Stereo amplifier short circuit issue - Page 1
Forum section: electronics / beginners

author: Guerissologue
author_rank: Newbie
timestamp: July 25, 2025, 03:33:07 pm
content: Hi,I have a Pioneer A-225 stereo amp that keeps blowing the fuse as soon as I turn it on even with nothing connected to it (there's only one fuse).The amp was working fine until I left it in my trunk for a month.My multimeter shows a short when I test the main plug.I desoldered and tested the diode bridge, but it seems fine.The capacitors look okay, but I’m a newbie, so maybe they’re not.There’s an STK4181V amplifier IC with 18 pins, but I’m not sure which pins I should check with the multimeter.The previous owner had added a Bluetooth module, which I removed just in case it was faulty, but that didn’t change anything.Here’s the service manual I found:https://akdatabase.org/AKview/albums/userpics/10007/Pioneer%20A-225%20Service%20Manual.pdfIf you need more info please let me know, I can upload pictures if needed.Sorry if anything I did seems dumb, and thank you for your time and help.

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 25, 2025, 03:53:55 pm
content: I assume that when you removed the rectifier the fuse no longer blows (of course there is no power to the amplifier). If it still blows the fuse then it's the transformer. Checking the winding of the transformer would result in low resistance reading and it may not be an indication of a short.

author: Guerissologue
author_rank: Newbie
timestamp: July 25, 2025, 04:28:05 pm
content: With the rectifier removed, the fuse does not blow when i turn the amp on and off. This means the transformer is fine if I understand correctly?

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 25, 2025, 06:35:33 pm
content: Then I would disconnect the amplifer circuit from the power supply circuit. I would try to see if the power supply circuit would produce the + and - voltage without problem under no load. If you have problem at this point then it would be relatively easy to find out.

author: MathWizard
author_rank: Super Contributor
timestamp: July 28, 2025, 04:32:03 pm
content: So the stereo was in a car trunk for a month? Were you driving around ? What sort of day/night temperature variation was there ?If you were driving around, I'd have a good look at the solder joints, especially under big heavy parts. Look up some images of cracked and broken solder joints.A broken solder pad on 1 of my stereo's, would trip the emergency shut-down circuits. I was fearing the worst, until I found the broken pad under a capacitor.

author: Sorama
author_rank: Frequent Contributor
timestamp: July 28, 2025, 04:55:44 pm
content: Quote from: MathWizard on July 28, 2025, 04:32:03 pmI don’t see how something like a broken pad can cause a short as is the case with TS.

author: Guerissologue
author_rank: Newbie
timestamp: July 31, 2025, 02:08:57 pm
content: Quote from: BeBuLamar on July 25, 2025, 06:35:33 pmI guess I have to connect the power supply to a light bulb or something after disconnecting it from the amp circuit? Or can I test it using only a multimeter?

author: Guerissologue
author_rank: Newbie
timestamp: July 31, 2025, 02:13:45 pm
content: The car didn't move much. It was during January in France, so I assume the temp variations were not that extreme, between 0°C and 10°C.

author: BeBuLamar
author_rank: Super Contributor
timestamp: July 31, 2025, 03:38:19 pm
content: Quote from: Guerissologue on July 31, 2025, 02:08:57 pmYou can just measure the voltage. However if you do that and it does not blow the fuse then the problem is further down.

author: Guerissologue
author_rank: Newbie
timestamp: August 01, 2025, 01:17:31 am
content: I'm sorry, I don't fully understand what exactly needs to be disconnected, or how and where to do it. I've attached a detail of the circuit schematic where I’ve outlined in red what I believe are the VCC+ and VCC- of the stereo amplifier.To test the power supply, I need to re-solder the rectifier I tested earlier (outlined in green on the schematic), and then desolder the amplifier’s VCC pins (12, 11, 9, and 4). After that, I would plug the whole unit into the mains, turn it on, hope it doesn't blow the fuse, and measure the power supply’s output. Is that correct?Desoldering those four pins seems pretty difficult. I don't see any alternative points to desolder from, since the connections are printed directly onto the board.Sorry if I'm talking complete nonsense, I'm a bit lost.

author: Seekonk
author_rank: Super Contributor
timestamp: August 01, 2025, 01:14:43 pm
content: This isn't that hard if you just do some analysis. It is stereo in the same IC and a comparison can be made with a resistance measurement between the left and right channel, power supply + and - to the output pin. Chances only one is shorted.I've always use a dim bulb tester with a 40-60W incandescent lamp in series with the AC power. Then you can make voltage measurements with power applied safely.

author: ramussons
author_rank: Contributor
timestamp: August 01, 2025, 02:06:39 pm
content: Quote from: Seekonk on August 01, 2025, 01:14:43 pmI agree. That's the way to track down the problem - powered ON, yet no blowing fuses or components.

author: themadhippy
author_rank: Super Contributor
timestamp: August 01, 2025, 02:30:46 pm
content: or powered up, fuse bypassed and see were the smoke /flames/bang appear first