Video jitter and crystal oscillators - UK Vintage Radio Repair and Restoration Discussion Forum

cmjones01 · 17th Apr 2012, 9:46 pm

I've posted this in 'Vintage Computers' because it seems most relevant here, though it's really a general tip with elderly digital electronics.

I've just been repairing a 'Moon Patrol' arcade game board dating back to 1982, as seen in this thread. Having replaced a faulty RAM chip, a few corroded chip sockets and repaired a via, it ran perfectly. But the display was marred by a slight horizontal jitter, about 0.5-1mm (on a 12" screen). I would have put it down to some effect of the game's odd vertical frequency (56Hz) with my monitor, except that I had another Moon Patrol which didn't exhibit the problem.

Almost by chance I found the source of the problem: it was the master 18.432MHz crystal oscillator that runs the whole game. Putting my finger on one of its pins removed the jitter and the display became crystal clear. The crystal oscillator is composed of a couple of TTL gates and capacitors, all of which seemed to be OK. I therefore settled for a modification: the usual trick of fitting a 22pF capacitor to ground where my finger had been. The jitter was gone and the display is now crystal clear.

I didn't have my spectrum analyser to hand, otherwise I'd have had a look at the output of the oscillator. Nothing untoward was visible on a scope. I don't really know what the problem was: maybe the crystal was losing its 'Q' with age, or the TTL chip is unhappy in some way. Maybe it came out of the factory that way and nobody ever noticed.

Attached are some photos which try to illustrate the problem, though it's hard to capture it in a picture. The pictures are closeups of the corner of the screen showing the number of lives left. See how the 'jittery' one is blurred horizontally on the diagonal of the '3' and between the truck's wheels.

I think a similar problem could occur in much vintage electronics which uses crystal oscillators and a video output, for example vintage computers. I hope it will help someone.

Chris

Refugee · 17th Apr 2012, 11:50 pm

That looks like it has had a new crystal fitted.
Is there any sign that the old one had the case earthed?
The capacitor would most likely not be required if the case had an earth.
There are two types of crystal too. They are cut differently.

cmjones01 · 18th Apr 2012, 8:05 am

I think the crystal is original. The glue holding it on to the PCB looks 30 years old, anyway, and there are no signs of rework around its pins. There is an earthed pad underneath the crystal, though, so if the crystal didn't have sleeving over it, it could be soldered down.

How would earthing its case stabilise the oscillator?

ppppenguin · 18th Apr 2012, 8:11 am

Crystal oscillators based on TTL gates are pretty crude devices. A crystal is capable of oscillating in several modes and it is quite possible that it's trying to oscillate in another mode (such as 3rd overtone) at the same time as its fundamental.

Years ago when I designed a Ch1 modulator using a 3rd overtone xtal in a simple Colpitts configuration it tended to oscillate on the fundamental. The simplest cure was a resistor across the crystal to damp it a bit. I admit this was crude but it was simple, effective and adequate.

Refugee · 18th Apr 2012, 8:57 am

There is a little bit of capacitance between the pins and the connections inside so an earth on the case is likely to work just as well as the added capacitor.
It does look a little bit new in the picture though and i must say there are some engineers out there that can make there work look pretty neat.

cmjones01 · 18th Apr 2012, 10:09 am

Quote:

Originally Posted by ppppenguin

Crystal oscillators based on TTL gates are pretty crude devices. A crystal is capable of oscillating in several modes and it is quite possible that it's trying to oscillate in another mode (such as 3rd overtone) at the same time as its fundamental.

That's interesting. I didn't try a resistor across the crystal to damp it, but have noticed that a lot of modern on-chip crystal oscillators in microcontrollers and the like recommend a megohm or so across the crystal. This could be the reason why.

Chris

ppppenguin · 18th Apr 2012, 10:18 am

A high value like 1M could well be just to give a DC path. I used 2K2 across my 45MHz rock!

Parabola · 18th Apr 2012, 10:44 am

Quote:

Originally Posted by Refugee

.... an earth on the case is likely to work just as well as the added capacitor.

