[solved] Problem with an SG-1000

I have this Sg-1000 and hook it up to a US tv using different types of RF cables and just displays a gray snowy color with a buzzing sound, tried both channels 95 and 96 since is a japanese console. I move forward and tap pin 36 from the TMS9918ANL that is a composite video and connect it to see if at leas give me a proper video with the game and still the same. Tried different games too with the same effects. I checked solders around to see if there was some problem but nothing, caps look fine and resistors too. I wonder if the Z80A CPU could be bad. I haven't tried yet to tap the sound from the SN76489AN to see if it work at least the sound.

 

Is there no video? Pal usually looks washed out, no colors but black and white.

 

The only thing it displays is like static and sometimes a pink color but with the same static and buzzing sound.
It was shipped from Japan so I assume is jap. I'm going to re flow the cart slot and also try different tvs to see if something change.

 

I have a couple of SG-1000 units. My theory has been new capacitors, but haven't actually got around to doing that yet.

 

It only has 4 electrolytic caps on the whole board.

 

Problems like that make me suspect vram. It's really primitive, but turn it on and put slight pressure on the the cpu and other chips, perhaps there's a solder break somewhere.

 

Well I reflow all the solders of the chips and still the same, I also do what you recommend of putting some pressure on the chips and nothing except when I touch the tms9918 it was hot, not even the cpu was hot like that one and make some color changes but the game never appear or sound changes the buzzing was still there. I put some photos of the board maybe you can see something I don't.

 

Just letting you know, TMS9918, TMS9118, TMS9928/9 and TMS9128/9 chips (all variations of the Texas Instruments VDP chip) have this tendency to run hot.

Some MSX computers had a slab of aluminium glued or clamped on top of it for that reason.

And if you have VRAM issues, you will hear the sound from the game. Bad VDP might give you no sync signal (meaning you would not even a black screen from the console).

On that case, the first thing to check is the NTSCx3 crystal oscillator that drives the TMS9xxx VDP chip.

 

Thanks oliveira I was thinking about that crystal too. I add a photo I don't know if it can be seen fine but is the only thing it display on the tv. I'll try to take a better photo.

 

you have any software for it which has easy means to check for sound?

 

if is software for the sg I only have two games and I have tried both with the same result. The only thing I haven't tried yet is to tap the sound directly to the sound chip and check if it gives the same humming sound.

 

I suppose start with the power circuitry, check output for proper voltage.

 

I checked the power and the regulator is giving the 4.89 to 5 volts and the components are getting voltage too. The interesting thing is I tap the audio from the sn76489 and I did not get any audio but when I connect the video from the chip too to the TV then I get the buzzing but this time with a black screen no image at all. Now I'm lost.

 

Black screen means the VDP is running. You have sync signals. That also means the master clock is running.

The next point of check is the Z80 CPU and it's work ram. You have tools to remove the Z80 chip from it and test? Unfortunately it's not common that the Z80 died up when a machine is not booting like yours...

 

I can remove the z80, but how can I dont know or have a way to test it. If that z80 is the same as the master system I have a master system board around I can use the z80.

 

the SG-1000 Z80 should work on the MAster System if it's not faulty. The opposite is also true.

 

Would the z80 from a md work? similar clock speed.

I have one here you can have for free if you like.

 

The Z80s used on those (SG-1000, Master System, Mega Drive) are identical. All of the Z80A (4Mhz) variety.

 

Not the Z80 I tried on the master system and it boots. I also use the moment to socket the z80 on the master system for a future testing.

 

Broken traces. Make sure you check all the traces on that board. Also the work ram the Z80 uses as operating memory on the SG-1000 need to be checked.