Hi all,
I've rebuilt an early Sansui AU-101 that was pretty much unusable -- it was affected by the notorious crackle that these things get.
So, by now I've replaced the transistors with modern equivalents and the capacitors with (almost) exact same values -- the only exception being the power supply cap, which I've upped to 2200µf.
Anyway, now it's sounding very good except for one problem -- it's very, very quiet. I can turn it up to 10 and have a normal conversation. The speakers are not terribly efficient (90dB), but prior to the fixes I was able to get it quite loud. I've set the bias to 9.4mV and checked the other voltages -- it looks pretty good to me.
The fact that it's quiet on both channels then suggests that it's a parts / gain problem, so I'm wondering if someone here could check my replacement transistor choices and, if I'm wrong, suggest alternates that may perform better.
2SC1060 -> TIP41C
XA-495BL (NEC A640) -> KSA992FBU
CDC9000 -> KSA1220AYS
CDC8000 -> KSC2690AYS
2SC871 -> KSC1845FTA
Does anything here stand out as being particularly wrong?
I've rebuilt an early Sansui AU-101 that was pretty much unusable -- it was affected by the notorious crackle that these things get.
So, by now I've replaced the transistors with modern equivalents and the capacitors with (almost) exact same values -- the only exception being the power supply cap, which I've upped to 2200µf.
Anyway, now it's sounding very good except for one problem -- it's very, very quiet. I can turn it up to 10 and have a normal conversation. The speakers are not terribly efficient (90dB), but prior to the fixes I was able to get it quite loud. I've set the bias to 9.4mV and checked the other voltages -- it looks pretty good to me.
The fact that it's quiet on both channels then suggests that it's a parts / gain problem, so I'm wondering if someone here could check my replacement transistor choices and, if I'm wrong, suggest alternates that may perform better.
2SC1060 -> TIP41C
XA-495BL (NEC A640) -> KSA992FBU
CDC9000 -> KSA1220AYS
CDC8000 -> KSC2690AYS
2SC871 -> KSC1845FTA
Does anything here stand out as being particularly wrong?
It's unlikely that modern replacement transistors will have lower Hfe and so provide less gain in the amps than the original parts. It would be more likely that you have made a systematic error in fitting them. Have you double-checked the pinouts on the transistors and the polarity of electrolytics? Japanese TO92 pinouts are ECB whilst Fairchild's KSC/KSA types have been known to be supplied in EBC or ECB format. All T0126 types are (from the label side) ECB.
If you look at that other forum that deals mainly with newbie vintage repair issues, you'll often see this point made about Fairchild K prefix semis. Also, their numbering system is not the same as KEC's K prefix numbering.
If you look at that other forum that deals mainly with newbie vintage repair issues, you'll often see this point made about Fairchild K prefix semis. Also, their numbering system is not the same as KEC's K prefix numbering.
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There will be a resistor in series with a capacitor in the feedback circuit that connects to earth. If there is an open circuit the gain would reduce to one. It is strange that would happen with both channels but check anyway - did this amplifier have a previous owner.
Keep the transistors that you removed - these might not have been the problem. Scratchy noises arise if dc gets into a potentiometer and the likely cause is an electrolytic capacitor that is leaky and needs to be replaced. There are contact cleaner sprays you can use to on potentiometer tracks to deal with noise.
If the output is a capacitor coupled the voltage on the inside connection should sit at near to half the rail.
If the output is a capacitor coupled the voltage on the inside connection should sit at near to half the rail.
The schematic of one channel of the power amplifier is here:
http://i245.photobucket.com/albums/gg66/pmd_2008/AU101amp.jpg
I think C807, R815 are what mjona refers to.
http://i245.photobucket.com/albums/gg66/pmd_2008/AU101amp.jpg
I think C807, R815 are what mjona refers to.
Thanks Ian, I had a warning not to open that link so I looked at the diagram on Hi-Fi Engine. C807 is one also C809 - these in the power amplifier. There is also C709 in the line stage and C605 in the phono stage. The series blocking capacitors should also be checked.
Thanks all, I'm just hitting the road for a week, but will check this when I get back.
It was an earlier model with no bias pots, so it's unlikely that was the problem. 🙂 I put in a 200 ohm pot so that I could adjust the bias now (R823 & 824).
The transistors were a large part of the problem. After I got it last autumn I re-capped it as a matter of course but the crackle was still there. The volume control didn't affect the output volume of the crackle, effectively narrowing down the problem to the power supply board. I read that the 'flying saucers' (CDC8000s) that came on these failed often, so I replaced them, but the crackle persisted.
After replacing the drivers and the CDC9000s the crackle disappeared for about a month and the amplifier worked perfectly but then the crackle came back with a 'bang'. That's when I replaced the A640s with 992s this past month, and it cleared it up but also introduced the volume problem.
I'll check for the open circuit. I had one of the 992s in backwards, which caused it not to function at all, so I'm pretty sure I have the correct pin-outs for that now.
Thanks again.
It was an earlier model with no bias pots, so it's unlikely that was the problem. 🙂 I put in a 200 ohm pot so that I could adjust the bias now (R823 & 824).
The transistors were a large part of the problem. After I got it last autumn I re-capped it as a matter of course but the crackle was still there. The volume control didn't affect the output volume of the crackle, effectively narrowing down the problem to the power supply board. I read that the 'flying saucers' (CDC8000s) that came on these failed often, so I replaced them, but the crackle persisted.
After replacing the drivers and the CDC9000s the crackle disappeared for about a month and the amplifier worked perfectly but then the crackle came back with a 'bang'. That's when I replaced the A640s with 992s this past month, and it cleared it up but also introduced the volume problem.
I'll check for the open circuit. I had one of the 992s in backwards, which caused it not to function at all, so I'm pretty sure I have the correct pin-outs for that now.
Thanks again.
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