Wyse data terminals contaning lots of TTL always had the cases of the crystals connected to earth, and they should know something about designing stable character displays.

I've had a number of occasions when repairing LCD computer monitors where jitter and line tearing have been caused by simply iffy crystals on the panel driver boards. Replacing them like for like with a new one always fixed the problem without modification, so crystals can become just plain unstable. The worst case of this was a monitor displaying a windows desktop with line tearing at the top like a bad video tape problem...very disconcerting!

cmjones01 · 18th Apr 2012, 1:42 pm

This is turning out to be interesting. Jeffrey, 2k2 is a bit scary! I'm tempted to have a play with damping resistors on this oscillator and see what happens.

Parabola, that's really interesting about duff crystals in modern monitors. I had no idea it was such a problem. I can see a good case for earthing the case of a crystal in general, though many surface-mount crystals don't have any means of doing this, which strikes me as a bad idea (so I don't design them into my work).

One of the things I find interesting about these arcade games is that they're designed to earn money, so they're generally built down to a price with the worst possible components consistent with enough reliability to get the 10 pence pieces flowing into the coin box. The effects of this are not just rubbish IC holders and electrolytics, but more subtle as well, it seems.

Chris

Refugee · 18th Apr 2012, 2:32 pm

If the crystal is close to flipping harmonics a little bit of 50hz on the case might just "set it off" to another harmonic for a few cycles now and again and lead most engineers to think it is hum pickup in the CRT driver PCB. That little bit of screening can help a lot.

cmjones01 · 18th Apr 2012, 11:46 pm

I had another play with the board this evening, with the following results:

- removed 22pF capacitor. Jitter returns.
- earthed crystal can. No effect.
- Tried various resistors across the crystal, from 10k upwards. Lower values make the oscillator slow down, higher ones (>220k) have no effect.
- Tried resistors from one leg of crystal to ground. No effect.

So I've gone back to the 22pF capacitor to ground...

ppppenguin · 19th Apr 2012, 7:49 am

Be a pragmatist: do what works

Another possible mechanism. As I said before, TTL based xtal oscillators tend to be rather crude. It's possible that the TTL devices are trying to work as a relaxation oscillator without the benefit of xtal control, hence causing jitter. The 22p (or Brisith Standard Finger) supplies just enough extra load to suppress this.

cmjones01 · 19th Apr 2012, 8:09 am

Quote:

Originally Posted by ppppenguin

Be a pragmatist: do what works

Done

17th Apr 2012, 9:46 pm	#1
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,681	Video jitter and crystal oscillators I've posted this in 'Vintage Computers' because it seems most relevant here, though it's really a general tip with elderly digital electronics. I've just been repairing a 'Moon Patrol' arcade game board dating back to 1982, as seen in this thread. Having replaced a faulty RAM chip, a few corroded chip sockets and repaired a via, it ran perfectly. But the display was marred by a slight horizontal jitter, about 0.5-1mm (on a 12" screen). I would have put it down to some effect of the game's odd vertical frequency (56Hz) with my monitor, except that I had another Moon Patrol which didn't exhibit the problem. Almost by chance I found the source of the problem: it was the master 18.432MHz crystal oscillator that runs the whole game. Putting my finger on one of its pins removed the jitter and the display became crystal clear. The crystal oscillator is composed of a couple of TTL gates and capacitors, all of which seemed to be OK. I therefore settled for a modification: the usual trick of fitting a 22pF capacitor to ground where my finger had been. The jitter was gone and the display is now crystal clear. I didn't have my spectrum analyser to hand, otherwise I'd have had a look at the output of the oscillator. Nothing untoward was visible on a scope. I don't really know what the problem was: maybe the crystal was losing its 'Q' with age, or the TTL chip is unhappy in some way. Maybe it came out of the factory that way and nobody ever noticed. Attached are some photos which try to illustrate the problem, though it's hard to capture it in a picture. The pictures are closeups of the corner of the screen showing the number of lives left. See how the 'jittery' one is blurred horizontally on the diagonal of the '3' and between the truck's wheels. I think a similar problem could occur in much vintage electronics which uses crystal oscillators and a video output, for example vintage computers. I hope it will help someone. Chris Attached Thumbnails

17th Apr 2012, 11:50 pm	#2
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Video jitter and crystal oscillators That looks like it has had a new crystal fitted. Is there any sign that the old one had the case earthed? The capacitor would most likely not be required if the case had an earth. There are two types of crystal too. They are cut differently.

