@root42 ground loop? -> https://en.wikipedia.org/wiki/Ground_loop_(electricity)

@root42 it thinks it’s an Atari

@root42 Bad bypass caps causing noise to backfeed into the data lines? Which is somehow coupling through the CIAs into the audio jack? I’d have to look at the schematic to have a better theory.

Let’s figure out where the noise is coming from. On the C16 and any other 264 series machine the TED chip does audio, video and I/O. Pin 33 on the chip is the sound output it seems. During a directory listing this stays flat on the scope. When I run a game we can see the waveform. So this rules out the TED chip I would say.
#commdore #c16 #plus4

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The IEC serial bus on the other hand is handled by the CPU it seems. There is an inverter and some ferrite beads in there, and that’s it. A few pullup resistors as well, but nowhere does the bus cross paths with the audio signal. At least in the schematics.

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@root42 3Vpp doesn't look like it's induced by accident, and 50% duty cycle looks like a clock signal 🤔

@lasse now that you mention it: looks indeed suspicious. The audio waveform should look a bit different.

@root42 The schematic is just a writeup of connections, it‘s important to know that it’s very well possible that the physical traces still cross.

I traced the audio signal through the whole schematic and this double diode bodge is super sus. The right one shouldn’t be there at all and connects to the IEC port. It feeds directly into the audio circuit. The left one should be there, but wired to the OTHER side of the resistor. What HAPPENED to this board???

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@root42 well, at least the electrolytics seem to be Audio Grade™, as they're labelled ''Grand Sonic'' 😜

I noticed that both bodges lead to the IEC port. So I quickly lifted the lower one. And guess what? The noise is gone. However I still have quite a bit of hiss in the audio signal even when idling. Also it might be that the two other diodes go through the resistor pack. I think I will still remodel this whole area to be the same as on my other board. Plus this board needs a good clean!

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Now the board is back to factory. I even removed the janky 3.5mm jack. I didn't like it very much. There is still an enlarged hole in the case, but that's not really fixable.

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@elosha obviously. There has to be some connection on the actual board. But if there was e.g. a shared IC pr something like that could have explained this.

@root42 It could even be AC coupling to a common plane/trace via bad cap …

@elosha nope. Much more evil.

@bytex64 no CIA, it’s a C16. Just the TED.

@root42 Aaah right.

@fozztexx haha! Do they do that?

@root42 @fozztexx Very much so. The pokey chip handled both serial port and audio and it made a beep for every byte transferred. Either super annoying or highly nostalgic depending on who you ask

@48kRAM @root42 @fozztexx More importantly, the Pokey handled the keyboard, and given these machines came with a membrane keyboard, it was the only way you got feedback that you triggered a key.

@uliwitness *on the Atari 400. The 800 had real keys. :)

@branch300bpm I would guess that a ground loop would be always audible, not only during bus activity?

@root42 no, this depends on electrical activity on either side. Since Commodore drives are self-contained, when they are idle they don't do much electrical interference that could pass on to your speakers through your machine.

@branch300bpm hm, I can beep out all the pins and shielding of the IEC port, I guess.