Famicom Grey Screen (no games running)

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BokuNES
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Famicom Grey Screen (no games running)

Post by BokuNES » Mon May 25, 2020 7:27 pm

hi to all, ok so, i have a old famicom with a hvc-cpu-06 motherboard, i try to run games but i only have a grey screen, so the first thing i tried is isopropyl alcohol to clean the cartridge slot, it doesnt work, tried to clean the motherboard with alcohol, it doesnt work, try to resolder al the contacts of the motherboard, it doesnt work, try to find broken contacts with a multimeter using the motherboard schematics (i found 2 of them) but it doesnt work D;, so i want to try a logic probe (i buy this on aliexpress) to find the bad ic but i dont know how to use it xddd, someone know how to use it? or find a solution of my problem?....btw sorry for my bad english, best regards to all!!! :P

PD: none of the ic have overheat problems so there is no an internal shortcircuit on the IC.

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Dwedit
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Re: Famicom Grey Screen (no games running)

Post by Dwedit » Mon May 25, 2020 7:43 pm

Something I see a lot in old console/computer repair videos is observing the address/data lines on an oscilloscope, just to see if the waveform looks "off" or not. You can sometimes tell by inspection if the square waveform doesn't look right, or has the wrong voltage range.
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lidnariq
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Re: Famicom Grey Screen (no games running)

Post by lidnariq » Mon May 25, 2020 9:40 pm

Super cheap 5V logic probe:

Code: Select all

+5V --- blueLED ---+
                   +--- 4.7k --- probe
gnd --- blueLED ---+
You could replace the blue LEDs with white ones, or two red/orange/yellow ones.

If one LED lights, the probe is connected to ground. If the other LED lights, it's connected to +5V. If both LEDs light you've got a changing signal. If neither light, the probe isn't connected to anything that's driving it high or low.

Since the PPU is drawing a grey picture, we can be confident that the 21MHz crystal is working.

Check if CPU /RST (pin 3) is high.
Check if CPU "M2" (pin 31) is alternating.
Check if CPU "A0" (pin 4) is alternating. If it is not, see what it does when you press and release the reset button.

BokuNES
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Re: Famicom Grey Screen (no games running)

Post by BokuNES » Mon May 25, 2020 10:43 pm

lidnariq wrote:
Mon May 25, 2020 9:40 pm
Super cheap 5V logic probe:

Code: Select all

+5V --- blueLED ---+
                   +--- 4.7k --- probe
gnd --- blueLED ---+
You could replace the blue LEDs with white ones, or two red/orange/yellow ones.

If one LED lights, the probe is connected to ground. If the other LED lights, it's connected to +5V. If both LEDs light you've got a changing signal. If neither light, the probe isn't connected to anything that's driving it high or low.

Since the PPU is drawing a grey picture, we can be confident that the 21MHz crystal is working.

Check if CPU /RST (pin 3) is high.
Check if CPU "M2" (pin 31) is alternating.
Check if CPU "A0" (pin 4) is alternating. If it is not, see what it does when you press and release the reset button.
my man, first to all, thanks for the reply, well thesting the pins that you say on the CPU i found this:

- the "/RST" pin is high
- the "M2" pin is always high
- the "A0" its always high and if i press the reset buton its still in high

you have any idea of whats happening??

lidnariq
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Re: Famicom Grey Screen (no games running)

Post by lidnariq » Mon May 25, 2020 10:44 pm

BokuNES wrote:
Mon May 25, 2020 10:43 pm
- the "/RST" pin is high
- the "M2" pin is always high
That's very bad and implies the CPU is broken. Sorry!

BokuNES
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Re: Famicom Grey Screen (no games running)

Post by BokuNES » Mon May 25, 2020 10:48 pm

lidnariq wrote:
Mon May 25, 2020 10:44 pm
BokuNES wrote:
Mon May 25, 2020 10:43 pm
- the "/RST" pin is high
- the "M2" pin is always high
That's very bad and implies the CPU is broken. Sorry!
my man, thanks a lot, i have a good cpu to change the broken one, but this cpu ends with the letter "G", its obviously a different revision, i have the "G" ppu too, i can install this version on the motherboard?, what do you think?

lidnariq
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Re: Famicom Grey Screen (no games running)

Post by lidnariq » Mon May 25, 2020 10:54 pm

What version CPU and PPU on are the existing board? As far as I know, you should only need to swap out the CPU - I don't know of any incompatibilities between different revisions of the NTSC CPU and PPU, but you could help us find out if there are any :D

BokuNES
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Re: Famicom Grey Screen (no games running)

Post by BokuNES » Mon May 25, 2020 11:05 pm

lidnariq wrote:
Mon May 25, 2020 10:54 pm
What version CPU and PPU on are the existing board? As far as I know, you should only need to swap out the CPU - I don't know of any incompatibilities between different revisions of the NTSC CPU and PPU, but you could help us find out if there are any :D
i have the "D" revision of the ppu into the motherboard, i will put the "G" cpu and see if there is any problems swaping different revisions!

Fiskbit
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Re: Famicom Grey Screen (no games running)

Post by Fiskbit » Mon May 25, 2020 11:26 pm

All 3 CPU-06 boards I've seen have RP2A03 and RP2C02D(-0), so I'd guess this is a letterless CPU. I have seen a board that had an E CPU and D-0 PPU, but that was on a CPU-08 board (which is surprising) and I suspect is a rare combination. I've been told that dropping a letterless CPU into later systems (eg CPU-07 with E-0 PPU) can lead to instability, but I don't know if there might be similar problems using newer CPUs in older systems.

BokuNES
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Re: Famicom Grey Screen (no games running)

Post by BokuNES » Tue May 26, 2020 3:25 pm

so well, i tested the "G" CPU that i have it on another NES, and voila, BAD CPU xdd, i want to buy a pirate nes cpu clone on aliexpress like this:

https://es.aliexpress.com/item/40004580 ... hweb201603_

this thing can work on a real famicom?, i saw that model on many many cheap old nes clones before, someone know if this works?

lidnariq
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Re: Famicom Grey Screen (no games running)

Post by lidnariq » Tue May 26, 2020 3:37 pm

... Two bad 2A03s? How cursed.

I have a page on the wiki here about Famicom/Famiclone/NES CPUs: https://wiki.nesdev.com/w/index.php/Use ... _revisions

The UA6527 would work with some caveats, but even though the photo you have there says only "UA6527", the listing says "UA6527P" which isn't the same part. The latter should work, but it'll be incompatible in subtle ways.

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Dwedit
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Re: Famicom Grey Screen (no games running)

Post by Dwedit » Tue May 26, 2020 5:00 pm

Two bad CPUs sounds unlikely, it might be a problem on a chip or trace elsewhere.
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pakosup
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Re: Famicom Grey Screen (no games running)

Post by pakosup » Fri May 29, 2020 5:31 am

Dwedit wrote:
Tue May 26, 2020 5:00 pm
Two bad CPUs sounds unlikely
It is quite real situation. I have repaired famicom where both cpu are gone due to short circuit in cartridge.

But it is better to buy replacement famicom board on ebay than buy chinese cpu. It is really high chance to get faulty cpu from aliexpess.

BokuNES
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Re: Famicom Grey Screen (no games running)

Post by BokuNES » Sat May 30, 2020 10:04 pm

Dwedit wrote:
Tue May 26, 2020 5:00 pm
Two bad CPUs sounds unlikely, it might be a problem on a chip or trace elsewhere.
the 3 cpus are extracted from 3 "junk" famicoms that i buyed on ebay, so its very posible jajaja

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