Roender's FC-100 prototype and builder's thread

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#830
AndriyOL said:
Does anybody have an idea what could be a reason of this amplifier topology starting to short at once from 5.6mV and higher VRe or from toching by test leads as a trigger (when VRe close to 5.6mV) or by itself after 5-10 minutes? During the short condition VRe shows 33mV, Vbe-s are in range.
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My amp works but when i put all the parts into a case and power up things start to go wrong ,One board Fuses blow after a short time for no reason i can see,i cannot set the bias current it just keeps on rising.
Then the other board starts acting the same .i was lost.So reading old posts i came across this and to get to the point are my mje 15035 /4 fake .
i have new ones in post . john
 
#835
The other side is on bench with heatsinks ,with power on all volts are correct .when i put fuses in and power up i adjust bias to 25mv it then comes down by itself to 0mv i adjust back to 25mv it again comes down to about 5mv when at that point both fuses blow,
Both boards where working just the other day
 
#838
lucylu said:
The other side is on bench with heatsinks ,with power on all volts are correct .when i put fuses in and power up i adjust bias to 25mv it then comes down by itself to 0mv i adjust back to 25mv it again comes down to about 5mv when at that point both fuses blow,
Both boards where working just the other day
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Have tried this one again but this time i switch on power and instantly both fuses blow
 
#839
I have had issues with an amplifier that was OK on the bench but not in the chasis. What had changed was the wiring. Out on the bench the wiring was different, the details of the grounding and the physical lead dress was different. The issue was oscillations, the amplifier was not stable in the chasis and the cause was wiring. One of the symptoms was unstable or difficult to adjust bias along with a sudden surge in current draw. It helped to have a current limited bench supply and an oscilloscope.

Another amplifier had stability problems when I increased the power supply voltage. In this case the amplifier was stable at low bias, but when I turned up the bias it went off the deep end. There were both wiring issues and issues with the amplifier implementation itself.

Is it possible you have some oscillations ?

In the range few hundred kHz it's likely a wiring issue. In the range few MHz it's a small-loop localized issue probably around the power device or the CFP driver.
 
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