A friend i know just bought a secondhand F5 clone. After listening for approximately 10 minutes at moderate volume the amplifier started smelling and we turned it off immediately. Upon closer inspection the 0.47ohm resistor are burnt and one has even desoldered itself.
Could anyone help with figuring out what might have caused this? And how to best approach a repair.
Thanks 🙂
Could anyone help with figuring out what might have caused this? And how to best approach a repair.
Thanks 🙂
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Bias set way too high, or obstructed ventilation?
Shorted speaker wires, or unusual high capacitance speaker wires?
It's unlikely the amp just suddenly decided to fail. There was a reason.
Shorted speaker wires, or unusual high capacitance speaker wires?
It's unlikely the amp just suddenly decided to fail. There was a reason.
Thanks!
Is there any chance the bias potentiometers might change during transport due to vibrations or something? Or maybe drift over time?
Ventilation was good and the output terminals was not directly shorted since music was still playing until we turned it off due to the faint smell of magic smoke.
From the schematic it looks like too high bias might turn off the channel? Or am i misunderstanding?
Is there any chance the bias potentiometers might change during transport due to vibrations or something? Or maybe drift over time?
Ventilation was good and the output terminals was not directly shorted since music was still playing until we turned it off due to the faint smell of magic smoke.
From the schematic it looks like too high bias might turn off the channel? Or am i misunderstanding?
sry but what the heck are the builder think about the resistor is populated!?! this is wrong!!
please see thread here F5 build guide
see pic
please see thread here F5 build guide
see pic
Good point.
When the amp arrived we did a quick inspection inside and did not see anything obvious out of line. A picture is attached.
While not apparant on the picture of the dead channel, both 0.47R resistors have a noticable brown discoloring compared to the good channel.
When the amp arrived we did a quick inspection inside and did not see anything obvious out of line. A picture is attached.
While not apparant on the picture of the dead channel, both 0.47R resistors have a noticable brown discoloring compared to the good channel.
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Which indicates too high a bias current on that channel. You will need to check the 2 output fets to make sure neither is shorted and probably change the 0R47 3 watt resistors, while you have the board out, check all solder joints. Once done then check and adjust bias on both channels as per the build guide.
Of course the resistors are discolored, since they had very high current and temperature.
This version does not have current limiting, so it would not turn off when hot.
This version does not have current limiting, so it would not turn off when hot.
Okay thanks guys.
Too high bias definetely seems the most probable cause.
The guy who sold the amplifier provided photos of the bias measuring correctly on both channels, but maybe something went wrong during transport.. 🙁
Now i will cross my fingers that no semis died also 🙂
Too high bias definetely seems the most probable cause.
The guy who sold the amplifier provided photos of the bias measuring correctly on both channels, but maybe something went wrong during transport.. 🙁
Now i will cross my fingers that no semis died also 🙂
seems it was never soldered.....
please follow build guide and --short input if you only heat it up....
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Also, with a DMM on resistance scale, check the bias trimpot operates smoothly over the entire travel, with no jumps as you slowly wind the pot.
Not likely to be a bad pot (unless a fake copy), but perhaps a bad solder joint connecting it.
The pot cannot shift setting during shipment.
Take off both boards and inspect all the solder joints carefully.
Good designs such as this can still be fouled by poor quality of assembly.
The pot cannot shift setting during shipment.
Take off both boards and inspect all the solder joints carefully.
Good designs such as this can still be fouled by poor quality of assembly.
It looks like the thermistor is contacting the metal washer holding the MOSFET to heat sink. The thermistor is known to conduct due to the paint on them being thin. Bend the leads so it's not touching and check the other channel to make sure it is only contacting the plastic case of the MOSFET. The thermistor will reduce the current through the source resistor when the MOSFET gets to hot.