Colour banding playback fault on Philips N1500 / N1700 - UK Vintage Radio Repair and Restoration Discussion Forum

collisr · 20th Sep 2023, 1:00 pm

Hello everyone.

Some 70s Philips VCR expertese needed here I think. I regularly experience a playback phenomenon that is bugging me. As the clip shows, the top 1/3 of the screen intermittently shows lines of colour errors horizontally across the screen. In my experience its only exhibited on Philips N1500 and N1700 series machines. It can happen to any length/brand/chemical composition of tape, tracking doesn't affect it and it seems almost impossible to correct. The only thing I have found that (sometimes) helps is to play the tapes outside of their cassette shell and manipulate the tape path to apply more back tension to the top of the tape than the bottom. And even this could be co-incidence. Wear / lack-of-wear on the video heads doesn't seem to make any difference, cleaning the tape path and drum either.

Does anyone have a definitive explanation of causes this - and tips for correcting?

Thanks in advance.

https://youtu.be/P4gU-ErXHUY

whaka54 · 21st Sep 2023, 10:27 am

hello,

here's a video with some (i hope clear

) explainations on how back tension work.
in very short, check all parts involved in back tension under the supply reel.
there's rubber which can go bad, and also the friction "wheel" can get a bad shape.

https://youtu.be/LHYSUKfYx5A

toshiba tony · 21st Sep 2023, 10:33 am

It appears that if the back tension is OK the entry guide is a tad out. That's if its a known good recording. Good luck!

collisr · 21st Sep 2023, 5:51 pm

Thank you Whaka54 for the video. The rubber brakes under the supply reel seem okay and I'm still not 100% sure this is a standard back tension issue. Do you think it is? If I play the tapes outside of their cassette housing and manually add varying amounts of back tension by putting jigs in the tape path, it doesn't affect the colour banding.

I can alter (sometimes fix) the problem by warping the tape path so the top of the tape has more tension that the bottom by use of a jig attached to erase head.

Perhaps, as Toshiba Tony suggests it is an entry guide problem. I notice as these machines get older, the whole lower drum now has some play in it (up and down) which I guess is affecting the alignment on the drum.

But why colour banding at the top 1/3rd of the screen - anyone hazard a guess?

TIMTAPE · 21st Sep 2023, 8:22 pm

Have you inspected the tapes at their very top edges? Any stretching (as shown by frilling) or creasing common to all tapes? Any sign of oxide scratched off at that top edge? Otherwise I'd mark the current position of the entry guide height with a fine Sharpie pen before temporarily adjusting it up or down a little. Is the entry guide advancing all the way in?

collisr · 22nd Sep 2023, 8:36 am

Hello TimTape.

In my experience the tapes that experience this issue all look fine in terms of frilling and oxide loss.

I am starting to wonder whether this is down to the amount of play (up and down) in the lower drum.

I'll try to rig up something that can manipulate this movement and see if it affects the situation.

Thanks all

corkers · 24th Sep 2023, 11:47 am

Hi,

This looks very similar to colour banding which oftern occurs on the Sony C7 Betamax, when the image has a lot of colour saturation, e.g. a lot of blue such as a sky scene, a band of colours flicker across the top.

A lot of trial and error took me to the AFC circuit. When adjusting one of the ferrite core inductors by about a quarter turn, the banding disappeared. Any further adjustment causes the colour to drop out completely.

Hope this helps...

whaka54 · 24th Sep 2023, 12:45 pm

Quote:

Originally Posted by collisr

I notice as these machines get older, the whole lower drum now has some play in it (up and down) which I guess is affecting the alignment on the drum.

they all have a bit of play, don't worry on that.

collisr · 26th Sep 2023, 4:15 pm

Hi Corkers.

Very interesting point. I too have had to adjust this on a C7. Not sure if such an adjustment on the Philips N1700 exists though. The service manual describes adjusting the Chroma amplitude in the Chroma-AutoGainControl (AGC)module and there is an adjust on the 4.43Mhz crystal but no specific AFC.

Do you or anyone know what the C7 AFC adjustment equivalent is on a Philips N1700?

Rob

Quote:

Originally Posted by corkers

Hi,

This looks very similar to colour banding which oftern occurs on the Sony C7 Betamax, when the image has a lot of colour saturation, e.g. a lot of blue such as a sky scene, a band of colours flicker across the top.

