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SukkoPera / LittleSixteen

Commodore 16 schematics and PCB, redrawn in Kicad
https://hackaday.io/project/182543-littlesixteen-commodore-16-mainboard
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New V3 board getting a black screen on power on #14

Closed MC6809E089 closed 6 months ago

MC6809E089 commented 6 months ago

Hi,

Sorry to raise this as an issue but I've just finished building a couple of V3 pcbs (one for me, one for my brother who can't solder so well), but neither board is working at the moment. I was wondering if there are any additonal instructions, or changes that need to be made to get a V3 board working, other than those written on the wiki?

On power up, both boards exhibit the same behaviour: They output a black screen with sync to my retrotink mini, there is no other picture or sound. The boards supply voltages seem fine, 5.1V on the 5V rail, and 6.5V-ish for the tape motor, so I think they're okay. I believe I can see a clock signal, and some sort of activity on the data and address busses (I only have a bitscope, not a proper oscilliscope), and some of the chips on the board get warm after a while, but only warm, not too hot to touch or anything. I've tried the kernal version of 264 diagnostics ROM, and left it running to see if there was any audio, but nothing on screen or via the audio. I wondered if I might have gotten something wrong relating to the ROM/RAM jumpers.

The boards are configured for use with an original 8501 (lower socket), separate ROMs (all jumpers soldered to the left) and 64k of RAM (both jumpers on the left). Do I need to do anything with JP1 or JP2?

All new components were sourced from Mouser in the US, and from the BOM linked in the BOM issue here. All original components were sourced from a pair working C16s, so that I had something to compare against. Because I built two pcbs, I've been able to swap all of the ICs between the two new boards and the original ones, and all ICs work fine in the original boards, so I believe the issue is somewhere on the new boards, either my bad soldering, or an incorrect component somewhere, rather than a failed one.

I understand that the problem is almost certainly something I've done, and could be pretty much anything but I thought I'd ask in case. Thanks in advance for any help or suggestions you can provide. :)

SukkoPera commented 6 months ago

First things that come to mind:

  1. Make sure that all resistor networks are properly bussed or isolated as they should be.
  2. Unsolder FB38/39/40. It's still unclear why but some people experience problems when using RC filters at these positions. If that works, you can replace them with ferrite beads.
MC6809E089 commented 6 months ago

Hi again,

Thanks very much for your help! It was the RC filters causing the issue, once they were removed, both boards booted up straight away, and pass all of the diag ROM tests.

Please may I ask another question? Although the new boards are working fine, the video output of both is very dark and fuzzy compared to original C16s, when connected via the same s-video cable and retrotink. I've just noticed that component link listed for Q81 and Q82 in the BOM is different for Digikey (BC550CBU), and Mouser (BC547B). I just clicked on the Mouser link when I ordered the components, so at the moment my boards have BC547B in them. The datasheet shows that the BC550C has a lower noise figure, and a much higher hFE than the BC547B, so would that be a better choice, and could my using the BC547B be the reason the video output is so dark and fuzzy?

Thanks again for your help, and sorry to keep asking questions. :)

SukkoPera commented 6 months ago

Glad you fixed it.

You can try but I don't think a 547 makes any difference wrt a 550. You have to work on the pots or even better remove the pots and just use the standard values as detailed here: https://github.com/SukkoPera/LittleSixteen/wiki/V3-Assembly-Notes#rf-modulator.