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).
https://imgur.com/gallery/PpqcqPY
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.
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).
https://imgur.com/gallery/PpqcqPY
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.
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