VFETs replacement, Sony TA-5650

[pietres]

Hi,
My first post... so "hello" to everyone

I have Sony TA-5650 with 2 broken vfets (both 2sj18 in left channel). I'm going to replace them... it's really not easy to find these vfets, so i think it'd be a good idea to ask simple question: HOW should i do it SAFELY?

I thought i can simply replace it, turn the amp on and set bias... but i found Sony bulletin about replacing vfets (http://www.audiokarma.org/forums/sho...highlight=4650) and there's procedure for replacing 1 unit that states a bit differently (point 4)... and what if I have 2 vfets to replace, not 1...?

I'll be grateful if sb who did it before guide me so i can replace them safely...
Thanks

[ilimzn]

5650 uses two pairs of VFETs per channel.
VFETs have a 'rank', which is basically a selection for pinch-off voltage (Vp), which should be the same for both VFETs.
Normally the service manual will state which ranks are applicable for the given amplifier. The best is to have both the J18 and the K60 of the same rank, but since these are so incredibly scarce these days, you might have to contend with the two J18 of one rank, and the two K60 of another - both have to be from the ranks permissible or you will not be able to set the idle current right. In general when one of the set dies, you should think about replacing all on that channel. Fortunately, VFETs are actually quite robust (in general problems elsewhere in the amp, such as the power supply and biassing multi-diodes that go open, are the cause of VFET deaths), so because they are so rare, you can get away with replacing just the ones obviously dead in most cases, but if you can get the parts, don't risk it.
Keep in mind that VFETs measure short from D to S, and diode from D to G and S to G when OK! These are depletion devices so out of circuit they are 'normally on'!
Replacement is generally as follows: remove old ones (they are socketed), replace with new ones making sure to clean the heatsink and reapply the greese and insulator (replace it if any signs of cracking or flaking are present), turn the bias pot to minimum, connect a meter to the appropriate test points, turn on amp, set bias.
The actual procedure is outlined in the manual. Of course, since you are dealing with components that are near unobtainium, you might want to check and double check the rest of the amp circuit, refreshing and replacing anything problematic, before you go replacing the VFETs.

[pietres]

Thanks for your answer

I've tested all vfets accordingly to Sony bulletins and all the info i could find and only two 2sj18 are broken (or rather "seem broken" as i know these tests are not 100% sure). The broken vfets were series HD-56 and I can replace them by two JP-56.

I've already replaced all the diodes, bought new silicon insulators, I'm going to check the voltage mentioned by EchoWars (http://www.audiokarma.org/forums/sho...65#post2166865) and then will try your procedure One question... "turn the bias pot to minimum" means counter-clockwise?