to limit peak whites like the brilliant white spots due to car ignition or motor brushes. Turn it too far and all the whites become grey, but not the sort of defocused, silvery grey of a flat CRT. __________________ Mike.