unclej Posted June 8, 2006 Report Share Posted June 8, 2006 i've got a fairly new fender deluxe in the shop for repair..basic case of motorboating in the overdrive channel. (if you play with the drive control knob you can make it rev up and then take off real fast) anyway, i remembered a thread i started some time ago and looked it up to find this quote: "In case you didn't think of this, one way to test for open filter caps is to hold a new capacitor of equal (or even a bit greater) value on underside of the pc board on the pins of the suspect capacitor (connected in parallel with it IOW) and see if it makes the oscillations stop." any reason i can't take two small jumpers with alligator clips and connect a new cap to the old one..pos to pos, neg to neg and see if it stops? makes perfect sense to me but then i've been known to blow stuff up while making perfect sense before. by the way..in the quote above he uses the initials IOW..whazatmean? Quote Link to comment Share on other sites More sharing options...
Mike Sulzer Posted June 8, 2006 Report Share Posted June 8, 2006 Sure, and I like your jumpers better than the method in the quotes. You can connect it up, turn it on, and see what happens. Maybe IOW means "it oughta work", but if you keep sticking caps across circuits with them hot, it will soon mean "Injured and Outa Work". i've got a fairly new fender deluxe in the shop for repair..basic case of motorboating in the overdrive channel. (if you play with the drive control knob you can make it rev up and then take off real fast) anyway, i remembered a thread i started some time ago and looked it up to find this quote: "In case you didn't think of this, one way to test for open filter caps is to hold a new capacitor of equal (or even a bit greater) value on underside of the pc board on the pins of the suspect capacitor (connected in parallel with it IOW) and see if it makes the oscillations stop." any reason i can't take two small jumpers with alligator clips and connect a new cap to the old one..pos to pos, neg to neg and see if it stops? makes perfect sense to me but then i've been known to blow stuff up while making perfect sense before. by the way..in the quote above he uses the initials IOW..whazatmean? Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 8, 2006 Report Share Posted June 8, 2006 I had a thought for those who might know: Can a bad bypass cap cause motorboating? Quote Link to comment Share on other sites More sharing options...
lovekraft Posted June 9, 2006 Report Share Posted June 9, 2006 I've never seen a bad cathode bypass cap cause oscillation, but that's no indication that it can't happen. Subsonic oscillations are almost always power supply related (IME), usually from insufficient isolation between the stages, so the filter caps are the first thing to check, since they are subject to fail over time anyhow. A test cap with alligator clips is a really good idea so long as you're very careful about keeping it discharged when it's not being used (remember, electrolytics have a nasty habit of "recharging" themselves when they're out of the circuit - dielectric adsorption, or something like that). Since this is a production amp with no well-documented issues, it's unlikely that there's a layout problem, or any other construction related failure, so the caps are the prime suspects, at least from where I'm sitting. Doesn't IOW stand for in other words? Quote Link to comment Share on other sites More sharing options...
Mike Sulzer Posted June 9, 2006 Report Share Posted June 9, 2006 "Doesn't IOW stand for in other words?" Yes, but what fun is that? Lovekraft is right. In guitar amp circuits, removing a cathode bypass cap lowers the gain and thus reduces the tendency to oscillate. I am not saying you could not design a circuit that would work the other way, but it would be exceptional. Quote Link to comment Share on other sites More sharing options...
unclej Posted June 9, 2006 Author Report Share Posted June 9, 2006 thanks gentlemen.. lk..you may remember that i posted some time ago that i had connected an electrolytic backwards and sent flaming shrapnel all over my work bench and my favorite grateful dead t-shirt. i did shock myself pretty good being a little careless with them once..now i'm cautious to a fault. i'll try the jumper test this morning when i go in and let everyone know how it works. Quote Link to comment Share on other sites More sharing options...
JoeAArthur Posted June 9, 2006 Report Share Posted June 9, 2006 Ah yes, motorboating. As LoveKraft pointed out, almost always due to some power supply problem providing a low frequency feedback path. Filter caps are almost always the problem... But sometimes clipping a good one across an existing one doesn't always mean that one is bad - just that it is one of the ones in the feedback path. You may wind up replacing a good one and find out you still have motorboating. If one is bad chances are the rest are about to follow so if the motorboating stops with a clipped on electrolytic anywhere I replace them all. You may find after clipping on a good one that the motorboating increases or the frequency changes. A friend of mine decided to change his own filter caps in his twin and after he was done found he had a motorboat problem whenever either channel's volume control was more than "2". He brought the amp over and we tore it apart. The cause was using a 250mfd 450 volt unit for the last or preamp stages - he thought it was a 25mfd because the price tag was covering the zero!! Replacing it with a 22mfd fixed the motorboating - so sometimes too much capacity can create motorboating. And if changing the electrolytics don't fix it... the next likely suspect will be the decoupling resistors - over time they can drift in value. Quote Link to comment Share on other sites More sharing options...
unclej Posted June 9, 2006 Author Report Share Posted June 9, 2006 well that was cooler'n fourteen little sonamaguns. worked like a charm. of course it was the last one that i checked instead of the first one but it was extremely easy and i didn't blow anything up and i didn't fry myself and i didn't even have to pull the pc board..just clipped the bad one off and soldered the new one onto the old leads. problem solved. by the way..i knew in theory that electrolytics would re-charge themselves but i'd never measured it until today. the first one i tested was 47uf, 500v and i discharged the whole system before i removed it from the test leads. out of curiosity i put the meter on it right after taking it out and sat and watched it build up a charge. in the book of electronics for dummies is there a simple explanation for why that happens? Quote Link to comment Share on other sites More sharing options...
Mike Sulzer Posted June 10, 2006 Report Share Posted June 10, 2006 "in the book of electronics for dummies is there a simple explanation for why that happens?" I like to think of electrolytic caps as 99% capacitor, 1% battery. Just to make it clear that there is some chemistry going on that I do not really want to think about. Quote Link to comment Share on other sites More sharing options...
lovekraft Posted June 10, 2006 Report Share Posted June 10, 2006 OK, I'm sure that there's a physics major or two around here that can correct this if I get it wrong, so here goes - in a cap, the electrical charge is mostly stored at the junction surface between the dielectric and the conductor, and is released when the cap is discharged. In electrolytics, some of the charge "leaks" into the dielectric material, and since the surface has the same charge, it repels the charge that's in the dielectric material so it can't escape. but when the cap is discharged, the barrier is removed, and the trapped charge can migrate to the surface, effectively recharging the cap. Anyway, that's my story, and I'm stickin' to it! The oldtimers I remember (from the days when one could actually repair electronic equipment) used to tell war stories about old CRTs that held kilovolts of power for literally years even after being discharged, just waiting to bite the unwary, but I'm much too young to remember any of that stuff. I'm sure I screwed at least some part of that up, but I'm sure that any gross errors or omissions will be corrected in due time. Quote Link to comment Share on other sites More sharing options...
Paul Marossy Posted June 12, 2006 Report Share Posted June 12, 2006 I know that the classical motorboating thing is usually due to a bad filter cap(s), but I just had wondered about the possibility of a bad bypass cap. That answers my question! Quote Link to comment Share on other sites More sharing options...
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