The This Korg DDD1 blew fuses one after the other. This often indicates a bad rectifier. The rectifier tested okay, but I noticed that there was no impedance when ringing through the filter capacitors with the multimeter.
I isolated the power supply board (unplugged everything from it), and the symptom did not change.
It was only a matter of time before the bad filter capacitor could be located.
I check filter caps with a continuity tester. In general, they should show continuity for a second or two, then the resistance should increase, until no continuity is shown.
If you are checking them "in circuit", then of course they may seem to be conducting slightly, but that is just a result of other components near them. It's important to see that curve, from low impedance to high impedance, which is a capacitor charging up. You should always be able to get it to beep for a second.
After replacing the filter caps, I also replaced the rectifier diodes.
I felt like it was possible that a leaky diode put more strain on the capacitors. This may not have been necessary, but it is cheap and easy to do.
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On the TR 505, which also didn't power on, I was able to easily repair the traces around the power input jack.
This is a very easy repair.
This is a repair man's dream: Easy to spot, easy to fix, and there are no lingering doubts that you may have not addressed the whole problem!
On this Alesis unit, I encountered a whole different situation.
It powered on, but there was plenty of noise on the outputs.
I started with the power supply again, because this noise sound like 60 cycle AC noise bleeding into the system.
This time I used the scope, and sure enough on the negative 12 volt rail, there was plenty of AC leaking in.
What do we blame here?
1) The diodes. The rectifier diodes may have failed and are letting AC into the circuit. 2) A shorted regulator, or a short downstream on the -12volt rail. Why? I have noticed that when there is too much of a load on a rail, the filtering potential of the filter caps is defeated, and they don't have the capacitance to "smooth out" the rail, because the rail is drawing too much current.
How do we check that? Do we lift a leg of the 12 volt regulator and stick our ammeter in series with the load, to see how much current the rail is drawing?
We can. I don't. I just put my finger on the regulator.
If it doesn't burn my finger within a second, then it's not drawing too much current.
Finally, option 3 is a blown filter capacitor.
That means there is no filtering going on, and this was the case.
Using my continuity ringer, I noticed that there was NO BEEP, not even for a second, no matter which way I connected my leads to the cap, and no matter how long I waited. With a 300 uF cap there should be a beep.
So I replaced the cap, and all OK: smooth power rail with the oscilloscope, nice clean sound coming out of the outputs.
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