SA-7500 2sk34

kingmace

New Member
I got myself a beautifull SA-7500 amplifier, still in good working condition.

I've cleaned all the pots and switches ultrasonicly. Also I've recapped all the boards, replaced the 2SC1451 transitors with the KSC3503, the 2SA725/2SA726 with KSA992 and the 2SC1312/2SC1313 with KSC1845 because I've read they can become trouble.

But now I found two 2SK34 n-fets (Q1 and Q2) on the Control Amplifier Board.
According to the Jaeger Elektronik table the 2SK34 is listed as (LF,Uni, 30V, Idss=0.3...12mA, Up=0.3...6V) and a possible replacement is BF245 (NF/HF...VHF, 30V, Idss>2mA, Up<8V) or 2SK381 (Uni,50V, Idss>0.3mA, Up<6V). But when I search the forums here I find that the 2SK34 is 50V. Is it possible to use the BF245 replacement in this situation (they are easy to get here in Europe)? I can't read schematics so I don't know whats the purpose is of the fet in the Control Amplifier.

n19e.jpg
 
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As far as I am aware, there isn't really a good replacement for the 2SK34. If yours is working, I recommend leaving it. I have done an amp where I replaced the rest of the transistors but kept the 2SK34.
 
Thanks for the advice. I already intended to keep them in place but they have black oxidized legs and I want to have a replacment just in case they go bad. If there is no replacement for them what should I do then when they go bad?
 
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In your case it is used in the amp. The JFET you quoted is a 30V JFET it looks like. I think the original 2SK34 was a 50V one. In cases where it is used in the power supply as a current source, there are other alternatives, such as a current regulator diode or custom built current source.

I am not sure if a 30V JFET would work in this position for you, but my best advice would be to leave it and only begin to investigate if it fails at some point. I don't believe the black tarnish on the 2SK34 leads is known for causing failures. I know on some other transistors has problems with cases that weren't properly sealed around the leads and moisture and corrosion could get up in there, but I think these were ok.

I know you want to have modern replacements for everything in there, but unfortunately there are a lot of custom ICs, transistors, bias diodes, etc, in vintage equipment that there is no modern equivalent.
 
In your case it is used in the amp. The JFET you quoted is a 30V JFET it looks like. I think the original 2SK34 was a 50V one.

I know the manual also mentions the 2SK30 (in the same place) wich is for sure is 50V (Jaeger rates this one also as NF/Uni, ra, 50V, Idss>0.3mA, Up<5V), but I can't find any conformation about the 2SK34 being 50V.

I am not sure if a 30V JFET would work in this position

Someone else has any thoughts about it?

I know you want to have modern replacements for everything in there, but unfortunately there are a lot of custom ICs, transistors, bias diodes, etc, in vintage equipment that there is no modern equivalent.

I know, I've killed a very nice Goldstar GSA-8500 by doing this.
 
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Leave the jfet alone. I do. DON'T sweat it...

They VERY rarely fail in those positions, they are mistreated when used in power supplies.

Sometimes if a 2sa725 that they are paired with fails, it kills them - but that would give you silence...
 
Thanks guys for all the advise. Is there anything else I could (should) do, to keep it playing as long as posible?

Beside the full recap and the transistor swap I've put in a new speaker releais and changed the bias/off-set pots for Bourne multi-turn pots.
 
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That sounds like good maintenance to me. I never do more than that unless there are some metal films that go hi-Z with age/heat. Make sure you deoxit all the switches and you should be good to go.
 
I've had good success replacing the 2SK34 in muting circuits with the BF256B.
Good to know, is this how they are used, in the schematic above?
No, the ckt, as shown, is a jfet/bjt composite amplifier with a AC gain of around 6.
Sub/replacing this jfet,2SK34, would require some ckt design/simulation/testing to come up with a working ckt. 2SK117 is a good candidate, which if you look was used on the SX-1280 to replace the 2SK30AW, as was used in the SX-1250.
Newer parts such as LSK170 probably could be made to work, but not to bother until someone has a failed 2SK34 & can not find an original part.
 
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