I just finished repairing a Carvin SX200 that was a very interesting repair.
It's possible that I would pass on an amp where all the PCB's, including the power supply and output boards are SMT, but this one had symptoms that seemed to indicate nothing worse than intermittent FX loop jacks and probably an intermittent contact in the headphone jack. I asked the guy if he ever used the headphone jack and would he care if I just jumpered across the interruptor switch to prevent future problems. He said go right ahead.
Fixed up the jack problems, and along the way marveled at the small size of an amp capable of cranking more than 100W before clipping. Son of a gun, it had a switch-mode power supply. Good thing that wasn't buggered, especially with all the surface mount stuff on that main PCB.
Unfortunately, final testing showed the amp exhibiting an intermittent high frequency oscillation that seemed to come and go when I wiggled an interconnect cable.
Long story short, it turned out that the VIPER100 switch mode regulator's legs were almost broken off, and in fact did come completely off with very little prodding. I think the howl was due to the switching frequency getting superimposed on the audio. Also replaced the +15V regulator which was hanging by a thread. I even noticed that one of the SMT zeners had come adrift and instead of tacking on a leaded component, I got it back in place with my long nose pliers and the same soldering iron I use on tube sockets.
Felt like a real swingin' dick I did. Fixed a switch mode power supply and did SMT rework with my regular amp bench tools!
Until I attempted to fire everything back up in the usual "power supply issue" mode, by bringing it up on the variac, and then on the lightbulb limiter.
Drawing lots of current, brightly lit bulb every time, but every voltage was what it should have been, given the limited input voltage. No indication of shorted output transistors, drivers, anything like that.
Bringing the variac up a little more resulted in smoke coming off an unidentified component that I now think is likely a thermal resistor. It's in-line with the main DC coming off the rectifier bridge.
Finally decided to chance just plugging the thing in and riding the power switch, and of course it worked fine.
I guess switch-mode power supplies behave that way in brownout conditions. Really never worked with them before, but techie beware: you can't use your usual soft-start techniques with a switcher!
In 5 years of taking in amps for repair, this is the first such amp I've encountered, and there are few posts on this forum talking about amps with switching power supplies, so it's probably not a huge issue, but man!
It's possible that I would pass on an amp where all the PCB's, including the power supply and output boards are SMT, but this one had symptoms that seemed to indicate nothing worse than intermittent FX loop jacks and probably an intermittent contact in the headphone jack. I asked the guy if he ever used the headphone jack and would he care if I just jumpered across the interruptor switch to prevent future problems. He said go right ahead.
Fixed up the jack problems, and along the way marveled at the small size of an amp capable of cranking more than 100W before clipping. Son of a gun, it had a switch-mode power supply. Good thing that wasn't buggered, especially with all the surface mount stuff on that main PCB.
Unfortunately, final testing showed the amp exhibiting an intermittent high frequency oscillation that seemed to come and go when I wiggled an interconnect cable.
Long story short, it turned out that the VIPER100 switch mode regulator's legs were almost broken off, and in fact did come completely off with very little prodding. I think the howl was due to the switching frequency getting superimposed on the audio. Also replaced the +15V regulator which was hanging by a thread. I even noticed that one of the SMT zeners had come adrift and instead of tacking on a leaded component, I got it back in place with my long nose pliers and the same soldering iron I use on tube sockets.
Felt like a real swingin' dick I did. Fixed a switch mode power supply and did SMT rework with my regular amp bench tools!
Until I attempted to fire everything back up in the usual "power supply issue" mode, by bringing it up on the variac, and then on the lightbulb limiter.
Drawing lots of current, brightly lit bulb every time, but every voltage was what it should have been, given the limited input voltage. No indication of shorted output transistors, drivers, anything like that.
Bringing the variac up a little more resulted in smoke coming off an unidentified component that I now think is likely a thermal resistor. It's in-line with the main DC coming off the rectifier bridge.
Finally decided to chance just plugging the thing in and riding the power switch, and of course it worked fine.
I guess switch-mode power supplies behave that way in brownout conditions. Really never worked with them before, but techie beware: you can't use your usual soft-start techniques with a switcher!
In 5 years of taking in amps for repair, this is the first such amp I've encountered, and there are few posts on this forum talking about amps with switching power supplies, so it's probably not a huge issue, but man!
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