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C64 black screen - Need an experienced opinion on the composite waveform

Scruit

Experienced Member
Joined
Apr 9, 2022
Messages
105
Hello all,

Working on my first C64 / Breadbin / 250407. Purchased as non-working. Getting black screen.

More detail below, but for those that want to get right to the question.... Here are a few waveforms comparing the Pin15 output on the VIC chip (luma/sync) to the pin 4 on the AV socket (composite signal).

My question is: Is the composite waveform range of 0.1v-1v too low for a c64?


More detail:

Symptoms are that that I get black screen on power up. The TV clearly detects a signal as it swtiches from the "no signal" blue background to a black screen. Have tested composite and s-video, same result.

I went through my typical checks with the schematics and confirmed the voltage rails, all of the ICs are getting power/gnd, the address and data lines all show activity, and the various clock signals are present and are at the expected frequencies.

- Pin 15 (which I believe to be "luma/sync") on the VIC seems to show a signal of good strength and reasonable for a b&w "composite" signal (0.25-2.5v, sync pulse present, but curiously no apparent back porch).
- Pin 14 (WIBTB "color") shows what appears to be data but hovers around 6v.

As I measure the composite pin on the AV socket, though, I see a signal that looks identical to the luma/sync, but with only half the voltage. Only between 0.1v and 1v. This does not seem to be high enough.


I suspect the composite signal voltage may be too low - was expecting to see closer to the same level that the VIC is putting out on pin15. I also, though, read the pin15 output as looking like it would produce a solid bright line on the left of the screen, and the rest of the screen being medium dark. (If that is the case, which can't I see the bright line? Or maybe that is a "start of screen-visible part of the line" pulse. I will do more research on the composite waveform standards.)

However, given that screen is all black, and the composite waveform looks like a blank screen, maybe the composite voltage level is fine and the VIC is faithfully producing a blank screen because of an upstream problem that is resulting in there being nothing to show.

I have a dead test cartridge on the way.
 
My thought was that if the RF modulator is muting the composite signal then I'd start there. If 0.1v-1v is a valid composite signal then I'd look at the PAL.
 
Generally composite video un-terminated is about 2v pp. When terminated, into the typical 75R load it drops in half to about 1v pp, because generally the video driver stage is designed for a 75R output resistance. About 0.3 V of that is the sync amplitude.

So if you just have sync it could be about 0.3V terminated or 0.6v un-terminated.

Your recordings shows sync, not really any video data, which is why your screen is black, but the TV detects the syncs.

That initial transient / spike at the start of H scan,just after the H sync pulse, is sometimes seen at the end of H blanking and, if visible, it is immediately on the left side of the scan, you might see it as a thin vertical line down the left hand raster edge, but most TV's and many VDU's are over scanned, so likely you wont see it at all, and just see your black screen. If you could reduce the scan width on the VDU or TV, you would likely see a thin vertical line down the left side of the raster because of that voltage spike.

I wish I could tell you more, but I don't have a C64 to play with.
 
Last edited:
Generally composite video un-terminated is about 2v pp. When terminated, into the typical 75R load it drops in half to about 1v pp, because generally the video driver stage is designed for a 75R output resistance. About 0.3 V of that is the sync amplitude.

So if you just have sync it could be about 0.3V terminated or 0.6v un-terminated.

Your recordings shows sync, not really any video data, which is why your screen is black, but the TV detects the syncs.

That initial transient / spike at the start of H scan,just after the H sync pulse, is sometimes seen at the end of H blanking and, if visible, it is immediately on the left side of the scan, you might see it as a thin vertical line down the left hand raster edge, but most TV's and many VDU's are over scanned, so likely you wont see it at all, and just see your black screen. If you could reduce the scan width on the VDU or TV, you would likely see a thin vertical line down the left side of the raster because of that voltage spike.

I wish I could tell you more, but I don't have a C64 to play with.

Awesome, I appreciate the response. This gives me direction, and the PAL chip is the next suspect.

Thank you!
 
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