A lot of trial and error took me to the AFC circuit. When adjusting one of the ferrite core inductors by about a quarter turn, the banding disappeared. Any further adjustment causes the colour to drop out completely.

Hope this helps...

20th Sep 2023, 1:00 pm	#1
collisr Pentode Join Date: Oct 2016 Location: Ware, Herts, UK Posts: 157	Colour banding playback fault on Philips N1500 / N1700 Hello everyone. Some 70s Philips VCR expertese needed here I think. I regularly experience a playback phenomenon that is bugging me. As the clip shows, the top 1/3 of the screen intermittently shows lines of colour errors horizontally across the screen. In my experience its only exhibited on Philips N1500 and N1700 series machines. It can happen to any length/brand/chemical composition of tape, tracking doesn't affect it and it seems almost impossible to correct. The only thing I have found that (sometimes) helps is to play the tapes outside of their cassette shell and manipulate the tape path to apply more back tension to the top of the tape than the bottom. And even this could be co-incidence. Wear / lack-of-wear on the video heads doesn't seem to make any difference, cleaning the tape path and drum either. Does anyone have a definitive explanation of causes this - and tips for correcting? Thanks in advance. https://youtu.be/P4gU-ErXHUY

21st Sep 2023, 10:27 am	#2
whaka54 Triode Join Date: Mar 2023 Location: Nancy, France. Posts: 33	Re: Colour banding playback fault on Philips N1500 / N1700 hello, here's a video with some (i hope clear ) explainations on how back tension work. in very short, check all parts involved in back tension under the supply reel. there's rubber which can go bad, and also the friction "wheel" can get a bad shape. https://youtu.be/LHYSUKfYx5A

21st Sep 2023, 10:33 am	#3
toshiba tony Heptode Join Date: May 2006 Location: Accrington, Lancashire, UK. Posts: 970	Re: Colour banding playback fault on Philips N1500 / N1700 It appears that if the back tension is OK the entry guide is a tad out. That's if its a known good recording. Good luck!

21st Sep 2023, 5:51 pm	#4
collisr Pentode Join Date: Oct 2016 Location: Ware, Herts, UK Posts: 157	Re: Colour banding playback fault on Philips N1500 / N1700 Thank you Whaka54 for the video. The rubber brakes under the supply reel seem okay and I'm still not 100% sure this is a standard back tension issue. Do you think it is? If I play the tapes outside of their cassette housing and manually add varying amounts of back tension by putting jigs in the tape path, it doesn't affect the colour banding. I can alter (sometimes fix) the problem by warping the tape path so the top of the tape has more tension that the bottom by use of a jig attached to erase head. Perhaps, as Toshiba Tony suggests it is an entry guide problem. I notice as these machines get older, the whole lower drum now has some play in it (up and down) which I guess is affecting the alignment on the drum. But why colour banding at the top 1/3rd of the screen - anyone hazard a guess?

21st Sep 2023, 8:22 pm	#5
TIMTAPE Octode Join Date: Jul 2006 Location: Perth, Western Australia Posts: 1,921	Re: Colour banding playback fault on Philips N1500 / N1700 Have you inspected the tapes at their very top edges? Any stretching (as shown by frilling) or creasing common to all tapes? Any sign of oxide scratched off at that top edge? Otherwise I'd mark the current position of the entry guide height with a fine Sharpie pen before temporarily adjusting it up or down a little. Is the entry guide advancing all the way in?

22nd Sep 2023, 8:36 am	#6
collisr Pentode Join Date: Oct 2016 Location: Ware, Herts, UK Posts: 157	Re: Colour banding playback fault on Philips N1500 / N1700 Hello TimTape. In my experience the tapes that experience this issue all look fine in terms of frilling and oxide loss. I am starting to wonder whether this is down to the amount of play (up and down) in the lower drum. I'll try to rig up something that can manipulate this movement and see if it affects the situation. Thanks all

24th Sep 2023, 11:47 am	#7
corkers Diode Join Date: Dec 2010 Location: Manchester. Posts: 2	Re: Colour banding playback fault on Philips N1500 / N1700 Hi, This looks very similar to colour banding which oftern occurs on the Sony C7 Betamax, when the image has a lot of colour saturation, e.g. a lot of blue such as a sky scene, a band of colours flicker across the top. A lot of trial and error took me to the AFC circuit. When adjusting one of the ferrite core inductors by about a quarter turn, the banding disappeared. Any further adjustment causes the colour to drop out completely. Hope this helps...