Hi to all my fans,
just completed repairs on one of these:
Found it fitted inside a mini-size bass instrument amplifier, which the own er had just purchased over the net and failed on him at first power up.
Smelling a rat, I checked the AC switch on the rear panel first - blow me d own, it was set to 120VAC and we live in a 240V country.
Reset the switch, then it drew zero AC current - no surprise.
Replaced the blown 5AT fuse in the IcePower module and then the PSU began h iccupping - drawing an amp or two very briefly then shutting off every few seconds.
IME feeding double AC supply voltage to a unit with a SMPS is almost always fatal, sure the AC fuse blows but semis in the HV part of the circuit fail in a fraction of a second. But these must be OK this time because the PSU was hiccupping - ie working but sensing an overload so shutting down.
Soon enough, multimeter testing showed two small power mosfets on the outpu t side of the SMPS tranny were shorted while two others seemed OK. But what the heck are mosfets doing there ?
Well, it's the fist time I have come across seen a *synchronous rectifier* in such an unit. IcePower had certainly gone all out to reduce losses and h eat in this module by using a synchronous bridge.
The particular TO-252 fets here are made by ON, number FDD86110 rated at 10
0V and 8mohms on resistance.Removing the duds cleared all shorts and after fitting some new ones ( deli vered to me by Element14, from their Singapore warehouse ) I gingerly Varia ced the unit up from zero.
No hiccupping this time and it began to run normally - after which it passe d all my usual bench testing, no problems.
FYI:
AC current draw was 120mA at idle, 2.4A at full power (340W rms/4ohms) No PFC and inrush surges were about 25A for a few milliseconds.
There was a residual sine wave signal on the audio output of about 1V at 50
0kHz - no biggie except it makes THD testing a right PITA.I find it odd the way the SMPS failed, just two mosfets in the secondary re ctifier, nothing else. For a brief time, the DC supply to the switching fet s must have been nearly double voltage.
Seems the two rectifier fets failed SHORT instantly and protected the rest of the circuitry - remember the power supply is NOT regulated, just a squar e wave inverter running at 100kHz.
I also feel that supplying an expensive amplifier to a buyer living in a 24
0V country with the AC inlet set to 120V and giving NO warning is criminal. BTW the amp was fitted with a regular IEC inlet which most of the world us es for 240VAC.Any comments?
... Phil