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Big V-FET thread

I got the thing pretty much done, I installed the 5 way output posts and they look great. I set the bias at 33 mV as the updates recommend.. I fired it up and it sounded fab, for almost an hour, then I heard clicking and some protection relay shut it down. I am taking it to a tech for final tweaking- but at least I didn't have to pay to get all of the caps installed. Remember, I only had this on for a minute when I first got it, so this could have been an existing issue.
 

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I need some help now! The amp worked great- but the tech said there was some "scratchyness" whatever that is. It worked and sounded great for over a week, then one side would start dropping out. Then it shut down on that side. The tech said he didn't think it would impact the VFET's... but sure enough they were blown on one side. I ordered some from China and this is a quote- " just put in the new transistors after checking them on the bench to see if they were good.
I ran the power up slowly and the unit woke up without excess current draw and the relay clicked in.
I let it sit for about a minute and I reached for my cords to start testing for output and the unit jumped to 2 amps of current draw and took out one of the output transistors.
At this point I am not sure what to do.
Please advise."
So I am wondering if any of my blog buddies have encountered anything like this or can give any advice?
 
Hope this works out well. I'd be suspicious of any V-FETS sourced out of China - hopefully the ones you sourced are genuine, and that you are able to find a replacement for the one that was destroyed by the tech.

I'm curious which 5 way binding posts you used?
 
Wire Nut said:
I ran the power up slowly and the unit woke up
Click to expand...

If by this your tech is saying that he brought the AC mains up slowly with a variac that would be a bad thing. V-FETs are depletion mode devices and as such it's important that they are biased at pinched off on power up meaning that the bias supply needs to be up and running before the B+/B- otherwise they will conduct themselves to death. From your description it sounds like this might be the case ... I'm hoping not.
 
I contacted J.W. at Rain City Audio, he is the tech in a couple of the links on the first page. He was pretty backed up so I let this project slide for a while.
Then along came an Ebay auction for a 4650 & the 4950 tuner together- I could not resits as I like matching components. I won the two pieces and it turned out all 4 of the V-FET's worked. I contacted J.W. again about taking out the V-FET's and sending them to him to repair my modded unit. He said just pull the output board and send that along with my unit and he would swap them after upgrading the board. I figured I could do that... and did. I have the board all done.
I did put in the recommended 3.3 resistors this time (I didn't find out about them until a few weeks ago). I also scored some Black Gate 100 mV caps for this one.
I am about to set the bias... does anyone have any advice of anything I should do before I set the bias and test the unit?
 
Wire Nut said:
Big Sony V-FET thread

4. This one shows how a tech used the old output caps to mount the board that is on top of it: http://blog.kf7lze.net/2014/11/11/sony-ta-4650-vfet-stereo-amplifier-inspection-and-upgrade/
Click to expand...

DO NOT _EVER_ DO THIS!!!
If this tech worked for me, he'd not only get fired but his palms glued onto his face in front of his eyes so that he can't even see let alone do something that stupid.
If old caps are to be retained for mechanical purposes, the proper way is to carefully open them up and hollow out the old contents then put the new ones in and make proper connections. 9 out of 10 good techs have no notion of VFET amps and should best not work on them, let alone any 'tech' like this one.

Also: DO NOT TRUST CHINA SOURCED VFETs unless you know how to measure them, and a test rig is not as easy to set up as one would think. The only definitive way is using a curve tracer. Several Shenzen sources can supply what they claim are VFETs and in most cases they are but in fact they come from only one or two sources. Some are recycled (which begs the question where they get the source material for recycling!) but all are re-marked and usually with whatever will sell. Neither the type or rank may correspond to what is actually in there. An experienced eye can spot the proper TO-3 case as Sony made them but then use a bit of alcohol on the markings and most will come off. This is because they were simply ink jet printed on the top. Most of the 1st gen VFET amps are not at all forgiving of wrong VFET and rank mix usage, and second gen (F7, N7) will have far reduced performance. The only VFETs that can be trusted on eBay you can get from Acronman Electronics however these are J28/K82 which are not directly usable in gen 1 amps.
 
Refurbing a Wega V4810 (TA-5650), have done all the caps on the power amp card plus the big smoothing caps. Also canged the values acc. to the service manual update. Strangely enough all components except the 4,3k resistors had the updated values and seemed to be original components. Even the resistor on regulator board was the right value.
Should I keep the 4,3k or change them back to the old value, 2,2k?
The bias was set to about 35 mV, instead of the higher 50 mV in the service manual.
How will that affect the sound?
It runs cooler but what abut the distorsion?
What worries me more is that it seems to be sensitive to radio signals. When the lid is off there is a noise like a mobile phone receives a call. When I put the lid on it disappears, but a faint high freq sound is still present. Seems it has smth to do with the volume control or in tis vicinity. Even if I move my hand closer to the pot, the noise decreases. Earth contact also improves.
Is it sensitive to Wi-FI signals? No mobile in the room. First I get this with a vintage amp.
Cheers
 
Guys,
I am very fortunate that a friend of a friend that lives here in the Twin Cities is the actual guy that built the anniversary VFET amps for Nelson Pass. I took my 4650 over to his place and he found an under rated cap and two cracked pieces... put it together, set the calibrations and it sounds sublime!
To give you an idea, I put it in the place of a Accuphase 202 with 100 wpc (the 202 has been completely redone with quality caps). The Sony had much more dynamic range and made my new Harbeth Compact 7's sound like they should.
I am a very happy guy indeed.
 
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I am helping a friend to make a 4650 he has had for years come to life, he just wants it simple. He has only one channel working, I have replaced the death diodes and the 4 resistors...but need a 2SK60 and a 2SJ18.
Does anyone have a matched pair they can sell us? If so please PM me
His was an odd fish, the wires for plugs 16 & 17 were hardwired??? Also his diodes had rubber standoffs; wonder it that is newer or older?

Thanks in advance.
 
Can someone explain how you remove the vfets from the 4650 and 5650 after you remove the bottom cover? When you remove the two screws can they then be pried off or is there something on the inside securing them that also needs to be loosened? What's holding the vfets in place besides the two screws? (Yes, I have read through every thread on the Sony VFETs but haven't seen this specifically addressed.)
 
The transistors should pull off when the screws are removed. The socket may be tight, or some goon may have soldered the legs to the socket.
 
Yes, from the bottom there is a screw on either side of the VFET, just remove those & the part should lift out. I think a better way is to do it from the top by removing the whole board so you can easily clean everything.
 
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