NES audio problem: Missing channel 3

[wakalabis]

What's up, guys?

I have some sound issues with my recently acquired NES (toaster).

Audio is OK, except for channel 3 (triangular wave), which seems to be absent. As a result the audio sounds shallow, as channel 3 is usually used to play bass lines.

The other channels, as far as I can hear, are OK, though.

Have you guys ever seen some seen something like this?

What do you guys think might be  causing this? Maybe a damaged cap? I have never opened the console.

I have checked the AV cable. It is OK.

Update: Tested RF out: same thing happens.

Update2: I have opened up the console. The electrolytic caps seem fine.

[Fix_Metal]

Hello, welcome.
If elecs are fine, I'd check the line itsef and the oscillator part. I suppose there must be a DAC with a built in (or not, preferibly for you) oscillator, then in some kind of mixer and then in the RF unit.
Odd enough since there's no trace of a public available technical manual for the nes (as opposite for almost all old Sega consoles).
How did you check elecs? Maybe just the decoupling cap for the line. It's not 100% accurate if you just look at elecs to wheter or not decide if they're good. They may be gone but look good.

[eidis]

 Sorry for resurrecting an old thread. NES modulates all sound internally with Ricoh RP2A03, which happens to be the main CPU. You can try probing the sound directly from pin 1 or 2 and see if it helps. I suspect that the CPU could be bad.

Nes schematics
http://www.freeinfosociety.com/electronics/schemview.php?id=2405

Short intro on Ricoh RP2A03
http://www.vgmpf.com/Wiki/index.php?title=RP2A03

Ricoh RP2A03 pinouts and their descriptions
http://wiki.nesdev.com/w/index.php/CPU_pin_out_and_signal_description

Keep the scene alive !
Eidis