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Luxman L-30 driver transistor

Oldsansui441

Oldsansui441

Super Member
Hi, l have just bought a Luxman L-30 that had a channel out. I found one failed op transistor 2sb531 and it's driver transistor 2sa816.
I was wanting to keep the amp original so am trying to source NOS transistors. I am having trouble finding a Toshiba 2sa816 with a y hfe ranking as original. Would it be a problem to use a o ranking in it's place (this is all l can find in a genuine toshiba). o hfe is 70-140 and y is 120-240. The transistor on diagram is Q206, please disregard coloured sections.
I know ideally l would just replace all with modern replacements but just a pity when others are all o.k. As an aside l had a tip42c in my parts at home that l used just as a test part for interim and the channels are indistinguishable, audibly at least.
Any help appreciated, thanks.
 

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I think it's OK, if that's what you can find and you don't want to replace the pair with modern equivalent parts.

I'd wonder why they failed. .. If they are reaching the end of their lifespan, other of the same type will fail.
 
Part with different sufex may still have same gain as the original, as hfe range overlaps.
 
Thanks for your reply elnaldo, yes that's a good point especially since the previous owner said he replaced them with NOS items a few years ago, maybe they weren't genuine, look to be but l have no way of knowing for sure.
 
Thanks jrc75, l am still trying to get my head around transistor gains and their influence in certain circuit positions. So what you are saying is because the hfe overlaps there is every chance the circuit will still perform as originally designed?
 
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yes, you could have 2 transistors, from different groups, with the same gain (for example, hfe 130 belongs to both groups..). The hfe group actually guarantees a minimum gain.
 
You won't find nos parts, so forget about that, just get modern equivalents. All so called nos transistors are fake, just like the ones you already had, and blew up.
 
Ok thanks for that zoom, so l guess same goes for the outputs, don't try and source one toshiba 2sb531 just replace all four with modern replacements?
 
TIP41 and TIP42 are usually used as output transistors. I think something like BD139-140 are more suitable for drivers. TIPs will work though.

I'd search AK for already suggested replacements for your transistors.
 
Thanks elnaldo for all of your help, you are correct, in fact l think l may have used some in a sanui a-40 as outputs. Come to think of it l may have some bd139/140's here as well, will check datasheets on those.
I have done a search and l think ksa940's have been mentioned but l was wanting a part l can source locally instead of paying overseas postage for $2.00 worth of parts.
 
Just to update, l used Fairchild bd-139's and 140''s as drivers and On semi mj15003's and 004's for the outputs.
Set Bias and dc offset and cleaned all controls, listening to it now, this little thing has great detail, very nice.
Thanks everyone for your help.
Just out of curiosity does anyone know how the bias circuit temperature compensation works on this little guy as is doesn't have any thermally coupled transistors (on heatsink) in bias circuit.
I assume it just relies on the feedback circuit from the output transitors?
 
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Well done! I've used those bd139/ Tip on a Sansui A-40 too, it's posted here at the DIY forum. But the MJ you used are newer and more linear devices. Compare the hfe/current graphs.

I didn't check the schematic, but if there is no other component on the heat sink, you are probably right and the thermal tracking is based on the outputs. ..

Edited: Clearly, the bias transistor is Q204, and I see it's placed on the PCB next to Q205, one of the drivers... I'd expect it to be mounted on the heat-sink... Perhaps the heat generated in that location is enough to make it work...
 
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Thanks elnaldo, l was just looking at your sansui a-40 thread, great resurrection there, great job, the a-40 l fixed was a basket case also.
Yes you are correct it is q204 thats why l was curious as to how it compensates, as you say it must gather enough heat in the area it is in.
 
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