Modded original Famicom, weird sound issue in some games - updated

[Finfami]

I have modded couple of these without any issues, but now it seems I have problem.

Some games have very strange sound issues in some spesific sound effects.

https://www.youtube.com/watch?v=chmJDXRCL5g

In that clip I play Mega Man 3 from a multicart, it is fine as far as I can tell. Castlevania is on the same multicart but as you can hear there's something strange going on with sounds, when you pick up item and enter to castle for example. Battletoads (also on multicart) has similar kind of problems with some sound just being way off. Mario 1 and 3 are fine when played from original carts and from the multicart.

I tried replacing the cable which connects to the RCA jack but it didn't fix the issue. This is strange since all other consoles have worked fine with all multicarts and they are modded with the exact same method. One difference is that in this one I use original transistor, the another Famicom I have has different type I bought from eBay.

Edit: replaced transistor, no difference. On the multicart Contra and Castlevania 2 seem also to have problems with certain sounds. The console has sometimes troubles playing sounds at all from the Battletoads multicart, but reseating the cart seems to help atleast sometimes. I tried to clean contacts of cartridge slot with contact spray but the sound problem remain.

[chowder]

How are you modding it? Sound only needs pin 46 on the cart slot via a 220uF capacitor, the transistor is purely for the video circuit.

[Finfami]

Yeah, I just replaced the transistor because it was the only thing I did different with others.

I have soldered a wire directly to pin 46 without caps, I have done it the same way as with others.

Mod instructions are from assemblergames forum, this board is older version, "non-gpm", just like the another Famicom I've had several months and still going strong.

It's bad that I only have two original cartridges to test.

I don't know if some of the caps and resistors I used to mod happened to bad and it would cause something like this, I have plenty of them so I could try replacing one or all.

I don't know if it's common for multicarts to behave like this but those same multicarts have worked 100% in all other genuine Famicoms I've had.

Edit: tried installing 220uf audio cap and also at the same I tried another RCA-port (just testing...) but no difference. If all goes well next week I should get original Famicom cart which uses expansion audio features, if that doesn't work then maybe there is something wrong with this machine.
Post Merge: December 21, 2015, 10:40:23 am
I googled a bit and read from these forums that you apparently could solder rca jack to CPU pin 1 and 2 to get audio from different channels, is that true?

I don't know what Castlevania, Contra and Battletoads would do differently compared to Mega Man 3, Mario 1 and 3, why do they have problems? Also why only spesific sound effects? I also tried Kirby's Adventure and during that short testing there were no problems.



UPDATE:

Received Akujamo Densetsu today (Castlevania III) and same problems persist, works fine except for some spesific sound effects.

Also tested soldering audio cable to pin 1 of the cpu, got sound and it seems that the effects are also on that channel. They sound the same, not correct, so is the cpu partially faulty? Soldering the audio cable to SOU pad produced worst sound of all options.