Sony TA-N7B VFET Help!! - diyAudio
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Old 16th August 2007, 01:49 AM   #1
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Default Sony TA-N7B VFET Help!!

Have a Sony TA-N7B with three (3) bad VFETs in one channel. The bad 2SK60s are rank JG-25 and I need two (2) of them. The bad 2SJ18 is rank JC-26. Am having trouble (surprise, surprise) finding the correct replacement parts. Thus far I've found 2SK60s with the rank of JG-34 and a 2SJ18 with the rank of KY-54. I believe these rankings are too far from the others to be useful, but would appreciate a better explanation of what will and will not work in replacement part rankings.

In addition, I have located six (6) pairs of Sony's 2SK82/2SJ28 VFETs, all rank KE-33. I am wondering if these larger VFETs from Sony's TA-N88B amp would work in the TA-N7B and what it would take to make them do so.

Any help would be greatly appreciated.

Thanks.
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Old 17th August 2007, 01:48 AM   #2
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Hate to see this drop off the radar without so much as one reply...

Paging ilimzn...where are ya buddy??
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Old 17th August 2007, 08:58 PM   #3
ilimzn is offline ilimzn  Croatia
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Right here!

Not sure if the JG-34 and KY-54 rank VFETs would be a problem - it is imperative that all the 2SJ and 2SK have the same rank between them. Since the TA-N7B works in an auto-cascode arrangement with it's output BJTs, the distribution of heat between the BJT and VFET in a cascode changes a little, but the total remains the same. Since they are all on the same heatsink, you should have no problem. The rank (pinch-off voltage) also influences how close the output can go to the power supply rail, hence influences maximum power, but not greatly. With higher pinch-off voltage, you lose a few W of output power, however, you should have no problem staying well above the declared power output as it is quite conservative. Now, take this with a grain of salt, I don't think anyone has tried it and can be 200% sure, these amps are so rare, but I think you are safe. Perhaps Axel at thevintageknob.org has a list of ranks? I will check...

Regarding the 2SK82/2SJ28, you most certainly CAN NOT use them in a TA-N7B. In fact, they can only be used in the TA-N88 and nowhere else, except perhaps a new design made just for them. The reason for this is that their pinch-off voltage is very high, several times higher than what you need - in fact so high that the auto-cascode arrangement of the TA-N7B would not work at all, because the power supply rails are only a few volts higher. Besides, they are not really built for linear operation.

Now, I hate to toot my own horn, but since I own a TA-N88B, I would be very interested in getting those K82/J28 pairs. If you can offer any help, I would appreciate if you could contact me at ilimzn(at)hotmail(dot)com. Thanks!
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Old 17th August 2007, 10:00 PM   #4
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Hi ilimzn - thanks for the reply. Is the info we needed to get the TA-N7B up and running.

I've sent you an e-mail concerning the 2SK82/2SJ28 devices.
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Old 17th August 2007, 11:29 PM   #5
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So, ilimzn, what you're saying then is that it is only the last number that indicates pinchoff voltage (rank)? I had hoped to find, somewhere, some sort of definition of what the numbers mean. Regardless, you have cleared the air somewhat. Appreciated.
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Old 18th August 2007, 10:15 AM   #6
ilimzn is offline ilimzn  Croatia
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There is some confusion about this. I have a service bulletin from Sony, regarding the TA4650 and 5650, which clearly states that only the number is what signifies the rank. However, in this bulletin it is said that ranks are numbered from 53 to 58, hence it would be impossible to find ones ranked 25, 26, 34.
My TA-N88B service manual shows a picture of a VFET with the marking 2SK.../2SJ... in the first line, and 4 characters on the second line, with only the last number being identified as the rank. This would actually be consistent with the first description, if we take the ranks for the 2SK60 and 2SJ18 to be 3,4,5,6,7 and 8. It would mean that the parts gpdavis found, actually have the same rank for P and n channel (shich is actually rare to find!) and the rank is 4. This is not too far from what he needs, but then all will need to be replaced.
Both the service bulletin and the manual clearly state the ranks are assigned based on Vgs0 = pinch-off voltage.
The TA4650/5650 service bulletin states that all but the two extreme ranks can be used in the amp (4 to 7), and I would expect this is to insure the range of the bias set trimmer can actually reach the desired setting. In the N7B, things are different, the BJT section defines the bias, however, the VFET rank defines current sharing at high currents and at clipping.
The N88B manual (keep in mind this is a VERY different construction) states that up to one rank difference is accepted between the two parallel pairs. Given the way the N7B works, I would go with all VFETs of the same rank (but it may differ between the J and K parts).
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Old 18th August 2007, 10:17 AM   #7
ilimzn is offline ilimzn  Croatia
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Quote:
Originally posted by gpdavis2
Hi ilimzn - thanks for the reply. Is the info we needed to get the TA-N7B up and running.

I've sent you an e-mail concerning the 2SK82/2SJ28 devices.
Thanks very much!
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Old 19th August 2007, 12:26 AM   #8
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Appreciate your input.

Seems like somewhere there ought to be a Sony bulletin that explains it in some detail, but I think we can get by on the basics you've outlined. Thanks!
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Old 19th August 2007, 01:27 PM   #9
GK is offline GK  Australia
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Hey, does any generous soul around here have a web link to the TA-N7Bís schematic or a file to email me. Iím really curious to see this circuit.

Cheers,
Glen
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Old 19th August 2007, 02:03 PM   #10
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Default Hi, G. Littlesmidt !

I have full service manual for these SONY .

Regards zeoN_Rider
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