18th Apr 2012, 8:05 am	#3
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,681	Re: Video jitter and crystal oscillators I think the crystal is original. The glue holding it on to the PCB looks 30 years old, anyway, and there are no signs of rework around its pins. There is an earthed pad underneath the crystal, though, so if the crystal didn't have sleeving over it, it could be soldered down. How would earthing its case stabilise the oscillator?

18th Apr 2012, 8:11 am	#4
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Video jitter and crystal oscillators Crystal oscillators based on TTL gates are pretty crude devices. A crystal is capable of oscillating in several modes and it is quite possible that it's trying to oscillate in another mode (such as 3rd overtone) at the same time as its fundamental. Years ago when I designed a Ch1 modulator using a 3rd overtone xtal in a simple Colpitts configuration it tended to oscillate on the fundamental. The simplest cure was a resistor across the crystal to damp it a bit. I admit this was crude but it was simple, effective and adequate.

18th Apr 2012, 8:57 am	#5
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Video jitter and crystal oscillators There is a little bit of capacitance between the pins and the connections inside so an earth on the case is likely to work just as well as the added capacitor. It does look a little bit new in the picture though and i must say there are some engineers out there that can make there work look pretty neat.

18th Apr 2012, 10:18 am	#7
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Video jitter and crystal oscillators A high value like 1M could well be just to give a DC path. I used 2K2 across my 45MHz rock!

18th Apr 2012, 1:42 pm	#9
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,681	Re: Video jitter and crystal oscillators This is turning out to be interesting. Jeffrey, 2k2 is a bit scary! I'm tempted to have a play with damping resistors on this oscillator and see what happens. Parabola, that's really interesting about duff crystals in modern monitors. I had no idea it was such a problem. I can see a good case for earthing the case of a crystal in general, though many surface-mount crystals don't have any means of doing this, which strikes me as a bad idea (so I don't design them into my work). One of the things I find interesting about these arcade games is that they're designed to earn money, so they're generally built down to a price with the worst possible components consistent with enough reliability to get the 10 pence pieces flowing into the coin box. The effects of this are not just rubbish IC holders and electrolytics, but more subtle as well, it seems. Chris

18th Apr 2012, 2:32 pm	#10
Refugee Dekatron Join Date: Apr 2012 Location: Worksop, Nottinghamshire, UK. Posts: 5,554	Re: Video jitter and crystal oscillators If the crystal is close to flipping harmonics a little bit of 50hz on the case might just "set it off" to another harmonic for a few cycles now and again and lead most engineers to think it is hum pickup in the CRT driver PCB. That little bit of screening can help a lot.

18th Apr 2012, 11:46 pm	#11
cmjones01 Nonode Join Date: Oct 2008 Location: Warsaw, Poland and Cambridge, UK Posts: 2,681	Re: Video jitter and crystal oscillators I had another play with the board this evening, with the following results: - removed 22pF capacitor. Jitter returns. - earthed crystal can. No effect. - Tried various resistors across the crystal, from 10k upwards. Lower values make the oscillator slow down, higher ones (>220k) have no effect. - Tried resistors from one leg of crystal to ground. No effect. So I've gone back to the 22pF capacitor to ground...

19th Apr 2012, 7:49 am	#12
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Video jitter and crystal oscillators Be a pragmatist: do what works Another possible mechanism. As I said before, TTL based xtal oscillators tend to be rather crude. It's possible that the TTL devices are trying to work as a relaxation oscillator without the benefit of xtal control, hence causing jitter. The 22p (or Brisith Standard Finger) supplies just enough extra load to suppress this.