SONY TA-4650 modification

TheBasilisk

TheBasilisk

Member
Hey all,
Have recently sold a 4650 which has had the output devices changed from the V-fets to MJ802 / MJ4502 transistors, and was working nicely with this mod (obviously without the V-fet sound, but still a nice sounding amp). It was suggested that I should post here as some might be interested that such a mod is possible. Unfortunately I don't know what else was done in the modification process, other than each transistor had three diodes in series connected between the base and emitter (I assume that's a mod as my 5650 doesn't have them).
I also have two NOS V-fets sitting here - does anyone know the correct way to test them with a DMM? I assume they're OK but would be nice to be sure... I can't try them in the 5650 as all eight are dead in that one!!
Cheers,
Sam.
 
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Hi Echowars,
Thanks kindly for that info. My V-fets check out OK as per instructions except that the resistance between drain and source measures only a few ohms, instead of 100+. Do you know if this is normal for a V-fet? The diode junction tests to the gate are fine, and results are quite different from the dead ones in my 5650.
Regards,
Sam.
 
Hi Sam,

S-D resistance -as some of my spares are- is in fact as low as 1-3 ohm if not short. There may be some other issues about vfet replacement on service manuals for specific models to ask. You can use diod measurement section of VOM.

attached is cropped bulletin 62

mesut
 

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Yes indeed...the D-S resistance of the high power devices is much much lower than what the doc I uploaded speaks of...it was written for small signal devices, not power devices.

Sorry, I should have mentioned that.
 
EchoWars said:
Here's a JFET testing procedure (VFET=JFET).
Click to expand...

I need to test a dual gate JFET 3SK45, is this procedure still valid? I get the ohm reading for the D/S, but the second part of the test (diode test) fails: it always reads open. I tried 2 transistors and both show the same results. Are they both toast?
 
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Thank you for posting the video. I tested a few other FET I had around following the video, but without success. The tech said that the multimeter voltage may not be enough to put the transistor in conduction mode, so probably the ones I tried to test (2SK23, 2N3819) require more voltage than the one provided by the multimeter. I will build a complete testing circuit on a prototyping board, it seems to be the only way for me to properly build a good relationship with those transistors :)
 
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