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Sansui QR-6500 restoration - What means the red dot on some 2SC871F ?

david winter

david winter

Classical music lover.
Hi,

I recently acquired a nice Sansui QR-6500 receiver.
There are a number of 2SC871 transistors inside (along with some 2SC458) which I intend to replace due to known failures.
However, several have a red painted dot on the top, which I think is a factory selection. Does it mean that they had a minimum gain (and what gain ?), or less noise than others ?
I would like to replace them by accurate equivalents.

Thanks.
 
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Coloured paint dots on transistors usually meant an indication of gain group, as for what the colour of the dot actually means, you'll have to ask Sansui. :)

'Red' might have meant '200 or over' - as the dots usually followed the resistor colour code. :idea:
 
Quick check of the four 2SC871F on the F-1267 board:
TR601: 340
TR602: 334
TR603: 470
TR604: 400

Not very indicative but seems to correspond to the KSC1845F which I usually find around 360-390 range. Some people also reported that the BC550C worked well.

The red dot can't mean a gain of 200min since the 2SC871 (no suffix letter) has a typical gain of 350. The 2SC871F should have an even higher gain, but not shure how much. Possibly 400 ?
 
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Red dot is the highest gain code.
The lowest one I've measured was around 450 and they can be found up to around 750.
I sub with BC550C, ZTX694B or 2SC2240BL.
BC550C are good, bonus is higher gain, frequency and current handling than 1845, pinout IS different!
 
This makes sense, as I took another two on a tone control board (F-1370) and both had an Hfe of 500/525.
I still have 10x NOS Zetex 694B but gains vary: 504 518 527 541 572 589 678 678 716 724. And pinout also differs: EBC vs ECB.
As for the BC550C, I could use it (as I did when replacing dual transistors) but I don't like swapping legs when all three pins are alinged.
I also checked the 2SC2240BL: not cheap if you want originals, but there are some new 2SC2240 (not BL) made by CDIL (Indian manufacturer). Not sure how well they can substitute those 2SC871F...
 
david winter said:
This makes sense, as I took another two on a tone control board (F-1370) and both had an Hfe of 500/525.
I still have 10x NOS Zetex 694B but gains vary: 504 518 527 541 572 589 678 678 716 724. And pinout also differs: EBC vs ECB.
As for the BC550C, I could use it (as I did when replacing dual transistors) but I don't like swapping legs when all three pins are alinged.
I also checked the 2SC2240BL: not cheap if you want originals, but there are some new 2SC2240 (not BL) made by CDIL (Indian manufacturer). Not sure how well they can
Click to expand...

I'm honestly not bothered by swaping legs, so long as they are insulated (normally one leg needs a sleeve) and twisted gently as not to damage a leg then I'm very confident with it.
For an option, you could use the higher close matching pair of ZTX for the fist stage pair followed by the next close matching pair of lower hfe, same with BC550C.
Only use Toshiba 2240's if you can obtain originals, but it's not a cost-effective solution if you do.
 
david winter said:
This makes sense, as I took another two on a tone control board (F-1370) and both had an Hfe of 500/525.
I still have 10x NOS Zetex 694B but gains vary: 504 518 527 541 572 589 678 678 716 724. And pinout also differs: EBC vs ECB.
As for the BC550C, I could use it (as I did when replacing dual transistors) but I don't like swapping legs when all three pins are alinged.
I also checked the 2SC2240BL: not cheap if you want originals, but there are some new 2SC2240 (not BL) made by CDIL (Indian manufacturer). Not sure how well they can substitute those 2SC871F...
Click to expand...
Use a sub with the right pinout, there is no reason not to. Look at the circuit and see what the requirements are of the transistor. There are plenty of alternatives which may not handle as much current, but depending on the circuit, it might be perfectly fine, as it often is, to use something which has a lower current rating.
Bending legs is just rubbish, too easy to short something else and just looks like a hack job for no reason.
 
As discussed many times the regarding the 2SC871, those are the go to subs. Leg twisting has yet to become illegal as far as I'm aware. It's your gear and your ears....
 
AnalogueBen said:
As discussed many times the regarding the 2SC871, those are the go to subs. Leg twisting has yet to become illegal as far as I'm aware. It's your gear and your ears....
Click to expand...
Didn't say it was illegal, no need to get all defensive and start something from nothing. I speak from experience and the fact that I have this sort of gear on my bench for repair and restoration pretty much every day.
All I am saying is do your homework, do the job properly and work to a standard befitting of the brand you're working on.
You do not need to use transistors with different pinouts, it is not necessary, do it properly, don't half arse it......
 
