Famicom not working

Started by amurphy245, January 26, 2020, 11:21:22 am

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amurphy245

I bought a famicom that the seller told me didn't work ,I took a gamble thinking he didn't know about the RF NTSC issue on UK TVs ,anyway i did the AV mod but when it came to testing I'm not getting anything.

I don't think it's a problem with my av mod because i tested with an everdrive which lights up when turned on however it isn't lighting up.

I replaced the two capacitors near the power port,the voltage regulator is giving out 4.8v not sure if it's supposed to be exactly 5v or not ?

Tested the main GND/VCC points on the main board that's showing 4.8v ,tested the cartridge connector that's also 4.8v

All solder points look good ,no corrosion anywhere.

Any ideas ?

amurphy245

After doing a diode test on all the chips I've discovered there may be a problem with one of the ram chips at U1 as well as both the chips at u7/u8 so unless I can find replacement chips I may be at a dead end

xIceMan

Could be a burnt CPU/PPU. If it was one of the RAMs you'd still have video but garbled up.

amurphy245

Quote from: xIceMan on January 27, 2020, 01:02:04 amCould be a burnt CPU/PPU. If it was one of the RAMs you'd still have video but garbled up.
Any way I can test it to identify which it is ?

xIceMan

First of all. What test did you do with those IC? Check for continuity? What's the matter actually?

Do they not get any VCC or is some trace missing? If so, route it and reconnect it properly.

amurphy245

i put my multimeter on diode mode,attached the positive lead to ground then checked the values of all the IC pins ,from what I read all pins should show a value other than infinity except for ground pins and those are the only chips that showed anomalies.

Not sure how to do a continuity test ,I don't have that mode on my multimeter.

P

What pins shows infinity in diode mode?
U1 should be CPU work RAM (U4 is PPU video RAM) so yeah I think you should get something even if that is broken. U7 and U8 are for controller I/O and I think it should work without those too, so I think it's too early to start replacing chips.

Continuity mode is among the most important modes. It's the mode with a sound wave symbol. It's the same as the diode mode on my multimeter. It should beep when you touch the two probes together (when resistance is close to 0 Ω) so you can check that a trace, cable or other connection is not broken with this mode.

amurphy245

Thanks for the advice I'm going to buy a better multimeter as mine really is missing the continuity test and il get back with the results in a few days time

xIceMan

I can really recommend the AN8009 multimeter. It's cheap and does an amazing job.

amurphy245

Ok I've done a continuity test and focused mainly on the cpu and ppu and haven't found a broken trace anywhere,at this point I don't what to do,if the cpu or ppu is dead how would I know?

P

Usually the CPU and PPU gets hot to touch when power is on if they are fried. If not they are probably ok.

I'm not sure how to check the chips without an oscilloscope. You could set your multimeter to voltmeter mode (set to the closest setting that is more than 5 V) and check power pins if they are 5 V. /RESET pins should also be 5 V. CPU pin 4 (address bit 0) should also be changing up and down if it's working.

For PPU I heard you can test composite video output (pin 21) without an oscilloscope by using a stereo or the TV speaker since HSYNC and VSYNC are audible frequencies (ca 15 kHz and 60 Hz respectively). You would need to connect pin 21 to the audio input on the TV or line-in on the stereo and listen if you hear anything.

CPU pinout
PPU pinout

You can also simply check the resistance (ohm meter mode) between a power pin and a ground pin and see that it isn't too low. I guess you basically did that with the diode mode though.

amurphy245

Both chips are warm to the touch but not hot although ppu is slightly warmer,both show correct voltage,I did the audio test on pin 21 and I get a continuous buzzing noise.

Like I said before I have an everdrive and it has a small led inside which lights up when you turn the system on ,but it doesn't light up for this system ,cartridge pins showing correct voltage so not sure what mechanism causes the led to light up.

P

Warm chips are normal, it sounds like there is nothing obviously wrong with either chip. There are many things that may break in the chips though so we can't say for sure that they are fine without more thorough testing, but for now I think we should focus on more common problems first.

Sorry I missed that about the LED. I think it should just lit up if +5V is connected. You already tested the power pin but also test the ground pin on the cartridge connector and make sure it has contact with ground. Otherwise it might also be as a simple problem as a dirty or possibly bent cartridge connector. Try cleaning it.

Cartridge connector pinout

amurphy245

The power and ground pins are fine on the cartridge connector ,I think for now I'm giving up as I've spent so much time on it and there is no clue as to what's wrong il keep it for spare parts.

Thanks for your help!

FAMICOM_87

you may get a solder pomp gun and desolder app ICs and put sockets :) so change them one by one to see the bad one ;)