Damage from out of spec line rates - UK Vintage Radio Repair and Restoration Discussion Forum

dominicbeesley · 9th Feb 2017, 11:31 am

A quick LOPT question...

I am building an 8-bit computer and would like to test my work so far. I am waiting for a 48MHz crystal to run the system clocks (divided down). At the moment I just have a 50MHz crystal and the line/frame signals generated are about 4% fast. Is that likely to damage my Sony PVM-2010 monitor, I know too slow can be bad....is 4% likely to be much too fast?

Thanks

Dom

kan_turk · 9th Feb 2017, 12:14 pm

Hi Dom

4% fast should not cause a problem as:

1. Magnitude of discrepancy is relatively modest

2. Faster generally has less impact than slower

Hope this helps

Rgds

John

dominicbeesley · 9th Feb 2017, 1:34 pm

Thanks John,

I thought not, but best to check...I'll give it a try tonight hopefully

dominicbeesley · 10th Feb 2017, 2:02 am

Thanks for the advice, I thought I'd report back

Well, I gave it a try and it didn't cause any smoke but wasn't overly stable. It seems the Sony is a bit fussy! At 61.4us a line it was definitely unhappy (see first pic). In the end I just faffed one of the registers on the CRTC chip a little and now it is closer to spec (63.36us) and happier. I should have thought of that earlier!

It seems that though the Sony is fussy it is not easily damaged, due to some mishaps it has been fed everything from 1kHz line rates to 40kHz and other than going out of sync didn't even make a squealing noise, I will have to dig out the schematic and see if this has a separate EHT oscillator.

It might be time to move on from breadboard tomorrow...

D

Argus25 · 11th Feb 2017, 4:14 am

The good news as Kan Turk points out, a faster line rate is no problem (If the set's horizontal hold/AFC system can lock to it). In this case the horizontal deflection transistor is held on for a shorter period of time than with a standard frequency signal. Therefore less magnetic field energy is stored in the yoke and line output transformer at the end of scan (beam at right side of the raster) so the picture width will be a little less and also at flyback, the flyback peak voltage on the collector of the H output transistor a little less and the EHT a little less too.

On the other hand a lower H rate signal, if the set could lock to it, increases the width and EHT , all other things equal.

However the beauty of TV's and most computer monitors, is that their internal horizontal oscillators and AFC system have a limited locking range, so that they simply won't lock to an H rate that is too low or too high, so they can't be damaged with a non standard video signal/sync or even when the video signal is just gross noise, like the noise between TV channels.

However, horrifyingly, there was a class of early computer monitor that didn't actually have an internal horizontal oscillator of its own. It relied on the feed from the computer. These are notorious for failure if fed with a non standard signal.

9th Feb 2017, 11:31 am	#1
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Damage from out of spec line rates A quick LOPT question... I am building an 8-bit computer and would like to test my work so far. I am waiting for a 48MHz crystal to run the system clocks (divided down). At the moment I just have a 50MHz crystal and the line/frame signals generated are about 4% fast. Is that likely to damage my Sony PVM-2010 monitor, I know too slow can be bad....is 4% likely to be much too fast? Thanks Dom

9th Feb 2017, 12:14 pm	#2
kan_turk Hexode Join Date: Mar 2014 Location: Dublin, Ireland Posts: 398	Re: Damage from out of spec line rates Hi Dom 4% fast should not cause a problem as: 1. Magnitude of discrepancy is relatively modest 2. Faster generally has less impact than slower Hope this helps Rgds John

9th Feb 2017, 1:34 pm	#3
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: Damage from out of spec line rates Thanks John, I thought not, but best to check...I'll give it a try tonight hopefully

10th Feb 2017, 2:02 am	#4
dominicbeesley Octode Join Date: Nov 2004 Location: Hebden Bridge, West Yorkshire, UK. Posts: 1,885	Re: Damage from out of spec line rates Thanks for the advice, I thought I'd report back Well, I gave it a try and it didn't cause any smoke but wasn't overly stable. It seems the Sony is a bit fussy! At 61.4us a line it was definitely unhappy (see first pic). In the end I just faffed one of the registers on the CRTC chip a little and now it is closer to spec (63.36us) and happier. I should have thought of that earlier! It seems that though the Sony is fussy it is not easily damaged, due to some mishaps it has been fed everything from 1kHz line rates to 40kHz and other than going out of sync didn't even make a squealing noise, I will have to dig out the schematic and see if this has a separate EHT oscillator. It might be time to move on from breadboard tomorrow... D Attached Thumbnails

11th Feb 2017, 4:14 am	#5
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Damage from out of spec line rates The good news as Kan Turk points out, a faster line rate is no problem (If the set's horizontal hold/AFC system can lock to it). In this case the horizontal deflection transistor is held on for a shorter period of time than with a standard frequency signal. Therefore less magnetic field energy is stored in the yoke and line output transformer at the end of scan (beam at right side of the raster) so the picture width will be a little less and also at flyback, the flyback peak voltage on the collector of the H output transistor a little less and the EHT a little less too. On the other hand a lower H rate signal, if the set could lock to it, increases the width and EHT , all other things equal. However the beauty of TV's and most computer monitors, is that their internal horizontal oscillators and AFC system have a limited locking range, so that they simply won't lock to an H rate that is too low or too high, so they can't be damaged with a non standard video signal/sync or even when the video signal is just gross noise, like the noise between TV channels. However, horrifyingly, there was a class of early computer monitor that didn't actually have an internal horizontal oscillator of its own. It relied on the feed from the computer. These are notorious for failure if fed with a non standard signal.