So I have reversed polarity my 100w amplifier from Dutchshop and sadly is not working, it powers up but no amplification. is it repairable and can it be checked just by using a multimeter as I am not technically minded I'm afraid, Does any one know if Dutchshop do a repair service or is it one for the Bin ?

Thanks Mick

I don't know exactly but it's probably shagged the mosfet, which is the black transistor (mrf101an). You could try changing it.

The first thing I would do is check to see if there is a voltage on the gate of the mosfet (with supply connected and dummy load), if not it probably means it is shorted inside. Take the mosfet out and re-check for voltage on the board terminals (it may come back with device removed).

There is a way also to test mosfets with a multimeter on diode test. Check this video...

Schematic is here https://cdn.shopify.com/s/files/1/0719/ ... 1689931102

Only thing to go wrong really is the FET. The voltage regulator is protected by the zener diode so will have survived. I guess you could have killed the 100u cap but given its still on the board and not shot halfway across the room its probably fine too

Thank you for both replies, i will look at it tomorrow when i get off work, thanks again
Mick

Remember - the FET is 90% of the price of that board!

If you're going to replace the FET, be certain the get exactly the same part number (and letters). Those devices are available as "mirror-image" parts, for making paired, balanced amplifiers. In a balanced amp, the two devices are legs towards each other, and bolted to the same heatsink, so you have to get mirrored parts!.

Change the MRF101AN

120Watt VHF Amp & Adjustment of BIAS - 87-108 Mhz -

thank you for the replies, i shall look into costings and see if it would be more cost efficient and buy a new board
Thanks again
mick

100% you've blown the fet. Just replaced with an mrf101an. Not B.