| Posted By
GreatWhiteRetro
on 2025-04-01
22:10:06
|  [Resolved] RF Mod Replacement / LCA / S-Video Help Needed
I've assembled one of two LittleSixteen V4s courtesy of SukkoPera and a lot of patience. I am stuck, however, on the S-Video signal and Composite signals only producing monochrome video.
The RF replacement is supposed to be based on mbarszcz's design from GitHub.
The RF replacement is on page 12 of the /devel/ branch PDF. Note that this version is missing JP18 that controls the composite video mixing, but this jumper doesn't need to be set for s-video output only as I get Luma and Chroma output on the video connector even without this jumper set.
When I build the two circuits for Luna and Chroma, the Luma output seems OK, going from around zero to 1V peak to peak but with a DC bias of about 0.5V. The Chroma output is adjustable and with a tuning resistor turned all the way down I'm getting almost 1VPP as well, but there's a very large DC bias of almost 2VDC.
https://github.com/gordonfpanam/FileRepository/blob/master/Busted%20Chroma.png
Looking around the S-Video spec I'm supposed to have a DC bias of close to zero for both of these, max 1VPP for Luma and max 0.3VPP for Chroma. I can dial back the Chroma VPP down to 0.3VPP but this still leaves a significant DC bias in that signal, and I still get monochrome video.
I would have loved to link to the relevant GitHub objects for the schematics but powweb's complaining about too many links in my first post.
I am way out of practice when it comes to analog electronics and I've forgotten what I need to reduce or remove DC bias. I may also have screwed up the assembly somewhere. All I do know is I'm getting monochrome output from this build, and my monitor otherwise works with either NTSC C64s and C16s, and PAL C64s and C16s, both of which I have. Any suggestions? |
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Posted By
SukkoPera
on 2025-03-29
05:29:37
| Re: RF Mod Replacement / LCA / S-Video Help Needed
Please raise this as an Issue on the L16 bug tracker.
To remove DC offset you add a series cap, but that should not be an issue in this case. Set your scope to AC coupling and the offset will be gone.
I suggest that you check the components in the chroma circuit: Q81, R81/82/83/84/85 and C81/84. Make sure the values match the schematics. Leave JP18 open for the moment, as you noticed it's only for generating the composite video output and that can wait.
Of course also check that the main crystal and KERNAL match the video standard you want to generate, either PAL or NTSC. |
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Posted By
GreatWhiteRetro
on 2025-03-29
14:15:48
| Re: RF Mod Replacement / LCA / S-Video Help Needed
I wanted to make sure I checked everything before submitting a GiitHub issue. And I have matched firmware and crystals for NTSC and PAL so I can test both. |
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Posted By
SukkoPera
on 2025-03-29
14:43:34
| Re: RF Mod Replacement / LCA / S-Video Help Needed
It's no big deal, you can open a GitHub issue anytime.
Another thing you can do is to use the V3 values for the Chroma circuit, have a look at this PDF. If you venture this way, please try to swap one component at a time so that we can identify the culprit. Start by removing C84. |
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Posted By
MMS
on 2025-03-29
16:54:48
 | Re: RF Mod Replacement / LCA / S-Video Help Needed
OFF
wow, I was not aware the Littlesixteen V4 is available, though I will surely not start from zero but to order it from PCBWay, when it will be available. Anyhow I will wait patiently, as it should notbe the machine of "121 shades of grey"  |
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Posted By
GreatWhiteRetro
on 2025-03-29
22:40:08
| Re: RF Mod Replacement / LCA / S-Video Help Needed
I have an update on this. On SukkoPera's advice I went back through these circuits and started swapping in parts one at a time. I'm starting to get colour video on certain monitors in certain conditions, but not always.
I converted these from the v4 circuits to the v3 circuits, even omitting the composite video combiner circuit and just having luma and chroma. As I swapped or removed parts I still didn't get any progress, until I got to the 1k pullup for the Luma-in coming from the TED chip. I stopped short there to avoid drawing too much current from the TED's Luma/Sync pin.
With essentially v3 circuits except for a 1k pullup on R86, I got to a condition where I would get solid PAL colour video (using PAL firmware and PAL crystal of course). But depending on the monitor I would get one of three conditions: Monochrome in both modes, colour in PAL but monochrome in NTSC, or colour in PAL but regular alternating between colour and monochrome in NTSC.
It's as though the LittleSixteen is using a colour-killer, or the monitor is misinterpreting something so it enables its own colour-killer. I've seen flat-panel monitors switch between monochrome and colour on an Apple IIc and IIe depending on whether the computer is in text mode or not, and this is the behaviour I'm seeing on the LittleSixteen, sort-of.
I still need to work back through the circuits and put back the v4 parts one by one to see where I lose colour, but I at least have a somewhat working baseline now, even if it's only in PAL and only on certain monitors. |
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Posted By
SukkoPera
on 2025-03-30
04:44:20
| Re: RF Mod Replacement / LCA / S-Video Help Needed
Please, let's keep all communication in a single place, otherwise it's impossible for us even just to understand the chronology of your messages!
Whoever wants to follow the discussion, just click here. |
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Posted By
GreatWhiteRetro
on 2025-03-31
23:23:44
| Re: RF Mod Replacement / LCA / S-Video Help Needed
I already reported to the GitHub issue, but just for closure here: I had to adjust the clock crystal circuit’s variable capacitor and that finally got me colour NTSC.
I’ll go into depth in the GitHub issue once I’ve had a chance to restore the v4 circuits. |
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