How do you know if your PPU is dead?

[claude]

Hi guys, I've been trying to fix my broken (AV modded) Famicom since I'm almost sure I fried the insides with something I shouldn't have. You know the story..... Anyway, I've replaced the 7805, the cap and the 1.5amp pico fuse, however I'm still presented with the same issue; no picture, no sound. 

Using my very limited multimeter experience, the replaced parts seem to be doing they're respective jobs correctly. I'm forced now to look at the AV mod itself (a capacitor and a few resistors attached to an AV cable) or the PPU. I'm using a 10 year old PAL CRT TV and have tested the RF connection, no luck with that either, although to be honest I'm not sure if a PAL TV will pick up an RF signal from a foreign device.....

I want to eliminate the issue of the PPU causing the problem before I tackle the AV mod.  My question is how do you know if the PPU is dead? What pins should I attach the multimeter to? What results should I expect to see if the PPU is working or not?

I'e been lurking here for 2 weeks trying to solve my problem and you all seem like great chaps, very informative forum.
Thanks.

[80sFREAK]

Welcome to the forum.

If you have PAL TV and NTSC signal, you will have b/w scrolling pictue.

Be sure, that AV mod done proper. If PPU is not hot as BBQ plate, it should be alive. Easiest way to check output of PPU is connect pin21 via 1uF capacitor to audio input of TV. You should hear HSYNC(15kHz) and VSYNC(60Hz). Check cartridge connector and reflow joints, if they doesn't looks good. USE FLUX.

[claude]

Thank you very much for your help. I connected the audio cable to pin 21 as you said and I can now hear a type of interference and crackling so I believe the PPU is working. It isn't heating up either so I'm inclined to think that the AV mod is the problem. I think I will follow your guide to AV modding rather than redo this guy's attempt.

Is the crackling sound expected or should I be hearing something else?

[80sFREAK]

 Well, i would describe sound more like high pitch cicades  And "crackling" is VSYNC.

Dead or wrong transistor probably.

Keep in mind some transistors are ECB type, but some EBC. I usually reused transistor from famicom PCB, it is ECB type, but any other general purpose silicon pnp transistor will do the job.

When everything connected proper, you should see stable or scrolling(if your TV don't know about 60Hz sync) picture. "Jailbars" is another story.

[infiniteNOP]

Have you got an oscilloscope? If yes, then check AV OUTPUT for a valid NTSC signal.

[claude]

The transistor maybe the problem, I have a few transistors, a 7805a which I think only puts out 1 amp instead of 1.5 and an LM317 which I was sent, perhaps by mistake from an eBay seller. Same results with each one. No oscilloscope unfortunately!

[80sFREAK]

1amp is fine, until you start loading it heavily via cart slot. What sort of transistors do you have?

[claude]

The only transistors I have are the 7805a and LM317.

[80sFREAK]

They are both voltage regulators. You need something like 2SA733, 2SA1015 etc.