Strange AV Mod Issue

[graces]

Hi,
I recently modded my Famicom. It's a revision 7 PCB.
I used this circuit:


and lifted pin 21 and put a 220uf Capacitor between pins 20 and 22 to reduce jailbars and the picture looks great for the most part. But some games seem to have a bad wavy picture. I've only seen it on Super Mario USA and some multicarts that I have. In Super Mario USA the picture looks like this when I start up (the waves in the image move back and forth):



But once I go to the character select screen the image clears up and stays fine when I play the game, if i press the reset button the title screen looks fine:



If I toggle the power switch it goes back to the bad image.

Does anyone have any idea what might cause this? I'm not using the 2SA937  transistor from the Famicom; I'm using a KSA733 (https://www.digikey.ca/product-detail/en/on-semiconductor/KSA733GBU/KSA733GBU-ND/1047436) instead. Could that be the cause? Any help would be appreciated...

[krzy]

1. What circuit did you use?
2. Why the hell everybudy puts capacitor between pin 20 (GND) and 22 (!RESET?).
You should between pin 20 and 40.

Yes, !RESET is connected to VCC but idea of decoupling capactor is to put them AS CLOSE AS POSSIBLE gnd pin, otherwise they does not function.

[graces]

I used the circuit in the diagram I provided; though it looks like I used a 330 ohm resistor instead of a 300...

RE: pin 40 instead of pin 22. That makes sense.  I was following recommendations that I read. I guess someone did it wrong, and a bunch of people just copied him without really knowing what they were doing; that's how so many things go in this world.

Anyway, putting the capacitor there did clear up the jailbars for me, and I don't think it has anything to do with the problem I'm having because I saw the issue before I put that cap in.

[krzy]

I used the circuit in the diagram I provided

Where is the diagram?

[graces]

here: https://assemblergames.com/attachments/kyuusaku_zps6a57f5ab-gif.10058/

[FAMICOM_87]

why lift the pin 21? you disabled the RF that way ? 

[graces]

lifting pin 21 helped with the jail bars. I was more concerned about that than preserving the RF. Also, I put a trrs jack in pace of the RF out.

[krzy]

here: https://assemblergames.com/attachments/kyuusaku_zps6a57f5ab-gif.10058/

White screen, cannot follow the link.

[graces]

ok. I don't know why you cant see the image. I've attached it instead of linking to it.

[krzy]

I just replicated this circuit on breadboard and I must say I am really amazed how good video it generates and how simple it is.
I tested it agains Super Mario Bros 2 and other games, which seem to output flickering video on some TVs and there were no problems.

Check if the right side of capacitor is "-".

Check transistor pinout - 933 has ECB, 733 has EBC (left to right)

[Pikkon]

That circuit  is based off of the av famicom.

As for the transistor I always use the one on the famicom and what kind of psu are you using.

[graces]

I get the same results whether I use a Sega Genesis 1 adapter or an original famicom adapter (I'm not using a step down converter with it; I'm in Canada with 120V AC)

I took a look at the circuit and the transistor is definitely correctly oriented. I rant a few tests on other televisions and I don't have the problem using an XRGB mini on my HDTV or on an old Sony Trinitron, so I suspect the issue might have to do with the TV I was testing with.

[famiac]

Seriously? That circuit is crap. No emitter degeneration and no biasing scheme. I could definitely see it having stability/linearity issues.

Try placing a 220uf capacitor at the base input, with the negative terminal connected to the base.
Otherwise you can try a more robust circuit

[graces]

Sorry for the long delay; I got busy with other stuff, so didn't get back to this until now. I tried adding the extra capacitor as you suggested; it did not fix the issue. The only effect it had was to subdue the colors, especially blue, making everything look a bit green....

What more robust circuit would you recommend?

[famiac]

Ok, which transistor are you using?

try this circuit

1000uF is probably overkill. 220uF is plenty
The 2SA1015 is a high quality part, but you can substitute practially any pnp general purpose transistor and still get a working image.