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SX-880 Output Transistor Replacements (B705A, 2SA1075)

Mightyhal

Mightyhal

New Member
I have a SX-880 with some issues at moment, partially self-inflicted when I let a probe slip while checking the voltages when powered on (building a dim bulb tester now that the horse has left the barn). So, I have a list of components which I'm buying, with the plan to test and replace with a stash of new parts at hand.

The issue that had me inside the unit though, was a sudden dip in output on one channel. That channel's NEC B705A chip had been replaced with a Fairchild 2SA1075 back in the 90s along with several other components (I only remember one servicing, for a lightning strike, but there may have been another). I desoldered the four output transistors and tested them for shorts. Probably due to my slip, the 2SA1075 is shorted.

That leaves me with a couple options at this point--based on what's available from Digikey and Mouser. I'm wondering what you all recommend for replacing these chips.
  1. If I only replace the B705A/2SA1075, Digikey has a couple dozen Sanken 2SA1494-ND (MT-200 mount) available still for about $7. That would of course, make it easier to mount on the heat sink--but should I be replacing the D754A chips as well? I can't find the complementary 2SC3858.

  2. If I replace all four chips, I have see two reasonable options--which would you recommend (or, do you have another suggestion)?
    1. FJL4215/2SA1943 and FJL4315/2SC5200
    2. MJL3281A and MJL1302A (I saw MarkTheFixer recommended these in one thread as a replacement on a SX-1080)
  3. Finally--how many does one need to buy to have a good shot at reasonably matching a pair of output transistors?
I have a bunch of work and testing ahead before I put these in—but since the stocks are rather low, figured I'd better jump if I want to buy the Sanken ones.

Thanks all!
 
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Tracked down the answer to #1:

markthefixer said:
The replacement advice wasn't transmitted to you properly.
Replace all outputs as a set when one of the originals blows up. THIS is to ensure ROUGH matching - same manufacturer / chemistry etc... so Vbe's are closer and loads are shared. Better than with one new and a bunch of originals.
PLUS the replacements will most likely have more power dissipation, higher voltages, different gain and such compared to the originals.

IF the outputs are OK, then it's up to you if you want to replace the outputs with probably more robust devices. You will not gain much in extra power out - that's a function of the power supply and the other circuitry. There will be very little change in distortion either. But the replacements may stand up to overloads better.
.
Click to expand...

So unless I track down 2SC3858, I’ll need to switch to the On/Fairchild chips…
 
Going back to the base assumption, that quote was addressing situations where there are two or more of the SAME polarity outputs in parallel driving (sharing) the SAME load.
In other words TWO NEC B705A IN THE SAME CHANNEL, especially when the bases of those transistors are directly connected to each other!!!

Look at the SX-1280 for PAIRS of outputs, (2 B706 + 2 D746 left and 2 B706 + 2 D746 right) or the SX-1980 for TRIPLETS (3 B706A + 3 D746A left and 3 B706A + 3 D746A right)
Those are situations where multiple replacements of the same transistor number are warranted. These are examples of the bases of transistors that are directly connected to each other!!!


The sx-880 has just 4 outputs, two complementary pair per channel. There are no transistors in parallel in the same channel.
One NEC B705A and one D745A for the left channel and one NEC B705A and one D745A for the right channel.
Thus only the one transistor needs to be replaced, NOT it's complement at the same time (unless the complement is provably damaged).
 
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markthefixer said:
Going back to the base assumption, that quote was addressing situations where there are two or more of the SAME polarity outputs in parallel driving (sharing) the SAME load.
In other words TWO NEC B705A IN THE SAME CHANNEL, especially when the bases of those transistors are directly connected to each other!!!

Look at the SX-1280 for PAIRS of outputs, (2 B706 + 2 D746 left and 2 B706 + 2 D746 right) or the SX-1980 for TRIPLETS (3 B706A + 3 D746A left and 3 B706A + 3 D746A right)
Those are situations where multiple replacements of the same transistor number are warranted. These are examples of the bases of transistors that are directly connected to each other!!!


The sx-880 has just 4 outputs, two complementary pair per channel. There are no transistors in parallel in the same channel.
One NEC B705A and one D745A for the left channel and one NEC B705A and one D745A for the right channel.
Thus only the one transistor needs to be replaced, NOT it's complement at the same time (unless the complement is provably damaged).
Click to expand...

Thank you for the clarification! I missed the fact that the big SX models had the double/triple transistors in parallel, and that definitely makes sense that they'd need to be replaced as a pair/triplet in those models (but that the SX-880 doesn't).

BDent has these available -- my guess would be the SA1215 would be the best fit, but maybe it's just splitting hairs?
  1. 2SA1215 https://www.semicon.sanken-ele.co.jp/sk_content/2sa1215_ds_en.pdf (2SC2921)
  2. 2SA1216 (and G/Y/P versions) https://www.semicon.sanken-ele.co.jp/sk_content/2sa1216_ds_en.pdf (2SC2922 and G/Y/P)
  3. 2SA1295 https://www.semicon.sanken-ele.co.jp/sk_content/2sa1295_ds_en.pdf (2SC3264)
  4. 2SA1494 https://www.semicon.sanken-ele.co.jp/sk_content/2sa1494_ds_en.pdf (2SC3858 not available?)

I'm tempted to buy the complements as well, to have on hand in case I did some amount of damage to the D745As.

Thank you for the help!
 
It would drive me crazy not replacing both transistors-PNP and NPN. Truthfully, it would make me crazy just having one channel different from the other so *I* would replace all of them! That's just *me* though!
 
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