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2SC871 (E,F) sub for Sansui AU-101

A

audiogymp

Active Member
Hi

I'm working on an amp with a noise problem. Some helpful threads have helped me identify BC550 as a possible sub for the x-sistors I think are the problem, eg:


I appreciate that some in the US are not fans of the BC series but they're easier to get than alternatives in my part of the world and I've never had a problem with them.

So here's the question - hopefully a simple one for someone out there. The specs for 2SC871 give the hFE as 350min. I've tested 25 BC550's and I can get just enough with hFE between 800-900 and the balance have values above 900. Are the high hFE's likely to be a problem?

Please let mew know if more information is required.
(Schematic: https://www.hifiengine.com/manual_library/sansui/au-101.shtml)

Thanks in advance!
 
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Anything with hFE above say 400 should be fine, I would try to use transistors with similar hFE to the types you are removing spec-wise, as the actual original transistors may have lost some gain over the years. I would avoid using the transistors with extremely high gain as this may cause problems if the circuit was designed to expect ~350 hFE min transistors. Also it goes without saying that when used as 'pairs', transistors should be matched.
audiogymp said:
I appreciate that some in the US are not fans of the BC series but they're easier to get than alternatives in my part of the world
Click to expand...
Interesting, so where is "your part of the World"? :)
 
I just used KSC1845 to replace the 2SC871s in my Sansui 4000.
Seem to work well.
Not available in your part of the world?
 
What @Hyperion says is sage advice to follow. Trying to replace like for like hFE is typically the target. That being said though, often our go to replacements come over or under these figures, and normally there are few mentioned issues here. So as for 900 hFE , yes it's a wee bit high but I'd personally use them in the pre amp stage. However for the phono section, I've read that this may be different and thus important to use correct hFE for correct RIAA equalisation curve, hence a lower hFE in the second position ( I hope my assumption is correct?? )
The BC550B may be a closer match for lower hFE.

BTW, I've measured the 2SC871 as high as 757 hFE.
 
moefuzz said:
Would this be a correct assumption and something to be kept in mind??
Click to expand...

Not certain there, Moe. Did see it mentioned that higher hFE can be of benefit to the next stage.

Looking at the data sheet on BC550C, mentions that max hFE is 800. Could just be yours are high or that your tester is perhaps reading them high.
 
Thanks for all the helpful replies.

If I had a few things I need, I'd just put together a Mouser order - sixty bucks and postage is free.
Otherwise the postage for domestic or international options is out of proportion to the cost of the parts needed (eBay parts from China aside, but that's another debate). So I attempted to source parts from a local retail chain - BC550; no letter suffix; min/max hFE 100/800. Bought 25 expecting I could select about 6 in the vicinity of 500. But nearly all the hFE are over 800 and a few are over 1000. My component tester could be to blame but it's generally given me values around what I expect.

The noise at issue is coming through phono and microphone (which uses the gain from the phono circuit and, as far as I can tell, also the equalisation). There is no noise on the line level input, so the key targets for replacement are in the phono preamp (but I might do the two line stage ones for completeness).

My part of the world is Australia - not as well serviced as many places for parts. But I've read a similar comment about availability of BC series vs alternatives from a Swede.

Cheers
 
audiogymp said:
My part of the world is Australia - not as well serviced as many places for parts.
Click to expand...

G'day, there's a few of us Aussies that visit the Sansui forums.
I've had good luck with orders from Mouser, Digikey and Element14.
Now what we seem to be noticing is the versatile small signals are drying up and what's left are in the lower gain codes, I used BC549C instead of BC550C because at the time I couldn't order the latter, so there are ways around it.
Perhaps pull the mentioned 871's and measure them on your tester, see how far apart they read.
 
bubhardy said:
So, all in all, the KSC1845, or the KSC1815, should be suitable for replacing 2SC781.
Or BC549.
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Yes and no, you would almost certainly need the BC549B/C, as Ben says, to be sure of getting enough gain, KSC1845 would be OK but the same applies. And useful though they are I haven't seen high gain KSC1815's from reputable suppliers, so that's probably a 'no' for them, but they are useful for PSU positions where gain is very often not as important as their other parameters. ;)

I have never seen a 'single' bipolar transistor with a gain above ~800, maybe I should get out more. :D
 
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Hyperion said:
Yes and no, you would almost certainly need the BC549B/C, as Ben says, to be sure of getting enough gain, KSC1845 would be OK but the same applies. And useful though they are I haven't seen high gain KSC1815's from reputable suppliers, so that's probably a 'no' for them, but they are useful for PSU positions where gain is very often not as important as their other parameters. ;)

I have never seen a 'single' bipolar transistor with a gain above ~800, maybe I should get out more. :D
Click to expand...

What is it in the specs for KSC1845 that you think makes it only OK, John.
Looking for transistor substitutions has always been difficult, without much of a frame of reference.
Meeting or bettering Ic, Vceo, Vcbo, and Vebo is fairly straightforward.
But Hfe numbers seem to sometimes be specified differently for different types that are commonly used in similar applications.
I am trying to learn here, about something that I have tried to teach myself about, but I haven’t been successful with yet.
I haven’t had the opportunity to work with discrete transistor circuits used as linear devices much, in the course of my career-ICs are SO dominant.

Not wanting to hijack this thread, so, please forgive me if I’m violating the rules.
I can post these questions in another forum, if more appropriate.
But the substitution issue is important here, I think.
 
bubhardy said:
What is it in the specs for KSC1845 that you think makes it only OK, John.
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I didn't mean to give that impression, they are "really OK" - it's just that I personally find getting KSC1845's with the kind of gains I am seeing from factory fit originals impossible. I tested 2 today from one of my amplifiers - one was hFE 666 :yikes: and the other was 616 - I have a few hundred fairly recently purchased KSC1845 and they all measure from around hFE 330-400, if you know a source for better ones than that I am all ears. ;)

I just noticed Mouser has KSC1845F (300-600 hFE) ;)
 
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I've seen mentioned the MPSA18 for use in audio.
Originally made by Motorola, then Onsemi and now I see Central Semiconductor are producing them.
Data sheet states max hFE 1500 but some googling around I see people find between 800-1000. Not tried them myself, but intersted to, digikey has some in stock.
The ZTX694B is a popular NPN that has gains of between 500-900 from those I have.

End of the day, finding a mid/high hFE small signal amplifier transistor sub (the 2SC871 is brought up often) poses a challenge. Still, it's not impossible if you are willing to expand your search criteria. Examples are, pay a premium for a NOS part if available, or an equivalent NOS part. There's still swaping the pin orientation to fit the board if you're ok with that (insulate a must), such as KSC1845F > BC550C as an example.
I was finding the almost the same readings of as Hyperion with the 1845, mid 300's to low-mid 400's tested with a Peak DCA75.
 
Hyperion said:
I didn't mean to give that impression, they are "really OK" - it's just that I personally find getting KSC1845's with the kind of gains I am seeing from factory fit originals impossible. I tested 2 today from one of my amplifiers - one was hFE 666 :yikes: and the other was 616 - I have a few hundred fairly recently purchased KSC1845 and they all measure from around hFE 330-400, if you know a source for better ones than that I am all ears. ;)

I just noticed Mouser has KSC1845F (300-600 hFE) ;)
Click to expand...

I’ve been buying KSC1845FTAs from Mouser.
The data sheet and graphs show hFE1=550 to 600, but hFE=300-600 for F type devices.
I am confused.
 
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