From gstopp@fibermux.com Tue Mar 12 17:54:17 1996 12 Mar 96 20:54:10 +0500 11 Mar 96 15:25:10 +0500 5: 28 -0800 From: gstopp@fibermux.com Date: Mon, 11 Mar 96 11:39:06 PDT Encoding: 99 Text Subject: Re: Another 'never thought it could happen to me' message Regarding dead power supplies in analog stuff - I recently fixed my Moog Taurus I pedals. They got squished in the Great L.A. Quake of '94 (when the music room turned into a giant rock tumbler filled with analog synths). The sloping metal front panel was pushed in so I took it all apart and hammered it out on the patio concrete. After re-assmebly no noise - some poking around with a scope revealed that the +12 was +zero. Unplugging the power connector to the main PCB showed that the +12 was fine coming out of the power supply PCB. First action was to replace the 7812 regulator (cuz it was easy). No change. I concluded that there was a +12 short to ground caused by some mechanical mis-alignment. Then I found that the short didn't go away even when the main PCB was totally removed from the case. The next action was the one that always gets tried last - to start cutting the +12 traces on the main circuit board. Using the "divide and conquer" approach I was able to isolate the short to a power bypass capacitor. Replacing the cap removed the short, +12 came back, noise was made, and the problem was solved. This approach has worked for me on several synths. I have no real good theory why bypass caps go bad - I've seen all types do it - tantalum, 'lytic, ceramic disk, etc. I suspect something mechanical since this particular failure coincided with a "high-intertial-force event". The trace-cutting tactic does sound a little gory but what with the advances in modern medicine there is usually a very high rate of recovery. To do it - isolate the PCB with the short. Find the bad power rail traces. Starting at the power connector, determine all the places it goes and note the major "branches" as the rail fans out amongst all the components. Try to divide the power rail in half by cutting the trace. Determine which half of the circuit board the short resides on. Keep on doing this until you get down to the single component, or a few components that you can lift one leg of. Once you find the short, repair all the trace cuts and you should be back in action. The way that I cut traces is as follows: 1. Using an Xacto knife or single-edged razor blade, make two cuts side-by-side in the trace, straight across. 2. Using a hot soldering iron, heat up the little trace segment that's between the two cuts. 3. Once the trace is hot, rub the soldering iron tip back and fourth on the trace segment. It will loose its adhesion to the PCB epoxy and fall off. Try to make it fall on your lap or the floor or just somewhere other than between the pins of some chip. Of course you can use a Dremel moto-tool or other drill-type thing if you have one, which is fun because it makes bystanders have bad dental visit flash-backs. To repair the cut, solder a jumper made from wire or resistor lead clipping across the gap in the trace. The same technique applies to dead bugs and other fried components. However sometimes the power rail is not dragged all the way to ground, and therefore is not a zero-ohms short, and therefore may not be traceable using a tool like a beeping continuity meter, so a scope must be used on the powered-up board. A longer process, but neccesary. Anyway don't be scared of cutting circuit board traces, it's not that scary after you do a few. YMMV, FWIW, the author of this email assumes no responsibility for botched jobs resulting from the information contained here, etc.... - Gene gstopp@fibermux.com p.s. ooops I just gave away one of my "buy broken synths for cheap" secrets but heck I don't need any more of 'em anyway. My apologies to tekkie dealers everywhere. p.p.s. gotta say this - it's a girl, 8 lbs 2 oz.! Subject: Another 'never thought it could happen to me' message Author: rmiller@pangea.ca (Aural Research Facilities) at ccrelayout Date: 3/9/96 10:12 AM To anyone with experience in servicing synths: Kevin? Gene?