Of course it's not illegal, that was intended as a joke.

We all know a substitute is a substitute, there will be compromise in one or more forms. And there is the need to be creative as OEM parts have mostly vanished, this adaptation is continually being spoken of on AK.

All the subs for the AU-999 which you recommend are of different pinout, is that saying that those are all boggy repairs?

To the OP and others, apologies for going off track. I'm mearly suggesting that pin twisting, if correctly done will not cause adverse effects.
Mind you I'm not an EE, but I have spent countless hours undertaking my own listening test on many available subs and rank them on performance, hence the original recommendation.

Good luck.
 
Thanks for your replies.
So far I removed the KST1845F I initially put on the phono stage and used the higher gain ZTX694 I had.
The problem is, the tone control boards would require more of these.

What I really need to know for future restorations is the gain ranges of those 2SC871(D) and 2SC871R(F). From this I could easily select subs.

As for twisting pins, this is a trivial job but I prefer the easier solution with same pinout or at least something which doesn't require twisting.
 
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It's no trouble offering my suggestions. I had some others in mind to share which are from well know "2N" line. Many of these would meet spec's but are of a different pinout variety too, as well as dificut to source.

I have searched everywhere for the 871 datasheet but to no avail. I have a pair with yellow dot that I think are the lowest gain (can't recall gain code), they were approx 200hfe.
 
After having tested several of those 2SC871 (with and without red dot) I can't rely on the Hfe.
Those without red dot had around 200-250 Hfe. I replaced them with KSC1845F.
I don't mind using 2N subs but if they are hard to come by and/or expensive I'll opt for the ZTX sub (or BC550C).
 
AnalogueBen said:
Of course it's not illegal, that was intended as a joke.

We all know a substitute is a substitute, there will be compromise in one or more forms. And there is the need to be creative as OEM parts have mostly vanished, this adaptation is continually being spoken of on AK.

All the subs for the AU-999 which you recommend are of different pinout, is that saying that those are all boggy repairs?

To the OP and others, apologies for going off track. I'm mearly suggesting that pin twisting, if correctly done will not cause adverse effects.
Mind you I'm not an EE, but I have spent countless hours undertaking my own listening test on many available subs and rank them on performance, hence the original recommendation.

Good luck.
Click to expand...
I'll send all my work to you so its done properly then....;)

No leg twisting on any of the transistors I put in 999's......but if you say so.......
 
david winter said:
After having tested several of those 2SC871 (with and without red dot) I can't rely on the Hfe.
Those without red dot had around 200-250 Hfe. I replaced them with KSC1845F.
I don't mind using 2N subs but if they are hard to come by and/or expensive I'll opt for the ZTX sub (or BC550C).
Click to expand...
What part of the circuit are they in?
The 871 is a high gain but pretty generic transistor, I'd be curious as to what its function is.....
 
There are four (red dot) on the phono stage, and four per channel on the tone control boards (so 16 for both boards, of which half have the red dot).
There are some in the two quadrophonic boards (along with 2SC458), so I don't think they will be mandatory to replace yet. Moreover those boards are difficult to access.
There might be some in the tuner section, but I'll work on this one once the basic stereophonic amplification section is ready.

Kev: I've done several AU-999 in the past. I wish I kept the first one I did some 15 years ago. Full of flying soccer transistors: some open XA-495, some shorted CDC-8001/9001, etc. Back then I didn't know the improvements the preamp sections and in the tone control, nor did I use ZTX-795A yet. But that was fun and it worked well.

As for the QR-6500, I'm curious whether I can simply use KSC1845F in place of those "red dot" 2SC871. Else I'll opt for the ZTX694B which I think is somehow better than the BC550C but I would be wrong. If you have any preference between these two, I'll be happy to know.
 
kevzep said:
I'll send all my work to you so its done properly then....;)

No leg twisting on any of the transistors I put in 999's......but if you say so.......
Click to expand...


Sure, why not, potential room for improvements.

All have different pinout to the original, therefore kind if a double standards quote imo.

Anyway, there are bigger issues in the world to be concerned with rather than care whether a .5mm transistor leg is twisted or not. I've got a few dozen of the 1845/992 that never met my expectations, of which I'll likely sell and dontate the $ to something more worthwhile.
 
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