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PostPosted: Fri Aug 11, 2017 6:03 am 
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First post here, hope i do this right.
ok, yesterday i just got my first Famicom, i had several PAL NES in my hands but never had a chance to play with NTSC hardware, i noticed that while i plug it in the 7805 gets very VERY hot, i almost burned my fingers by touching the heatsink, none of the other chips get warm on the main board.. i measured voltage on the board and it was less than a volt, it was also showing a short between VCC and GND.
first thougt was a bad 7805 but even after removing it the board still shows a short..

Any clues? also i can't get any replacements because the local electronics store is closed for holiday..

maybe i should start pulling chips from the board..

The board is a HVC-CPU-07.


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PostPosted: Fri Aug 11, 2017 10:02 am 
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I'm a little surprised that something else didn't get warm when the 7805 was driving 1A into 1V . That should still be roughly 1W being dissipated somewhere.


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PostPosted: Fri Aug 11, 2017 10:06 am 
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This kind of troubleshoot tends to be a little time consuming, since it can be anything: from an electrolytic or decoupling capacitor to an IC or a bad soldering doing a short.
I would suggest you to look to things that get hot, but seems you already have looked at the ICs...
I once had a problem just like this on a NES clone, and after removing almost all ICs I found it was the RF modulator. :evil:

Edit: I just remembered a nice trick!!
You don't need to desolder the whole IC in this case, just take the soldering out of the Vcc or GND pin.
Make sure the pin is not touching the board. Check with the continuity test.
It's kind of tricky, but it's a lot faster than removing all of the IC's pins!!
It also works with any kind of componnent, not just ICs.


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PostPosted: Fri Aug 11, 2017 10:57 am 
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lidnariq wrote:
I'm a little surprised that something else didn't get warm when the 7805 was driving 1A into 1V . That should still be roughly 1W being dissipated somewhere.

That. At work we'd solve the problem using a thermal camera, but I doubt you could afford one. If you came pass by ours, I'd gladly help solve the problem but I don't think it's possible for you.

In those case you should visually analize the board, solders and everything. If all else fails, just insert several amperes between VCC and GND and the short-circuit should melt by itself. In cases where a tiny wire of copper causes a short, this is a life-saver trick.


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PostPosted: Fri Aug 11, 2017 11:07 am 
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Bregalad wrote:
If all else fails, just insert several amperes between VCC and GND and the short-circuit should melt by itself.
Just to be extra paranoid: do make sure that it's still voltage limited. When the short ceases to be a short it wouldn't do to destroy anything else afterwards.


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PostPosted: Fri Aug 11, 2017 11:24 am 
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A very tedious but workable method to find a short is to measure the resistance at different parts of the board (on the chips). If you meter is accurate enough and has enough resolution the closer you get to the short the lower the resistance gets.

edit: I dont recommend disconnecting the power/ground on chips and not removing the whole chip. This way the chip might get powered through an i/o pin and that's bad news(tm)


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PostPosted: Fri Aug 11, 2017 12:04 pm 
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lidnariq wrote:
Just to be extra paranoid: do make sure that it's still voltage limited. When the short ceases to be a short it wouldn't do to destroy anything else afterwards.

Yeah, basically I'd use a lab 5V supply with limited current, and increase the current knob until the short melts. It is not guaranteed to work, but it can work if the short is caused by a tiny hair of metal.
If it fails, you can still see which part heats and this is where the short circuit is.

And there is nothing paranoid about not wanting to use +30V on chips supposed to be powered in +5V, this would burn them quickly in smoke.


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PostPosted: Fri Aug 11, 2017 1:36 pm 
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Looks like you can try to turn an cheap camera into a infrared one, as said here.
I just don't know how usefull this can be, if usefull at all.
Jeroen wrote:
I dont recommend disconnecting the power/ground on chips and not removing the whole chip.

Looks like I had luck and didn't destroy anything. :shock:
Another bad practice I'm going to leave!!
Thanks for pointing out. :beer:

Don't a heatgun makes easier to remove DIP ICs?


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PostPosted: Fri Aug 11, 2017 1:38 pm 
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Another part you might want to check that fails often is the bridge rectifier. I believe it resides in the RF modulator which might be why you didn't notice it getting hot.

edit: found a pic, its the part labeled D4 (upside down) http://i48.tinypic.com/4tprmr.jpg

edit2: just thinking some more, but the bridge rectifier sits before the 7805, so that wouldn't explain why the 7805 is getting so hot. Stil a good idea to check it though.


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PostPosted: Sat Aug 12, 2017 7:43 am 
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1. Check cartridge connectors, especially pin 1 and 31 (its GND and +5V and they're opposite themselves) - if they were bent to make better contact, they might short if no cartridge is present
2. RAM memories might be shorted
3. CPU or PPU might be shorted
4. 74HC368 might be shorted
5. Something inside metal box with modulator might be shorted

During repairing tens of famiclones, no other chips were cause of any problems.

BTW. Famicom does not have bridge rectifier.


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PostPosted: Sun Aug 13, 2017 6:02 am 
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thanks everybody for the good advice, i appreciate active communities like this one :D

lidnariq wrote:
I once had a problem just like this on a NES clone, and after removing almost all ICs I found it was the RF modulator. :evil:

i removed the RF modulator but it made no difference..

Jeroen wrote:
A very tedious but workable method to find a short is to measure the resistance at different parts of the board (on the chips).

i was going to try measuring the chips resistance but i misplaced my multimeter and i am left with a crappy analog meter that can't even measure Ohm's.

i think the best thing i could do is get some sockets and transplant the CPU and PPU in my good PAL nes and see if they work.
in the event that one of those is fryed i don't know where to get replacements..


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PostPosted: Sun Aug 13, 2017 6:45 pm 
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One way you could do it is to cut power to the pins one by one, and measure between power and ground with a multimeter. Do not power it up like this.

If a chip has been verified to not be the culprit you can use some solder to rebridge the pins on the chip.

Downside is that this is a bit of a destructive process.

edit: check if its not something like a capacitor first though.


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PostPosted: Mon Aug 14, 2017 11:40 am 
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Jeroen wrote:
One way you could do it is to cut power to the pins one by one

Maybe instead of cutting the pins he could try to disconnect them, being a lot less destructive.
If no current goes through the circuit he has no risk of powering the chip by an i/o pin, as stated before.


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