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PAL 7800 resurrection - technical help needed!


ongikong

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Hey guys, maybe someone is able to help me, I'm a tech-rookie... :-)

After I did a successful S-Video mod with Magic Knights PCB my 7800 died 2 weeks ago.

It didn't want to start up anymore. :_(

I measured voltages at the power socket: About 14V when turned off and 9V when turned on.

So the power button is not the culprit.

The MJE210 transistor near the power socket got very hot,

The 7805 voltage regulator behind the cartridge slot had an output of about 5V but strangely enough i measured an input of 0V...

I replaced both with new parts, as well as the 2 smaller capacitors on the mainboard.

But nothing's changed... It's still dead.

Yesterday I noticed that the inductor(?) (circled red in the attachment) has continuity/might be "open"?

Does someone may have the exact term for that part? Maybe that's causing the problems...
Or are there any other ideas? Hope, someone can help! Thanks!

 

 

post-35449-0-90317000-1557245169.jpg

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What you're describing might be more indicative of probe issues. There's no way you'd get 5V out with 0V in.

 

Also, to make sure you have terminology correct: Continuity means that there is conduction. "Open" means that the conduction is blocked off.

 

With that in mind, inductors should show continuity and not "open".

 

Now, there are two probable issues that would cause MJE210 to get hot. One would be if it's not turning on all the way, which makes it a resistor. The other is if there's a short afterwards. Does the 7805 get hot too?

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There is a flip flop in the 7800 just like the 5200 that I've had to replace at least once that I can recall.

 

Also to measure the 7805, you should be measuring the input voltage from pin 1 to the far left on the 7805. Center pin is ground and the right pin is the output that should be around +5.

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Thanks for your answers and sorry if my explanations weren't that precisely. my mother tongue isn't english... :-(

 


Yes, the inductor is NOT open. It shows continuity. So I think this is all fine!

I measured the 7805 again. Sorry for reversing the in/out-voltages in my first explanation.

I get about 5V in and about 0.2V out... And yes, it also gets hot pretty quickly.

But more frightening was the fact that the solder under the MJE210 even began to melt after a few seconds (I guess the heat comes mainly from the center pin)... so this one is VERY hot....

So being afraid of damaging even more on the board i promptly pulled the power plug.

 

I couldn't find that 14013 flip flop chip that often causes problems in NTSC machines.

Here I found a picture of my PAL mainboard:

753c5e64c3824ed9881ea3a571cba3fd.jpg

 

 

 

 

 

 

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Then you have short there somewhere because 5 volt to the input of the 7805 isn't likely enough to kick it into action as it really needs about +7 in most cases I've found. But the MJE transistor shouldn't be getting hot and definitely not to the point of melting its own solder underneath it?!

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If the short is after the 7805, it will pull the output voltage close to zero while straining the input for all it's worth, along with that power transistor.

 

Well, a short that bad probably has visible evidence, so take a very close look at your board, especially around the areas where the modding was done. Look for solder blobs and stray wires/melted insulation.

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Success!!! It's alive again - thanks guys! I appreciate your help a lot! :thumbsup:

Stupid me... I had a short under the S-Video PCB where I forgot to cut off some longer blank wires. :sleep:

Next time I will be more investigative.

 

Thanks again and good night from Germany

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