arfink
Offline
Minneapolis

I've been working on modding a Famicom (HVC-CPU-07) to have a composite video output using this circuit seen here:

http://jpx72.detailne.sk/modd_files/fc/avmod.htm

Thus far I have constructed the circuit properly, and it appears to work... mostly. The final image has "waves" in it, such that the image seems to move from left to right in bands which bend like a sine wave. There are also bands of light and dark moving through the image too. I have triple checked my own wiring and have even made sure that my hookup cable is shielded and so on. I suspect the problem lies in my power supply.

The PSU is a really cheap wall wart I found in my box of spares which happened to have the correct voltage and polarity, but is under-rated in amperage and is probably incredibly noisy, although I have not had the opportunity to check it. Has anyone here done mods like this before? Is it possible this problem is related to my power supply? The symptoms seem indicative of 60hz line noise just based on the sine-wave shape of the defects in the image.

jefftheworld
Offline
Toronto, Ontario, Canada
arfink wrote:

I've been working on modding a Famicom (HVC-CPU-07) to have a composite video output using this circuit seen here:

http://jpx72.detailne.sk/modd_files/fc/avmod.htm

Thus far I have constructed the circuit properly, and it appears to work... mostly. The final image has "waves" in it, such that the image seems to move from left to right in bands which bend like a sine wave. There are also bands of light and dark moving through the image too. I have triple checked my own wiring and have even made sure that my hookup cable is shielded and so on. I suspect the problem lies in my power supply.

The PSU is a really cheap wall wart I found in my box of spares which happened to have the correct voltage and polarity, but is under-rated in amperage and is probably incredibly noisy, although I have not had the opportunity to check it. Has anyone here done mods like this before? Is it possible this problem is related to my power supply? The symptoms seem indicative of 60hz line noise just based on the sine-wave shape of the defects in the image.

I'm currently fighting through the exact same issue with my TI-99/4A. I'm still not sure what the solution is but if I figure it out I'll post here and hopefully it'll be helpful to you as well.

arfink
Offline
Minneapolis

Well, after examining the output of the wall wart with my meter I kinda guessed something was up. I was going to throw it on the 'scope and see just how bad things were, but rather than dig it out of storage just to test, I instead decided to go rummaging through storage (the irony!) and just find a different wall wart to try out. I eventually found an old Tandy power supply which happened to have the correct voltage, polarity, plug size, and enough amperage to do the job. It seemed far more substantial than the tiny crappy one I had been using before. Plugged in the new power supply, and everything just worked. smile

I suppose if people are really curious I could do some more analysis, but I figure it's not necessary. The fix makes sense too, because if the 5v line supplying the video amplifier is really noisy, the television will just interpret that extra noise as something wrong with the signal, which it was in this case.

Moral of the story: use a good power supply with your Famicom.

jefftheworld
Offline
Toronto, Ontario, Canada
arfink wrote:

Well, after examining the output of the wall wart with my meter I kinda guessed something was up. I was going to throw it on the 'scope and see just how bad things were, but rather than dig it out of storage just to test, I instead decided to go rummaging through storage (the irony!) and just find a different wall wart to try out. I eventually found an old Tandy power supply which happened to have the correct voltage, polarity, plug size, and enough amperage to do the job. It seemed far more substantial than the tiny crappy one I had been using before. Plugged in the new power supply, and everything just worked. smile

I suppose if people are really curious I could do some more analysis, but I figure it's not necessary. The fix makes sense too, because if the 5v line supplying the video amplifier is really noisy, the television will just interpret that extra noise as something wrong with the signal, which it was in this case.

Moral of the story: use a good power supply with your Famicom.


You just fixed my TI-99/4A. Thank you kindly, good sir!

arfink
Offline
Minneapolis

So you tried a different power supply and it just worked? Awesome. smile

jefftheworld
Offline
Toronto, Ontario, Canada
arfink wrote:

So you tried a different power supply and it just worked? Awesome. smile

I had an identical issue. I did an AV mod and everything seemed to be by the books but the output seemed to be distorted in a sinewave pattern. I changed out caps and tried all matter of black magic. Thought maybe it was an impedance issue so I matched the impedance of the RF unit I was replacing and still nothing.

Switching the PSU worked instantly.