MC78T12 3Amp regulator - do I need it, where to buy?

alejandromad

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MC78T12 3Amp regulator - do I need it, where to buy?

I have an English Mark Darts (Series 6300) with a non-working monitor.
I was told it was working and simply quit.

OK... I measured the 12vdc in at the monitor (it's the Orion monitor) and got no voltage reading.

I moved back to the power supply output and read all the voltages out.
The only output that is missing is the 12vdc to the monitor.

Looking at the schematics, it would seem like the only possible failure point would be the voltage regulator 'U2'

I noted that it is very, very hot to the touch (while U3 for example is not).

The game itself, when hooked up to an external display, plays fine.

So, here are my questions...

1. Is my troubleshooting conclusion ... well... conclusive?
2. If not, how can I check this transistor to determine if it failed?
3. If so, where can I get a replacement or alternative part (google was not much help)?
 

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if your 12vdc is low and the reg. is hot, sounds like something is pulling the voltage down and thus the reg. is trying to work too hard. Desolder the ouput pin on the reg. and see if the voltage goes back up to 12vdc, showing the reg is good. Then you'll need to determin what is pulling down the 12 volt line.
 
if your 12vdc is low and the reg. is hot, sounds like something is pulling the voltage down and thus the reg. is trying to work too hard. Desolder the ouput pin on the reg. and see if the voltage goes back up to 12vdc, showing the reg is good. Then you'll need to determin what is pulling down the 12 volt line.

I disconnect J1 from the power supply PCB
I measure all the outputs on the PCB... they shouldn't be under any load since the connector is removed... and they all read as expected, correct voltages EXCEPT pin 8 which feeds 12vdc to the monitor

Pins 5 and 7 read 12vdc
Pins 4 and 6 read 5vdc
Pin 8 reads 0vdc

What do you mean 'desolder the output pin'?
Haven't I effectively done that by pulling the edge connector?

If I trace farther back in the schematic, towards the transformer, seems like the 5vdc circuit shares the rest of it, and that works, so that's why I suspect this voltage reg.
 
Once you install the new regulator on the power supply board, measure the output voltage without J1 plugged in. This will show you that the 12 volts for the monitor is good again. Now shut off the power and plug in J1. Now turn on the power and measure the 12 volts to the monitor again. It should still be there. If it drop way down or goes to zero, that means there's a short on the monitor chassis (usually a diode or the horizontal output transistor goes shorted).

There were only TWO of the 9" monitor manufacturers used in the 6000 series machines: Kristel (tan colored circuit board) and Omnivision (green colored circuit board).
 
Once you install the new regulator on the power supply board, measure the output voltage without J1 plugged in. This will show you that the 12 volts for the monitor is good again. Now shut off the power and plug in J1. Now turn on the power and measure the 12 volts to the monitor again. It should still be there. If it drop way down or goes to zero, that means there's a short on the monitor chassis (usually a diode or the horizontal output transistor goes shorted).

Excellent advice. The regulator failed for some reason so I will do as you suggest.

There were only TWO of the 9" monitor manufacturers used in the 6000 series machines: Kristel (tan colored circuit board) and Omnivision (green colored circuit board).

Ahhh... that makes sense. 'Orion' was the tube.
 
Alright Ken... I removed the VR and reinstalled the new one from Mouser.
Believe it or not, there was no change.
I left J1 disconnected and measured pin 8 - still no volts
All the other pins are still outputting the correct voltages.

Looking at the schematic I can't see any other possibility.
there's nothing but caps in that device's circuit (that would affect ONLY that and not U1 or U3)

Could U1 actually be causing the problem on U2 while simultaneously supplying the lamp voltage it should be providing?

Color me stumped.

... oh yeah, for historical purposes, the monitor PCB is green - so "Omnivision" it is.
 
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With the output connector J1 removed, the only thing connected across the output of 12 volt regulator is electrolytic capacitor C9.

Originally, C9 was a 1 uf @ 25 volt value. Later, Arachnid issued a service bulletin saying to bump the value up to a 4.7 uf @ 35 volt dipped tantalum or a conventional electrolytic is fine. As noted in my dart monitor repair guide, this capacitor is the second most troublesome part on this power supply.

Radio Shack # 272-1024 will work as a replacement. Be sure to observe the correct polarity as the board is not silkscreened with polarity markings.
 
OK... so I popped out C9 and replaced it with C11 for a quick test... tested it and sure enough, pin 8 now has 12vdc on it.

So, I am off to the shack!
 
Thanks for the assist Ken.
She is up and running again.
 

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A closeup picture of a modern dipped tantalum replacement capacitor for C9 on the power supply board.

Also a picture of the 12 volt regulator that provides the 12 volts only for the monitor.
 

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