No Video - Checklist to narrow down the issue?

[Krystman]

I imported 6 Famicoms that I wanted to AV mod for use in Europe. I cleaned them all up and de-soldered the RF modules. I am now in the process of attaching new custom replacement RF modules. But to my dismay, 3 of the Famicoms seem not to work (Picture). When I power them on I get no Video or Audio signal.

What are some basic checks I can do to narrow down the issue.

The replacement RF/Power module I checked them all with is the Power Vamp module from The Backoffice. It comes with a 5v voltage regulator and an AV out. I tested them all with the same module to eliminate the module being the issue. All of the non-working Famicoms are HVC-CPU-GPM-02 boards. Some of the working ones are the same type as well. This is the first time I was really able to check them for function because I don't have the setup to display the Japanese RF signal. The seller sold them as "Confirmed to work". But perhaps something went wrong in the cleaning process or during shipment.

I have access to a soldering iron, a heat gun, a multi-meter and a simple oscilloscope.

[Skawo]

Check if there's 5V going to the CPU and PPU (pin 40 on both)

Check if you have clock (pin 29 on CPU, 18 on PPU)

Check if the reset line is properly working (Pin 3 on CPU should be high)

Check if pin 21 on PPU is outputting composite signal (provided rst is high, there is 5V and a clock, you should be getting a gray screen at the very least)

If it's getting out of reset properly, there is 5V and there is clock, but pin 21 has nothing then the PPU is toast.

[Krystman]

Perfect! Exactly what I needed. I will report with results

[Krystman]

Check if you have clock (pin 28 on CPU, 18 on PPU)

CLK is 29 on the CPU, right?

[Skawo]

Uh, yes, 29, sorry.

[Krystman]

Ok, so I did some poking around. Haven't run the checklist fully yet. Just soldered the connections to all of the AV/Power boards I have. While doing so I noticed that on the boards that had no video also seemed to have some issues with the vias that the AV/Power boards solder to. It seems like the pads around the vias have been lifted and no longer connect. This is especially true for the audio and video pads. Possibly that happened when I de-soldered the original RF module. It was a pretty stressful de-soldering job.

So I poked around with a multimeter to find good places to grab video and audio in that area.

I grabbed video from the leg of the black diode (?) - Yellow Cable

You cannot see attachments on this board.

I grabbed audio from the leg of a resistor (?) - Green Cable

You cannot see attachments on this board.

Are these good alternate locations to get those signals from? I am particularly skeptical about the audio. I based this choice on the fact that one of my Famicoms already came with that kind of bodge.

[Krystman]

Ok so I just gave it a go and it worked. But I first poked some more with a multimeter. I think the audio should be grabbed from the other leg of that resistor? I measured the resistance between both legs and the audio pad on a working pcb. It looks to me like the audio signal goes through that resistor, entering on the left. In any case, I moved the green cable to the other leg and I'm getting clean audio and video.

And it turns out that was the problem with all of the 3 Famicoms that didn't work. Just sketchy pads around those vias. I added more solder to get a better connection or used bodge wires in cases where the pads came off completely. All of the Famicoms are in working order now!