How to figure out unknown amp wattage???

[Bull]

My uncle gave me a power amp he built from a kit ages ago.

It's 3u 19 inch rack mountable,pretty heavy and has 2 very huge capacitors,4x Hitachi mosfet transistors and a large torridal transformer plus other components.

How can I figure out how many watts in rms it gives???

Per channel there are two metal mosfet transistors.
Hitachi J50 [japan 4J2] protected by 3.5 amp fuse and Hitachi K133 [japan 4H1] protected by 9.5 amp fuse.

So could you tell me how much power in rms per channel and @ what ohms it gives it??

Also the torridal transformer sadly has a tiny piece of the insulation [about a few mm hole] on one of the secondary windings[exposing the copper wire itself]. Can I repair this without replacing the transformer instead???

[amplifierguru]

From memory Bull, the Hitachi MOSFETS were 100W case dissipation and as such the amplifier would be no more than 50 to 60W at the outside for 8 ohms/channel.

You really need to test the power delivered into an 8 ohm load as the high on resistance and power supply limitations could impact the power output considerably.

[Leolabs]

Or use a DC voltmeter and measure the MOSFETs supply voltage

[anatech]

Supply voltage may be inaccurate depending on the driver voltages and layout. Best measure into an 8R load.
-Chris

[Leolabs]

Measuring supply voltage just a quick and dirty trick to know output power,without the need of applying input signal.

[amplifierguru]

That is also a poor indicator because the MOSFET could impose large or small voltage losses depending on the output stage configuration. With 1.7ohm Ron and 8V at current it's significant.

[Bull]

So I recon its btw 100w-200w per channel.



But how can I repair the torridal transformer??? its got a nick in one of the turns of the insulation on the secondary winding exposing a few mm of copper wire.

I was thinking heat shrinking but I don't wanna unwind the whole transformer. Or electrical tape maybe.... Its not high voltage but can still cause a short maybe.

[Zero Cool]

Soundcraftsmen uses the J50/K135 output devices, 2 pairs per channel at they produce 205 watts @ 8 ohms both channels driven etc. 300@ 4 ohms.



Zc

[matth]

If its a kit from the UK with the J50 and SK135 then pennies to a pound of salt its the good old Maplin 150W mosfet modules (see below).
These were optimistically rated at 100W @ 8Ohm and 150W max @ 4Ohms with a max supply of 55-0-55v.

If the transformer windings still have the enamel (spelling?) intact on them then I would not worry to much, just tape it up.

[Bull]

Yes the modules per channel look exactly like that.