Very low volume and loud hum

Yes, the dot is the pin 1 marker. That's perfectly normal. I cannot see a hole or bulge on your pictures either. The IC looks intact. It can still be broken, but at least it has no typical "reverse-plugged-in burn-mark". So maybe I was on the false track, which is somehow good I believe.

Can you show a picture of the DIAGNOSTIC and EXPERT screen in ConiGuru. Just to check the basics.

Do you have a Voltmeter to measure the 9V supply?

Oh, how I "love" this intercontinental debugging...

Thanks for the pictures! At first glance it all looks OK. SID1 is set to 8580 mode, SID2 to 6581. You are using pseudo stereo. Everything fine.

To measure the supply voltage, I think it's best to measure it directly at the SID socket:

1. First remove the FPGASID from the socket and disconnect the cables from it (you can leave the clamps connected to the board)

2. Check this pinout here:

6581_pinout.jpg

you will have to measure between pin 14 GND and pin 28 VDD.

The expected voltage is either 9V when the original SID was a 8580 or 12V when a 6581 was installed before. I'd say +/- 1V should still be OK. FPGASID can cope with both voltages.

3. When the voltage is OK, plug the FPGASID back again.

The next part is a little bit risky. So in case you don't dare to do so, please just don't do it. The voltage measured in step 2 should be OK then.

4. Now measure again. You can access the pins from top of the FPGASID. But you will have to be VERY CAREFUL not to accidentally produce a shortcut between pin 28 and anything else. The pin28 supply voltage can easily damage other parts when they are connected to it.

I suggest that you connect GND not from pin 14 but from the metal box of the modulator. Then you have all hands free to tap the pin 28 of the FPGASID with the second wire of the voltmeter. Please practice this with the C64 turned off before you do the actual measurement with the C64 powered on.

When the voltage is OK (what I believe because the original SID seems to work quite well), the symptoms are still pointing to something outside the C64. Maybe some faulty contacts of the AV breakout adapter or the cables you are using. I know, you certainly checked this, but please check it again and also try to move the cables and plugs during operation to see if that changes anything.

Did you try pressing the FPGASID (gently) into the socket during operation? To do so, touch the metal box of the modulator first to get rid of any electrostatic charge of your fingers, then put two fingers on the two big square ICs on the top side of the FPGASID and push down with slowly increasing force to see if it is making any change. That usually helps against faulty contacts in the socket (well temporarily at least).

I'm still scratching my head. The only thing FPGASID is bypassing on the mainboard are the filter caps. Otherwise, it relies on the same signal amplification on channel 1 as an original SID would. For channel 2, it must have its own amplification. It would be interesting how much both channels if otherwise set to the same sound parameters differ in output level. Also, checking the setup with external battery driven speakers will only allow to operate the machine without causing a hum loop on the sound output. It wouldn't help in diagnosing amplification problems. We'd probably need a test tone and a "good" (reference) output voltage reading for this precise tone.

In other words: the original 8580 is 4 times louder than the FPGASID?

The sound quality of your video seems OK. In case there would be a hardware problem I'd also expect some kind of distortion which does not seem to be the case.

The output level of original SIDs can vary a lot. I found that 8580s are usually a little weaker than 6581s. Since I wanted the FPGASID to work with the 9V voltage of the 8580 as well, I reduced the level slightly against typical 6581 SIDs. But here we are talking of something like 10-20% and not 400%.

Maybe your 8580 has an exceptional high output level so compared to the FPGASID it seems much louder.

I think we need a more reproducible way of checking the output level than just your ears.

Do you have a recording device that can be connected to your C64 and which displays the input level? It has to be a wire connection. Recording via microphone is not an option because this usually has automatic gain control involved. You could then compare the FPGASID recording level against the 8580 level. An exact figure like "FPGASID is 6dB lower than 8580" would be helpful to judge if the effect is still normal or out of spec.

I am using a PC (with an external Audio interface) to capture the SID output. For recording I use Audacity which has a rather decent level display.

OK, Voltage is perfectly OK.

Checking cables, pushing and wiggling the FPGASID did not help.

I think we did what we could.

I contacted Kryoflux. They will exchange your FPGASID when you send it back to them.
Please contact them (https://webstore.kryoflux.com/catalog/contact_us.php) by refering to this discussion. Ask for the shipping address and guidlines how to declare for the customs ("defective return") etc.

Before you send it away, please reinstall the original 8580 to see if the volume is OK then. This would give us the final hint that nothing in your cabling and setup is wrong.

-33 dB ist definitely not ok. I suggest to exchange the FPGASID with a new one.

Ok, your decision.

You wrote that initially everything was fine until the hum started and the volume dropped. At that moment something must have happened. The symptoms still indicate that the output amp is broken. So maybe you find someone to exchange that for you. I could do it. But that would still require shipping from US to EU and back...

Great! Thanks for the feedback!