SID with grounding issues

Testing/Troubleshooting
1

I just put together another sid, and I am not quite sure how to get it grounded properly.

Right now I am powering it off of a 9VAC wall wart, and I notice that from the audio ground to the chassis there is  7VDC, and some serious hum. Nothing is grounded to the chassis yet, except the midi jack cause I am using plastic standoffs and plastic audio input jacks. I suppose I could sheild the midi input somehow, or switch to a plastic jack - but I would like to see it done properly.

here is a pic of the guts:

where should I begin?

2

The whole case should be grounded somehow. Always take the ground from the core / the SID for that (not the ground from your wallplug!).

To get it even better, use shielded cables for the audio plugs. Meaning: The Audio In and the Audio Out of the SID should be made with shielded cables (shielding naturally to ground).

Best is if you put a metal piece between the core and the SID, which is grounded, too. This way you should get away all the rest (the hum should come from the core and the power cable to it and the MIDI stuff always brings in some distorted sound, too).

Try all these steps in order until you´re confident with the result (not all are must-do´s).

Greetz!

3

when I grounded the audio out to the chassis it made the wallwart burn out - so thats out.

I am thinking of lifting the ground to the audio and moving it to the chassis.

like I said, I measureed 7V from the SID ground to the case ground - its just putting too much strain on the power supply.

4

when I grounded the audio out to the chassis it made the wallwart burn out - so thats out.

That mustn´t happen regardless of any grounding issues. Ground loops don´t have enough current to cause this, so there must be an error in your power supply cirquit that causes a short from audio ground to the wall wart. did you build the cirquit from http://www.ucapps.de/mbhp/mbhp_core_power_fix.pdf? BTW do you really use a 9V AC supply??? Even if you have an 8580 (and the photo looks more like 6581) you need more voltage.

About ground loops: I had a hum problem because I use the housing as a cooler, so I found out I had 3 different ground connections to the case that were:

  1. the back of the 7809

  2. Also the outside of the Alps encoders are connected with ground, so when they´re mounted on a metal front panel, there´s a connection.

  3. the outer rim of the DC connector was in contact with one of the power leads.

My fix was to use a rubber “cable lead-thru” (don´t know the english word) to isolate the DC connector from the housing.

But I think you should first check the power cirquit again.

Seppoman

5

9VAC, after its rectified and regulated puts out 12VDC no problem. But now I am left with a 12VAC transformer, so that is out.

that schematic looks really useful - I will send the AC to the Core only, then use a a regulator to bypass the bridge on the SID module - that might be whats causing this problem.

yeah, right now the only metal on metal contact is the encoder body, but I am not even sure if its grounded to anything in circuit

my AC inlet is also metal, but the wall wart itself is isloated from ground.

I will change around the supply and see if I can’t fix this issue - thanks guys!

6

ok, I did the mod and it made some differene, but I am still getting 7VDC from the SID ground to the chassis. I think I am just going to have to live with it.

my encoder has no electronic connection with the chassis, and midi is optoisolated so I should not have any problems as long as an audio cable coming out of the SID does not short against the chassis. Its a weird problem, but everything works properly so I am just gonna roll with it for the time being.

7

my AC inlet is also metal, but the wall wart itself is isloated from ground.

So if your AC inlet is metal, isn’t that carrying one leg of your low voltage AC to the chassis?  (can’t tell from the pics)

Power down and do a continuity check between the case and both legs of your AC input, one at a time.  That would be my first guess…

Metal power jacks should never be used with AC, due to these kinds of issues.  With DC a metal jack is a bonus since it carries the ground effectively to the chassis…

ok, I did the mod and it made some differene, but I am still getting 7VDC from the SID ground to the chassis. I think I am just going to have to live with it.

No no don’t just live with it!  This is dangerous since some xformer designs can sink a lot of current…Let’s say your chassis is now connected to the hot leg side of the transformers output, and it comes into contact with anything on the neutral leg (most things in your house!), the potential -could- be full 110vAC depending on the transformer design.  Seen this many many times with the coin op stuff, two games next to each other with one line cord flipped in relation to the other where they plug into the outlet.  Both work fine, but as soon as a patron touches metal on both they get a full 110v shock. 

Keep in mind it takes more than 7vdc to meltdown a transformer…you might want to see how much AC voltage potential you have there…

my encoder has no electronic connection with the chassis, and midi is optoisolated so I should not have any problems as long as an audio cable coming out of the SID does not short against the chassis. Its a weird problem, but everything works properly so I am just gonna roll with it for the time being.

We can get this through process of elimination, and I definitely would not run it until you have at least isolated the chassis.  Next run might mean your SID chip not just another transformer… :-\

Another thing that’s not completely related, thought the MIDI is opto-isolated there is still a ground connection on the MIDI out, so an improperly wired MIDI cable (shield wired to ground) or non-standard MIDI ports on other gear can cause some real issues like wall wart supplies smoking (worst) and flaky MIDI data from ground loops (best).

More info on the power connection please!  :slight_smile:

Best

Smash

8

oh. Yeah. That.

I will check into that tomorrow if I get a chance. I have a plastic power inlet - I sould switch to that

looking at the current one, yeah - I think one side of the AC line is connected to the chassis - that would explain the mysterious 7V too - I will also check it for AC voltage.

I kinda scrapped this whole thing together with stuff I just had left over from other projects, I did not even think about the metal power inlet.

I will post my findings soon!

9

yeah, the AC line was totally shorting with the chassis. My plastic inlet does not match the inner diameter of my supply - but I will order one that does this week, and call this SID done!

thanks smash - as usual, an outside opinion is the best thing for pointing out the obvious.  :slight_smile: