Muff's Modules & More

[Neutron7]

I have a triple VCA with a strange problem. (since i finished building it, but have had time to look at it today)

the CV inputs affect the volume a bit, but the gain controls are like they are full on. on further testing i noticed that the -0.6v branch which the gain pots use at one end, was at 15v. in fact it is shorted to 15vB somewhere.

I thought there might be something under the gain pots, so i removed them (difficult!) and just to be on the safe side i removed r1, C1,C2, D1, and r35.

so now there should be nothing at all connected to the -0.6v branch right? well it is still shorted to 15vB (tested with multimeter, did not plug the module in!) and i cant find the short ANYWHERE! the 15vB is no longer connected to +15v through R1, and that is correct, i removed it because my continuity meter still beeps with the 22r there, and i thoguth it would help narrow down possible places to look.

Any tips on trying to find it? i am getting frustrated, and tempted to put a bit of current across the shorted 15B and -0.6 and see if i can burn the short like a fuse

[Synthbuilder]

The most likely places where those two lines are going to be joined are just below the lower pad of either of the three pots.

If you look at the board with the board held in its normal orientation as mounted on the panel you'll see the -0.6V line from R35 sneaks right past the lower pad of the lowest pot and moves to the front of the board and then onto the top pad of the pot. It then moves up the board to the next pot and does the same snaking about before moving up to the next pot.

The problem is that the track is under the pot on the lower two pots and only on the top pot will the track be visible on the solder side of the board. But if you have taken the pots off you should be able to find it. However, track shorts are usually very very small. Use a bright light and a magnifying glass to help you locate it.

You can try and fuse it out and I have had some luck doing this in the past on multilayer boards. But make sure that it isn't a thick short before doing this.

Thankfully all new Oakley boards are now electrically tested so these sorts of shorts are not possible now.

Tony

[Synthbuilder]

Just a thought. Don't try to find it. Assume that it is there and scrape with a sharp knife that space between the -0.6V track and the lowest pad on each of the three pots.

Tony

[Neutron7]

Thanks, I found it! It was under the solder mask between R1 and D1

[update]
Got it all back together, and now it is working fine!