Part Compatibility & Amplifier Repair

Thread Starter


Joined Jun 17, 2004
OK, I'm not a genious with electronics, but I'm darn good with a soldering iron & have repaired more things than I would have expected. However, some things still elude me. I was wondering, I have an amplifier that uses IRF540 transistors & they are blown. I was surfing ebay & saw that there were some IRF540N transistors for sale at a very good price & was wondering if these were interchangable & how I would know if they were? Or for that matter if I can use a completely different one or one with higher power capacity?

Also on a different note, I replaced some blown transistors on a different amp that I have & now when I turn it on, it gets really hot real quick (seconds). Of course it still doesn't put out any sound, where would I look next for the fault? I'm sure that it would help to do all sorts of tests with my multimeter & and osilloscope, but I really don't know if it's worth spending hours troubleshooting this thing.



Joined Jan 22, 2004

let's tackle first the IRF540 issue. there are several manufacturers of this 540 series. IRF & fairchild has an Rds - 0.077ohms 28A 100v while ST has an Rds - 0.055ohms 22A 100v and SGS-Thompson has an Rds - 0.065 & 0.050 30A the Rds is your reference for proper biasing of this device.

another parameter is the 540 has a transconductance of 8.7S

the question is, whose is the manufacturer of your blown power mosfet.

the IRF540N is rated at 100v Rds - 44mohms Id - 33A gfs - 21

personally i would only replace that with the exact spec of the original. likewise replace also the complimentary pair of that device. if this one has blown the complimentary mosfet is also affected.

there's no point in replacing that with a higher capacity, you are only wasting the mosfet capability by still having the same supply rail and biases.

as to your other problem of fast heating output transistors. these are some possible areas to look at:
1. output transistor emitter resistor has change value
2. driver transistor base resistor has shorted or change value
3. check the voltage amplifier stage trimmer resistor
4. your output transistor is probably oscillating, check the miller caps.

that will suffice for the moment. :rolleyes: