Hi all,
I'm in the middle of restoring a Sansui AU-101. It had the common problem (for these models) of crackling in the output, so I started by replacing all of the capacitors, both power and pre. I then tested it, and while sound came through it was still crackling.
I then replaced all of the 2SC871 preamp transistors with KSC1845FTA transistors. While I was mucking around with that, I also changed the amplifier to a star ground scheme to help get rid of a tiny bit of hum that was coming through.
However, now there is no sound on either channel. I have tested all my solder joints and they seem to have contact with the next component, so I think they are all ok.
At this point, I could use some pointers on where to look to troubleshoot the amp. I have it set to auxilliary input, which seems to bypass the first stage of the preamp. I've verified that the transistors are in the right position (I think the pinouts on the older and newer models are the same).
Unfortunately the service manual doesn't seem to have voltages to check at certain points, so I can't tell if there are any loose connections.
The service manual. (HiFiEngine has a much better version of the schematic than the 'free' one that's floating about elsewhere on the internet.
If anyone has any pointers, I would be grateful. Thanks!
I'm in the middle of restoring a Sansui AU-101. It had the common problem (for these models) of crackling in the output, so I started by replacing all of the capacitors, both power and pre. I then tested it, and while sound came through it was still crackling.
I then replaced all of the 2SC871 preamp transistors with KSC1845FTA transistors. While I was mucking around with that, I also changed the amplifier to a star ground scheme to help get rid of a tiny bit of hum that was coming through.
However, now there is no sound on either channel. I have tested all my solder joints and they seem to have contact with the next component, so I think they are all ok.
At this point, I could use some pointers on where to look to troubleshoot the amp. I have it set to auxilliary input, which seems to bypass the first stage of the preamp. I've verified that the transistors are in the right position (I think the pinouts on the older and newer models are the same).
Unfortunately the service manual doesn't seem to have voltages to check at certain points, so I can't tell if there are any loose connections.
The service manual. (HiFiEngine has a much better version of the schematic than the 'free' one that's floating about elsewhere on the internet.
If anyone has any pointers, I would be grateful. Thanks!
The 2sc871 are base,collector,emitter from left to right with the flat side facing you and the ksc1845 are emitter,collector,base so you may have them reversed.
You've made 2 alterations at the same time which will be confusing but at least you haven't yet changed the power amplifier transistors. That should still function so give it a direct input signal from another source (with attenuated output, not full blast) to R802 and verify it works.
Obviously, if the amp. doesn't respond to any sort of signal (even a cautious touch with a finger) the problem lies with your star earthing modification. If it's working OK, then your preamp transistor changes were wrong. As I understand, all KSC1845 are shown as Japanese ECB pinout but I have seen some with EBC pinout. Start by checking those parts and all the other transistors for correct connections and operation in-circuit, by measuring the B-E diode voltage of each, whilst powered. You should measure ~ +or - 0.65V from Base to emitter in all cases.
Obviously, if the amp. doesn't respond to any sort of signal (even a cautious touch with a finger) the problem lies with your star earthing modification. If it's working OK, then your preamp transistor changes were wrong. As I understand, all KSC1845 are shown as Japanese ECB pinout but I have seen some with EBC pinout. Start by checking those parts and all the other transistors for correct connections and operation in-circuit, by measuring the B-E diode voltage of each, whilst powered. You should measure ~ +or - 0.65V from Base to emitter in all cases.
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Thanks for your responses.
I'm fairly certain the star ground scheme works.
The best information I could find was that the 2SC871 and the KSC1845FTA were equivalent pinouts.
2SC871
KSC1845FTA
It could be that my spacial reasoning is crap, but I think these both show ECB connections. I will double check the voltages and report back.
I'm fairly certain the star ground scheme works.
The best information I could find was that the 2SC871 and the KSC1845FTA were equivalent pinouts.
2SC871
KSC1845FTA
It could be that my spacial reasoning is crap, but I think these both show ECB connections. I will double check the voltages and report back.
Reversed transistors -- that was the problem! Flipped them around and everything worked fine.
Thanks again.
Thanks again.
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