Rasterlist Archives: RE: Breakout PCB & Motorola monitor

From: Paul Sommers <Paul.Sommers_at_cwo.com.au>
Date: Tue Sep 26 2000 - 16:49:57 EDT

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Rodger,

Thanks. I'm not up to the challenge of re-winding a transformer, so I will take your second suggestion and get the 12 volt job. These are cheap and easy to come by.

I assume if I get the phasing wrong, I just change it.

I'll give it a try tonight.

Thanks
 Paul

PS: The transformer isn't running hot at all - although the voltage regulator gives off a LOT of heat. I've even installed a fan in the back door.
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 From:   Rodger Boots
 Reply To:       rasterlist@synthcom.com
 Sent:   Tuesday, 26 September 2000 17:45
 To:     rasterlist@synthcom.com
 Subject:        Re: RASTER: Breakout PCB & Motorola monitor - questions for everyone!!!!

If I remember correctly, the H series of voltage regulators were good for 5 amps.á No need to change it.á You might try using BOTH the 8000 AND the pair of 4700 uF capacitors together to minimize the valleys of the filtered DC ripple.

OR you could rewind the existing transformer, if it isn't currently running very hot.á (I'm not necessarily joking about this, but don't try it unless you think you're up to the challenge).

If there is still room on the transformer for additional turns (not usually).á On a large transformer they might be getting out as much as a volt per turn, so we aren't talking much here.

Another possibility is to boost the primary voltage to the transformer.á You want 16.5, you have 15.á That's a boost of 16.5*100/15 = 110% of the current line voltage.á So take a small (at least one amp) 12 volt transformer, connect its primary to the line and the secondary in series with your large transformer's primary.á If you get the phasing wrong it will further reduce the output of your large transformer.á If you get it right, though, the output should slide right up where you need it.

á

Paul Sommers wrote: 

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á

Rodger,

I pretty much came to the same conclusion last night. Here's what I did...

Turned it on with no game board - there was small ripples oná screen edges (no sync or signal) but no humbars.

I replaced the old 8000uf with 2x4700uf. No difference.

I went back and looked at the schematic for the paddle problem. I then tried adding a 1uf lytic between pin 5 of the 555 and GND. There is still paddle flutter, but it is constant across the screen at about a half inch rather than up to 2 inches on the extremes.

I then tried a 10uf in the same spot but it went back to the big paddle flutter. I suspect the 10uf was big enough to affect the timing.

There was nothing left to look at except the voltage regulator - UA78H05S (of which I can find no info). Checking against the schematic this is not the original - it should be a LM323 which is rated at 3amps. All the 7805's that I could find were rated at 1 amp or 500ma.

As I can't get a LM323 in Australia I started to look on the web for an equiv and that's when it hit me.....

The original transformer put out 16.5vac to the board. The transformer I was able to find is only 15vac. I thought it would be enough as most voltage regulators have a bit of tolerance.

Checking the 5v at the chips gave me 4.94v... pretty close.

I did a check of the local electronic distributors catalogue and they have one at 18vac. I'm thinking I should try this. The spec sheet on the common 7805 and the LM323 says that it should handle both voltages, but maybe it doesn't have enough grunt.

But now looking at your mail I could run into the problem if I go too high as well....

No-one has a 16.5vac transformer. Why on earth did Atari used such weird voltages????

Thanks for working with me on this one.

Cheers 
 Paul 
 á

---------- 
 From:áá Rodger Boots 
 Reply To:áááááá rasterlist@synthcom.com 
 Sent:áá Tuesday, 26 September 2000 03:16 
 To:áááá rasterlist@synthcom.com 
 Subject:ááááááá Re: RASTER: Breakout PCB & Motorola monitor - questions for everyone!!!!

You know, I didn't really think about it, but the problem might be the replacement transformer!á Check your +5 volts on the game board and see if it is too high or too low.á Voltmeter OK, but scope is even better.á None of this applies if you have only ONE humbar.

Possibility one, too low.á In this case either the capacitor is bad OR the transformer voltage is too low, causing the regulator to drop out of regulation.

Possibility two, too high.á In this case, since this is an early Atari board, the voltage peaks (if high enough) can cause the regulator to quit regulating.á What happens is these old boards run most of their current through a large resistor connected across the regulator with the regulator picking up the last little bit of current.á Otherwise the regulator will overheat and shut down.á Now, IF the transformer voltage were too high, the regulator current would drop to zero and ALL the current would go through the resistor.á This would happen during voltage peaks and cause the +5 volts to increase (but only during the peaks).

What got me thinking about all this is that the problem didn't start until the transformer got changed.á If you do have a high voltage condition and you are lucky enough to have a higher voltage primary transformer setting available (example, you currently are using the 110 volt tap and switch to using the 120 volt tap) you can fix it that way.

á 
 á

Rodger Boots wrote: 

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        á

Paul Sommers wrote: 

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        ááááááá The 8000uf at 80v won't fit on the board (the one that is their is surface mount) so I'll get a couple of big ones in series - that should do it. BUT, before we get too carried away with this, I have a question.á Is there ONE hum bar or TWO?á Very simple rule here:

ááááááááááá ONE hum bar = bad rectifier (or fuseholder in series with rectifier) 
 ááá TWO hum bars = filter capacitor problem.

That's a good tip to remember. No hum bars. It's a continuous wave.

Imagine a grayscale light to dark - then put another on with the light side butted to the dark edge. There is no break - it just keeps coming. It's a vertical monitor - so it is going across the screen(or down/up if it was horiziontal). I guess if two hum bars mean a filter cap problem - 8 could mean a bad filet cap problem.

á

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        OK, you're saying you have TWO dark waves?á Or two greyscales?á Whatever, if there are two of anything the capacitor (8,000 uF) is bad.
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-- 
 Windows:

32 bit graphical interface for a 
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 4 bit processor by a 
 2 bit company that can't stand 
 1 bit of competition. 
 á

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Received on Tue Sep 26 14:08:59 2000

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RE: Breakout PCB & Motorola monitor - questions for every one!!!!