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Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
Hi all. I know this isn't the sci.electronics.repair newsgroup but thought someone might be able to advise.
Got a nice 150W MOSFET amplifier here I was hoping to modify for SL work, unfortunately it has a bad channel. One side works 100% fine but the other just crackles and hisses.
Took it apart and the 4.7 ohm resistor in series with a PP capacitor on the output side is "cooked" and has a big hole burned in the side.
Any ideas what might have caused this? I checked the capacitor and it looks OK, however from reading other notes on these it seems they have a nasty habit of failing under load and shorting, then "self healing".
thanks in advance, -A #include "No_Nukes_on_Mars.h"
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The Zobel network usually burns up because the amp has gone unstable and cooked it with high frequency oscillations. This may have been caused by the user putting a bad load on it, but once the Zobel resistor has burnt open, the amp will be unstable regardless of the load. I'd try replacing it and see if the crackles and hisses go away.
Registered Member #96
Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061
OK, thanks. Found references to this effect being caused by bad joints on the output as well, apparently the semiconducting properties act as a little radio transmitter and lead to the amp bursting into 100+ kHz oscillations.
On some amplifiers they are unstable no matter what, the fix is to add a 10K resistor on each input to ground which helps reduce the oscillation potential without affecting anything else.
Another interesting effect I noticed, some amplifiers use a pair of yellow LEDs and a photoresistor as gain control. If the resistor goes low value it will cripple the affected channel. Just don't ask how I found that out
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Zobel network fried, cause?
