NES Repair

Discuss hardware-related topics, such as development cartridges, CopyNES, PowerPak, EPROMs, or whatever.

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teaguecl
Posts: 211
Joined: Thu Oct 21, 2004 4:02 pm
Location: San Diego

NES Repair

Post by teaguecl »

I powered on x-mas '85 NES for the first time in years a few weeks ago and got nothing - not even a light on the power button. I opened it up and started probing voltages and very quickly found the 17805 was not only producing the wrong output - but it was burning hot.
I purchased a replacement 7805 plus a capacitor kit from Console5 and installed it all - even the ones in the RF modulator. It still doesn't work. New symptoms:
- 7805 is producing 5v as expected
- power light blinks at 1hz with no cartridge in, as expected
- power light stays solid with cartridge in, as expected
- I get nothing on the screen at all - grey screen.

My unit has been stored safely in my living space (reasonable temps / humidity) for the entire 37 years. It's clean inside - I clean it every few years. Any suggestions as to what might be wrong or what to do next? My suspicions are:
- Something could have fried when the 7805 was malfunctioning - but I don't know what. I don't see any obvious burn marks or anything.
- I could have messed up the soldering on the RF module. I'm competent with a soldering iron and I don't think I messed anything up - but it's possible. It's a real pain to open up the RF module, so I haven't re-inspected it. I suppose I could have a capacitor backwards - but I thought I was careful :)
- Maybe a faulty CPU or PPU. Is there any way to diagnose those? I could swap them (I have spares) but I don't really want to do something drastic until I've exhausted other options.
lidnariq
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Re: NES Repair

Post by lidnariq »

teaguecl wrote: Sun Aug 21, 2022 6:38 pm - power light blinks at 1hz with no cartridge in, as expected
- power light stays solid with cartridge in, as expected
CIC and its 4MHz resonator work fine
- grey screen.
21.5MHz clock works fine, PPU at least partially works.

The CPU is probably crashing. The most likely culprit, historically, has been its RAM.

The simplest test is to use a logic tester and see if CPU A0 oscillates at all shortly after reset.
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Memblers
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Re: NES Repair

Post by Memblers »

Check the datasheet of the exact 7805 used as a replacement. The input and output capacitor values are critical, you must use what it specifically calls for. Otherwise, the regulated output may oscillate.

Test the bridge rectifier. Seems like a failed one could take the regulator out.

Transformer failures aren't uncommon. I've had a dead NES transformer myself, though the NES did nothing when it was used. Voltage test of a dead A/C transformer will still look normal, when there is no load on it.

Not common, but I've seen ceramic disc capacitors fail short before (not in the NES). For a bypass cap, that would result in 5V shorting to ground. It's not easy to track down which one failed, though kevtris showed me an ESR meter can be used to locate what general area of the board has the short, at least.

Power travels through the power button. I've had to disassemble and clean an NES reset switch before, probably a power switch can get dirty as well.

Good luck.
Wyatt150
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Joined: Fri Aug 26, 2022 2:27 pm

Re: NES Repair

Post by Wyatt150 »

I had an issue similar to this, unfortunately the solution was painstakingly testing all the traces until I found a bad one but that one had literally been outside for multiple years so I had reason to believe there was a bad trace, here you say it's been treated well so that might not be it, but fortunately the fact you get a grey screen does mean your video/power box or whatever it's called is good assuming you have 5 volts at the mobo.

The particular trace that was causing it was that happens to go from the ppu directly to the work ram. If I remember correctly nes's just produce a black screen when the ppu or cpu are either faulty or not present, so there probably good. I would look at first the obligatory 72 pin connector, bad traces, bad ram as they mentioned earlier, or the 20 pin chip between the vram and ppu (74ls373) or the 16 pin chip above the cpu (74ls139), I can't remember for sure about the 74ls373 but I had a faulty 74ls139 and it did pretty much the same thing. The cic has also been known to cause this if faulty but the if the light is still blinking then I wouldn't look there.

This issue could really be anything but I'd start with double checking the 72 pin then checking for any suspicious traces around corrosion if there is any, then look into bad chips.
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