Re: Amplifone jitters

On Wed, 24 Jun 1998 10:32:50 -0400, Joel Rosenzweig <joel-r@an.hp.com> wrote:

>Last night I had a few moments so I started to do a little
>troubleshooting. Hooking up the scope proved that the video output is
>clean, therefore it's the monitor that has some problem.
>
>I actually noticed two different problems, the jitter is one, but the
>other is that now all of a sudden, those fireball explosions which are
>bright, cause the screen to expand and contract about an inch. It's
>just like what some people report when the Death Star explodes, however,
>my system has never exhibited this behavior until now, and it happens
>not just on the Death Star explosion, but on any explosion that's
>bright.
>
>Didn't someone say that the expanding and contracting occured because
>the 24 volt regulators couldn't supply enough current at that moment?

That's the reason, Atari added pass resistors across the regulators to supply
the extra current needed to run the HV when the screen is bright, but it also
means it's unregulated under high current loads. The 78xx/79xx regulators are
guaranteed to a certain current, but I've noticed a big difference between them
when it comes to maximum current. How bright your screen is before it starts to
bloom will be dependent upon the maximum current the 78xx/79xx puts out before
going into current regulation mode.

>If so, could this effect be eliminated by having a more robust +/-
>24volt supply replace those regulators? Could the LV2000, for instance
>be used instead to supply a nice regulated +/- 24 volts?

You could use a LV2000, but you need to add the pass transistor that are present
in the WG that are not there in the Amplifone.

>With respect to the loose wire issue, I'll take a look tonight. I did
>check the ground wires on the power supply, and they were still firmly
>in place.
>
>The good news is that I can swap in any component in the game with
>others, so that will at least tell me which assembly is at fault if it
>comes to that.
>
>Anders Knudsen wrote:
>>
>> I seem to remember that Jeff Hendrix had this similar problem with an
>> ampliphone. It turned out that some wire (might have been the HV wire) off
>> the transformer was loose. I am speaking from faded memory here though.
>> Perhaps Jeff can remember. If you could stick a scope in XY mode off the
>> PCB outputs and they are "clean", then your jitter problem is the monitor.
>>
>> -Anders.
>>
>
>Zonn wrote:
>
>>Just out of curiosity, do you still have the metal cage around the HV? Not
>>having the cage will cause the displays to jitter.
>
>I didn't realize there was a metal cage around the Amplifone's HV board?
>Is this a typo?

I've seen cages around some HV sections on some Amplifones, and just assumed the
ones that didn't were lost. Maybe Atari quite supplying them, who knows.

>I guess the absence of mine is not helping things any. However, I do
>own the metal cages for the Wells Gardner monitors, but I don't use
>them, but their absence has not had any negative effect (jitter wise at
>least) on the video quality. The Wells Gardners seem to be rock steady
>as long as the ground wires are solid, and the caps in the LV supply are
>not leaky. At least, that has been my experience with my Tempest which
>once suffered from jitter problems until I checked these two things. I
>actually had the problem in two different Tempests, the ground wires
>fixed one, and the LV cap replacement fixed the other beautifully.

When removing the cages on any of my WG's I notice a small triangle wave ripple
slightly appear around the edges. Some worse than others, but I always see some
type of interference.

>If anyone has any other leads to follow up, that would be great! If
>someone could comment on the utility of replacing those 24v regulators
>with a better supply, that would be good too.

The "jitters" on Amplifones are always worse than the WG monitors. Amplifone
didn't bother to regulate the low voltage section of the deflection circuits. I
thought "Great Idea" no low voltage regulator, no low voltage problems!

So I experimented with WG's by regulating only the HV, like Amplifone does, and
I ended up with jitters all around the edges, like Amplifones do.

My guess is that there is interaction between the axis that bleed over through
the non-regulated power supply, that and a lack of constant voltage also causes
the lines to shake a bit.

I assume Atari thought the reliability of removing the low voltage regulators
was more important than the occasional customer complaining about "jitterness"
around the edges.

-Zonn

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Received on Wed Jun 24 13:31:42 1998