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Pioneer SA-506: how realiable is my Xref japanese transistor book ?

david winter

david winter

Classical music lover.
Hi,

I student brought his Pioneer SA-506 amp with one fried channel. All transistors after the dual 2SA798 are either shorted or resistive.
The problem is, several transistors are not that easy to find, he has not much money and he wants it repaired for sentimental reasons.

I use a japanese book called "The Transistor Manual with Cross Reference", edition 2008/2009 which I bought from ACME. Everything else is in japanese characters. The first section of the book gives the characteristics, the second section gives equivalents by manufacturer, just like if a same transistor had a different reference for each manufacturer (please correct me if I'm wrong on this).

I need the following originals and don't know whether the cross refs are really 100% compatible:

2SC1438B = 2SC2229, 2SC1890A
2SA850D = 2SA817A, 2SB1238
2SC1919G = 2SC2240, 2SC1740S
2SC1735D = 2SC1627
Output = in stock since ordered during past repair on another Pioneer

Any suggestions ?

Thanks !
 
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I checked Fairchild and ONSEMI against your originals. Only came up with a xref for the 2sc1438 which Fairchild xref's to a KSC1845. Good transistor.

The 2sc2240 is also a good one. BDENT.com has them. Unfortuneatly they are discontinued @ mouser and I've heard Digikey.

Try www.alltransistors.com It'll give you xref's for most everything, even a lot of germaniums.

Larry
 
Thanks for the links, very interesting.

My main problem is that I still don't really know how to choose an equivalent transistor.
When I check the differences, I never know what should be close to the original transistor and what can differ.
As the two sites that you mention give me different subs, I wondered if there is a typical method to choose an equivalent transistor.
Should I rely on the gain and power output as long as the basic voltages are same or higher ? What about the internal C-E capacitance ?

Thanks,

David
 
Ok, here's the upate.

I found a last pair of 2SC1438 from a french supplier.
A quite well commended schematic of the 506 is found here (although in french).

What about the BC640 / BC639 instead of the 2SA850D / 2SC1735D ?
What would you recommend for the 2SC1919 ?


Thanks,

David
 
david winter said:
What about the BC640 / BC639 instead of the 2SA850D / 2SC1735D ?
Click to expand...

Close enough. By the way you can use the same BC639 to replace Q205 and Q206. It is very common in audio amplifiers to use the same transistor for the voltage gain stage (pre-driver), as one of the driver transistors.

david winter said:
What would you recommend for the 2SC1919 ?
Click to expand...

Any small signal transistor that you have on hand will do the job. Ideally that transistor should have the same Vbe drop as the diode D201 (or D202). One good way to accomplish this is to replace the diode by a "diode connected transistor" (B and C connected together), and use the same transistor on both side of this current mirror circuit.
 
Thanks.

Still a few questions:
- I noticed that R240 is not a 4.7R as on the schematic, but 1.2R. What's the difference ? One was burnt so I ordered a couple along with other parts that I needed.

- I already found a pair of 2SC1438 but if I put a BC639 instead, will I need any changes around ?

- Finally, I have seen that the BC639/BC640 exist in many versions. The originals have a lower gain than the original 2SA/2SC but some others with a suffix have a higher gain, sometimes in the range of the 2SA/2SC. Should I care about this or just use the cheaper and simple version ?


Thanks,

David
 
As you see on the schematic, there is no real bias adjustment on this ampp, like many Pioneer amplifiers, by the way. Probably that there was too much bias with the 4R7 resistor and someone replaced it by a 1R2. In principle this resistor should never burn because the is a very small current through it, of the order of 7mA. Check the varistor D206 and diode D204.

Try to get the BC639/BC640 with the highest gain. I saw that they usually have a lower gain than the original transistor.

No need to make any changes if you replace the 2SC1438 by the BC639
 
Ok I changed all transistors except the 2SC1919 and 2SA798 and guess what ? One of the two 2SA798 is bad and makes the speaker voltage raise 300mV. I found it by swapping both and see the same volage on the other channel.
The 2SC1919 are both ok.
I replaced those 798 by a pair of BC557B and the speaker voltage became ok again (almost 0mV on R and around 7mV on L). I paired them and used transistors from the same manufacturing batch.
The only drawback is that you have to cross the B & C pins and isolate one of them, but that's minor IMO.
Now I'll have to test it and see how far the amp goes.
Let me know if you think those BC557B aren't good enough :)
 
Last edited:
Time to close this thread with my thanks to your help.
The amplifier works like a charm.
Just to summarize for those who will have a Pioneer SA-506 to repair, here are the transistor cross references which work without trouble:
- BC640 / BC639 (make sure gain is at least 100; 120 is closer) instead of the 2SA850D / 2SC1735D drivers
- BC639 for the 2SC1438
- A matched pair of BC557B for the 2SA798 (swap B & C pins)

Thanks again !

David
 
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