STK0039 to replace STK0040 ??

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The 0039 is a quasi-complementary design (which is OK), but you'll be running it at its design maximum in the SA-400. I'd look for the proper part.
 
It seems hard to find the 0040 down here... and I should get 2, to replace the 0039 fitted now and the blown 0040.. It's rumning at +-38V.
 
Well, doing some phone calls, located STK-0040 and 0050 replacements... I'll get the one that looks better, knowing they are chinese copies...
 
They are fake, 100% guaranteed. Throw the entire receiver in the garbage, it's not worth the effort - you will never find real, functioning parts.
 
I bought 2 fake STK-0040 for $14. I'll install them and give them a try. As I posted before, the unit is working for some years now, with a fake STK-0039 in one channel, and the fake one survived the original 0040.

I don't think the unit doesn't worth any effort. Those amps sell for good money down here. A working unit, in mint cosmetic condition as this one, can be sold easily for $300 or 400. The only one of this model published at a local auction site is asking $500.

It belongs to an old lady who want to hear to her music in her beloved amplifier, that's another reason to fix it. And it looks very nice, it's a nice design.
 
Hopefully you can get the repros working ok. It's doubtful the old lady will be pushing the limits of the amplifier anyway so I bet it will be fine.
 
I'll check it with a dummy load at 1/2 power for some hours before sending it back. Perhaps 12 or 24 Hs while monitoring tenperature.
 
How did the fakes sound? I assume all the stk0039 $14 a pr. on the bay from HK are fakes. Do they sound ok or are they junk?
 
Not yet installed... Perhaps tomorrow... I hope everything will be fine. I've used "replacement" STKs several times, and until now, they are working...
 
ZOOM said:
They are fake, 100% guaranteed. Throw the entire receiver in the garbage, it's not worth the effort - you will never find real, functioning parts.
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There is a high percentage probability that the replacements you acquire will not be original Sanyo devices.

I have a handful of SX-780s running on these replacement modules. Are they originals? No, they are reproductions made with completely different dies and layouts.

Are they as robust as the originals? Probably not even close. Then again, considering that the original Sanyos would be killed by a single errant strand of speaker wire for a few milliseconds, they would not really fit my definition of "robust" either.

Do they work acceptably? Well. that depends upon the definition of "acceptable" but they do work without noticeable crossover distortion (poor bias), without sending the unit into protection, and without having to reinvent the wheel. Running one pair of good 8Ω speakers they seem to do just fine, considering that the original amplifier design had limitations anyway.

If that's good enough to save a unit from the scrap pile, so be it.
 
Well, everything is going fine. No crossover distortion visible in my oscilloscope, I was playing a test tone at 11 Volt RMS measured on a dummy load for some minutes, the heat-sink gets normally warm, so the dummy load.

1KHz Square waves look perfect.

Now it's playing FM at the dummy load, volume knob at 1/2. I don't think the owner will listen that loud for long time...

It has 46 and 48mV DC offset, L and R. I'll replace the differential pair transistors and see if it makes a difference.

I see the differential pair is the double transistor 2SA798, which I usually replace because they tend to fail ...
 
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Update: Didn't have success replacing the 2SA798... I did what I always do, to fit 2 x 2SA992, hfe matched with my meter, but in this unit they don't work. rolled like 10 different transistors, all in the same hfe range, and DC went up, never lower than the 48mV of the original pair. The "replacements" have lower gain, all in the 200-230 range, and the originals are around 400. Perhaps that's the problem. I think I'll leave it alone by now.
 
redk9258 said:
Maybe something else is causing the offset? Resistor or something changed value?
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I don't think so. Replacing again with the original transistor, the offset comes back to 48mV. Both channels match, 46 and 48mV, L and R. I think it's the circuit topology. I'll check the resistors in that part of the circuit anyway.

I see this circuit has another transistor biased by the differential pair (Q603), perhaps the replacement with lower gain can't bias it properly.

Tried swapping the original differential pair (you can flip it to swap the transistors), and the offset stays the same, 48mV.

Another detail, the offset is always positive, regardless of the transistors I fit or swap.

w1jim said:
Did you try cutting (or reconnecting) jumper wires A & B?
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What jumpers are you referring to ?
 
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elnaldo said:
Update: Didn't have success replacing the 2SA798... I did what I always do, to fit 2 x 2SA992, hfe matched with my meter, but in this unit they don't work. rolled like 10 different transistors, all in the same hfe range, and DC went up, never lower than the 48mV of the original pair. The "replacements" have lower gain, all in the 200-230 range, and the originals are around 400. Perhaps that's the problem. I think I'll leave it alone by now.
Click to expand...

Since the 2SA798s are 50V devices, it should be safe to match and install a thermally-bonded pair of BC560C devices in their place. They have a much higher current gain factor.
 
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