Famicom problem

Started by Earthshaker, March 16, 2016, 02:39:28 pm

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Earthshaker

Hi guys,
I have a problem with a Famicom that I AV modded some time ago. I modded many Famicom in these years and didn't hane any trouble. The mod works fine but the console sometimes doesn't turn on.. I replaced the capacitors next to the RF box (one was 1000uf and the other one around 22 uf). I also replaced the voltage regulator and the resistors, capacitors and pnp of the Famicom Av mod. The console seems to work.. but sometimes doesn't turn on at all... for hours. Then suddenly works perfectly.
Do you have some problem for the issue?
Sorry for my poor english >.<
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chowder

Have you tried a different power adapter?  Reflow the solder on the DC connector on the console too.

famifan

what did you mean under doesn't turn on ?

have you seen anything on the screen? gray or black screen maybe?

my bet goes for 0.1uF capacitor, which resides very close to the reset button. Inspect it and try to change to the similar any 0.1uF (even ceramic one).

the capacity of this capacitor shouldn't be too high, it's needed for resetting the CPU right at the moment when power has been turned on or the reset button had been unpressed. It adds a delay for reset signal, the actual time depends on the capacity. 0.1uF is highly enough in terms of amount of time which's required for the charge (or discharge) of this capacitor. The reason for having such a delay in use is that CPU needs some amount of time for startup. AFAIR, famicom's CPU doesn't have built-in reset capabilities, so it uses external reset circuitry like it was mentioned.

If replacing of this capacitor didn't have any effect, then you might to replace some of ceramic capacitors about quartz oscillating circuitry. They reside very close to the oscillator.  Also, there's an adjustable capacitor, it could be faulty. Try to replace it too. Even if all of that won't help, then try to replace quartz resonator itself.

zmaster18

Maybe your power switch and the wire connecting the switch to the board has a bad connection.

Earthshaker

Thank you for the replies!
I tried different power supply but nothing change.
When I say that it doesn't turn on, means that the console doesnt work at all. doesn't show anything on sceen..dead. (just sometimes the console is hot even if it doesn't work)
All the cables works fine..
I will try to change the capacitors next the reset button during this weekend as famicom suggested, and I will let you know!
Thank you very much guys!
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famifan

Quote from: zmaster18 on March 17, 2016, 11:23:56 am
Maybe your power switch and the wire connecting the switch to the board has a bad connection.


my bet goes for clocking issues. If quartz oscillating circuitry needs significant amount of time to produce stable results, then it could result in such a huge delays for startup. So it could probably and unpredictably work from time to time, but it should work on every very single powering on without the issues at all.

Quote
(just sometimes the console is hot even if it doesn't work)

yep, that's ok. when power switch is turned on, then 7805 is powering PPU and CPU and other components. Could you confirm that PPU and CPU are getting hot too even if the console doesn't work?

do you have a multimeter around?

if yes, have you measured the output voltage from 7805 when it's hot and the console doesn't work?

it should be very close to 5V.

Earthshaker

I just want to inform you that the problem was the whole power pcb.. I replaced the voltage regulator and the main caps, but it didn't want to work at all. Luckily I had another broken famicom at home, then I swapped the power pcb and now it works. Thank you to you all guys.
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