Dual JFET- 2SK100 - Yamaha

[Hifibythekg]

Can anyone tell me how, or direct me to a page, that explains how to test a dual jfet?

I want to test the 2SK100’s I have in my Yamaha C4 and A1 with a DMM.
Apparently, if they’re working well, they’re interchangeable. However, I have issues in one channel in each amp and confirming their functionality would be very helpful.

Any advice on best substitutes in these specific locations happily collected!

 

[Hifibythekg]

Question:
Swapping one or both of the 2SK100’s from one amp to the other.. can this cause damage to either the amp or the Dual JFET?

 

[whoaru99]

Yes, it could. If there is something wrong with the amp any parts you put into it could be damaged.

 

[Hifibythekg]

Thanks Whoru99, much appreciated.
I’m pretty sure that the amp is ‘fine’ - I’ve spent a lot of time cleaning all the switches, reflowing solder etc etc. It sounds great except for a loud hum. Swapping the dual jfets from one channel to the other channel made the hum swap channels.
So it would appear that I have a faulty dual jfet.
But I’d like to see if the amp’s hum disappears completely when this faulty DJ is replaced by a good one. I suppose this prompts the next question:
The C4 and A1 2SK100’s are apparently interchangeable. If there is any mismatch should it just result in bad audio sound rather than damage to something else in the amp?