Amplifier channels cutting out

daddyjt wrote: »

Scenario #2 sounds like a bad relay. I’ve fixed this on may vintage (and some not so vintage) amplifiers. The contactors get corroded and/or oxidized, and at lower volume (voltage) the signal is not sufficient to overcome the insulating effects. Increased volume (voltage) will often “bridge the gap” as it were.

What if it's not corroded? What if it's someplace where a speaker lead could be stressed like an outdoor speaker that gets moved around by wind or, say, a car dash or trunk that vibrates from road noise?

This isn't going to go as well as I had hoped.


So here's what happened to me and what it ended up being.

- One, the channel just isn't playing. Increased volume sort of "wakes it up" and then it operates fine for the duration of use. It may or may not occur again on the next start up after sitting idle. It seems to be random but it will occur again. The greater the length of time that the amplifier is off seems increase the chances of this occurring.

This ended up being a capacitor on the channel. It was working but tested out of spec and lower volumes were not enough to charge it because it was out of spec. So it was an open circuit when it wasn't charging. Higher volumes forced it to charge and once charged it would operate in spec and teh channel would "wake up". This is more common of a problem in older, oil filled capacitors but it can also be a problem in newer, film based capacitors. Newer capacitors will see this problem more from corrosion. Oil based capacitors can get slow leaks which causes a slow failure. In my case, it was a car audio amplifier, the capacitor had overheated at some point and there was a small overheated spot on the top of the can but it didn't burn out completely. It corroded and developed into a pinhole that let corrosion get in. It was taken to an authorized service dealer who checked the main board and replaced just that capacitor as nothing else was wrong. This can happen often in head units on the output end because they will routinely get over-driven. Probably happens often in receivers too.




- Two, The channel is playing quietly but it is playing. Increased volume ends up "waking it up" and it will play at full capability. If the volume is reduced too soon, it will go back to playing quietly again until the volume is increased again to "wake it up" again. If played at a high volume for a short period of time, it is much less likely to return to playing quietly again.

This one ended up being a failing output transformer. It was a cheap stereo receiver from the late 80's or early 90's and the output transformer was partially open due to burned windings from being over driven in a harsh environment (auto repair shop). Like carbide dust from a grinder was shorting out some windings on the outer wrapping. I don't even think it was an actual power transformer as it had stampings like a loudspeaker transformer or a guitar amp transformer would have had. The electronics tech said he never saw an output circuit built that way. It was like they cobbled together an amplifier network with whatever parts bin parts they had. I don't even remember the name. Something like Kenphon or Kenton or something. I don't even know why we were trying to repair it. The shop owner was just cheap, I guess. Anyway, the output transformer was not putting out enough power at certain tap points because of it. When volume was increased, it would, kinda, get past that problem if you increased it slowly and the capacitance on the output circuit would charge correctly. Then the channel would play at full output. If you increased the volume too fast, it would trigger a protection circuit which would cut power to the channel and you would have to shut it down and turn it back on to reset the protection circuit. It was only a matter of time before a capacitor on the protection circuit blew and killed the channel completely anyway. It was only one tap point that was affected on the output transformer for that channel and this was found through testing on a bench. The whole unit was replaced as it was "free" to begin with and the shop owner didn't want to put any money in to it.




- Three, one of the drivers is not playing and the other is playing or the other is playing at an attenuated level. Increased volume will "wake up" the other driver and if the functioning driver was attenuated, it will strengthen performance as well.

This happened twice. Once it was due to a crossover capacitor failing and not working until a high enough power load forced it, temporarily, into spec by being over-driven. It eventually failed completely and took a resistor with it. It was an older Realistic kit speaker from Radio Shack. The entire crossover had to be replaced.

The other time, it was a burned voice coil on a woofer. The speaker component set was bought and installed but was using the factory amplification. The car had a separate, external amplifier from the "entertainment system" which was in the dash. The owner had used a pigtail kit to hook everything up to the factory wiring at the speaker locations. I thought, when diagnosing it, that the tweeter and woofer were connected but didn't see that the tweeter didn't piggyback off the woofer connection because it's wiring was coming out of the loom. When I looked up the wiring schematic, I found that it actually had a 9 channel amplifier. 4 high range outputs, 4 low range outputs and a subwoofer output. The stock speakers were all 10 ohms and the aftermarket speakers were presenting 6 ohms and when run through their crossovers would present a 4 ohm load to the amplifier. Individually they were 6, though, so they must have had some circuitry in the crossover network to present 4...anyway, it was goofy, not the point. The lower impedance on the factory amp meant it was being over-driven, likely to clipping knowing the owner and his self-destructive habits with EVERYTHING. So the woofer on the driver's side burned the voice coil and at low initial volume, there wasn't enough power to move the voice coil from rest. Jack up the power and instead of just vibrating slightly on lower power, the voice coil would start moving as the rest of the working part of the voice coil would create a large enough field to work right again. But, since the VC was effectively a fraction of the original size, it was overheating regularly and driving that channel into clipping regularly. The owner asked for help because he would drive it into clipping so hard the protection circuit would trigger and the whole stereo would die.

The solution was to replace the bad driver and re-wire the entire stereo to an aftermarket amplifier that could handle the loads. We also incorporated the crossover networks. The speakers were some weird Czech made brand with a German name like Brucker or some other Becker knock-off name that he got from one of those shady car audio stores that you see at, like, indoor flea markets or the meaner parts of town. He bought a Lanzar amplifier from the same place to replace the factory unit he burned up.



Just figured it'd be a fun thing to see everyone's stories. Not everything is a corroded potentiometer. Some stuff doesn't even have potentiometers to corrode.