Moderators isaac42 Posted October 7, 2019 Moderators Share Posted October 7, 2019 Opened it up, looking for visible issues. Checked all of the fuses. One of the power supply caps was bulging. Noted that, but couldn't think of any reason that would cause the speaker to fail completely. Couldn't find any other issues. Tested the drivers, and they worked, as expected. I wouldn't have thought that both would fail at the same time. Wiring looked good. The cap was the only thing I could see. My thinking was that a bad cap ought to cause noise problems if open, or a blown fuse if shorted. Neither of those was the problem. It turned on (power LED lit), but no sound. So, I did what a good tech ought to do. I got a new cap and replaced the cap. Fix the known problem, even if you don't think it could cause the symptoms. It might be masking another problem. Put it all back together, powered it up. No smoke, good. Light on, good. Applied a signal, and voila! Sound. No idea how that cap could have caused the lack of sound, but it works now. I must have the magic touch. Link to comment Share on other sites More sharing options...
Members DeepEnd Posted October 7, 2019 Members Share Posted October 7, 2019 Congratulations on the fix. Any explanation I might come up with would be half baked. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted October 8, 2019 Author Moderators Share Posted October 8, 2019 My best guess, at this point, is that the bad cap pulled the voltage down enough that there wasn't enough for the regulator to do its job, resulting in no voltage for the rest of the amp. Link to comment Share on other sites More sharing options...
Members nice keetee Posted October 8, 2019 Members Share Posted October 8, 2019 voila! Link to comment Share on other sites More sharing options...
Moderators daddymack Posted October 8, 2019 Moderators Share Posted October 8, 2019 a bulging capacitor is an unhappy capacitor Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted October 9, 2019 Author Moderators Share Posted October 9, 2019 6 hours ago, daddymack said: a bulging capacitor is an unhappy capacitor Tru dat! Link to comment Share on other sites More sharing options...
Members Mr.Grumpy Posted October 9, 2019 Members Share Posted October 9, 2019 Sometimes, dumb luck works in one's favor. I repaired a "vintage" (early 70s) Acoustic 170 (?), a HUUUGE solid state combo amp with a single 15" driver in a "horn loaded" cab. I used it as a bass amp, then found out it's more of a 'general purpose' amp for keyboard or PA use. When I bought it, it made sound, but the volume was extremely low. Opened it up, and there were a couple of bulged electrolytic caps, I replaced them, and the amp worked perfectly. I wish I hadn't sold the damn thing, it was cool. Electrolytic caps aren't just used in the power supply. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted October 11, 2019 Author Moderators Share Posted October 11, 2019 On 10/9/2019 at 8:24 AM, Mr.Grumpy said: Sometimes, dumb luck works in one's favor. I repaired a "vintage" (early 70s) Acoustic 170 (?), a HUUUGE solid state combo amp with a single 15" driver in a "horn loaded" cab. I used it as a bass amp, then found out it's more of a 'general purpose' amp for keyboard or PA use. When I bought it, it made sound, but the volume was extremely low. Opened it up, and there were a couple of bulged electrolytic caps, I replaced them, and the amp worked perfectly. I wish I hadn't sold the damn thing, it was cool. Electrolytic caps aren't just used in the power supply. That's true, but I do know a little bit about this stuff, and this was a power supply cap. Link to comment Share on other sites More sharing options...
Members WRGKMC Posted December 6, 2019 Members Share Posted December 6, 2019 A cap is supposed to pass AC and block DC. This is how its used in most power supplies to smooth the ripple created by the bridge rectifier diodes. When a cap goes bad it begins to pass DC and taking your DC supply to ground which blows the fuses. Unfortunately in many high powered SS amps taking the DC to ground also raises the current up on anything that DC is passing through, namely the power transistors. What happens the Power transistors overheat and wind up blowing out if the amp doesn't have fuses in the circuit (which many don't) On certain amps that have the standard Button type power transistors the End result is passing AC voltage through your speakers and cooking the voice coils. All of that can be avoided by simply spending 10~20 and replace the power caps every 5~10 years depending on how much you use your amp. If you run it every day you should get 10 or more years out of them. If you rarely power it up they wind up going bad much sooner. Think of them as batteries that need to be charged up regularly. If you let your car sit the batter eventually dies because it isn't being charged back up by the alternator. Caps use aluminum foil for plates. When new the plates have an oxide coating which act as an insulation barrier. When they aren't passing electricity, that oxide layer gets thinner and thinner until the plates begin to short. Once they start to go bad they may simply overheat and that's when you get the cans to pop as the electrolyte boils off and escapes through that safety valve. A full short out is what typically occurs next. Yours had made it to the point of shorting out and the fuse did its job. You're lucky it didn't take out any power transistors in the process. Allot of it has to do with the circuit design, some amps have very little protection beyond maybe being a transformer primary to prevent fires. Yours has a fuse in the DC circuit that saved it from doing any damage to the output transistors. I wish that old Peavy bass head I was testing had that fuse. The power transistors blew and it took out my prized vintage Altec speakers. Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted December 7, 2019 Author Moderators Share Posted December 7, 2019 No, there was no blown fuse. I thought I made that clear. Link to comment Share on other sites More sharing options...
Members Voltan Posted December 9, 2019 Members Share Posted December 9, 2019 even a blind hog finds an acorn once in a while, isaac... lol... (what a terrible metaphor considering they use sense of smell for such hunting to begin with.) Link to comment Share on other sites More sharing options...
Moderators isaac42 Posted December 10, 2019 Author Moderators Share Posted December 10, 2019 On 12/9/2019 at 5:55 AM, Voltan said: even a blind hog finds an acorn once in a while, isaac... lol... (what a terrible metaphor considering they use sense of smell for such hunting to begin with.) They say that it's better to be lucky than good! Link to comment Share on other sites More sharing options...
Members Voltan Posted December 11, 2019 Members Share Posted December 11, 2019 6 hours ago, isaac42 said: They say that it's better to be lucky than good! agreed! no matter how good one is, theres always that off day... unless of course, if one is lucky... maybe “curiosity filled the cat”? Link to comment Share on other sites More sharing options...
Recommended Posts
Archived
This topic is now archived and is closed to further replies.