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Sony TA-N7B Transistors

Thanks Harolda, I have ordered all the Parts and hope to recap the unit by the end of the month and will do the same to my other N7 later on.
 
I have the chance to get another N7 but the owner says that there is no sound. The unit has not been in use for 20 years, could the caps just be dried out?and the Vfets could be ok?
 
The only way to be sure is to get to the amp and pull the VFETs out and test them.
I have attached a Sony service bulletin on how to do that.
As you well know finding replacements is near impossible, finding the matching rank even harder.
If they are good there is an entire procedure to power the amp back up again.
As is well documented everywhere, a variac power-up will kill your V-FETS
If you find the problem, DO NOT power the amp back up without a full understanding of procedure.
There are threads here by Echowars on how it's done.
This holds true for the 8550 and presume it does for the N7 as well.

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rottalpha said:
for second generation V-fet amps too?
Click to expand...

I am not sure, but would assume so!
VFETs run away without the proper bias voltage.
As in deliver full current and fry themselves.
That is why you need the right voltage for the right rank.
Also why you start with a lot of bias and dial down, rather than the other way around.
A engineer/tech could probably explain it better than I.
Best to check the N7 service manual, they do have the same VFETs as the 8550, which is what I am basing this discussion on.
 
Is it ok to replace the Q130 with the C1962 and the Q131 with the A968 (higher power rating than the A835)? Pair matching of transistors at these locations on both channels is necessary?
 
Vinylcafe said:
I am not sure, but would assume so!
VFETs run away without the proper bias voltage.
As in deliver full current and fry themselves.
That is why you need the right voltage for the right rank.
Also why you start with a lot of bias and dial down, rather than the other way around.
A engineer/tech could probably explain it better than I.
Best to check the N7 service manual, they do have the same VFETs as the 8550, which is what I am basing this discussion on.
Click to expand...
yes but not the same bias circuit
 
Hello @ilimzn I have a question about bias current on ta-f7
It seems a bit weird that on N7 bias is set to 60/150 mA for each Vfet and on ta-f7 only 17 ma for each Vfet
I'm right or I'm misundertanding something?
 
Harolda said:
I restored two of those, the only transistors I replaced are the 2SA473/2SC1173 with d44h11/d45h11, as per ilimzn's advise. No other transistors are replaced and they both are dead quiet. First i used "nos" 2sa473/2sc1173 from littlediode. But I wasn't able to adjust the bias, now with the d44/d45 no problem at all. Both are running at 80-85mv.
Click to expand...
how can I adjust the bias and do this measurement? Ah I see it's in the service manual
 
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