5000 bias diode placement
Krikke,
It appears that, for whatever reason, the clips that once held the bias diodes onto the complementary pre driver transistors were removed. Surprisingly , this is not necessarily a bad thing. I say that because placing those diodes on those particular transistors was the worst place Sansui could have put them, virtually in the entire receiver.
The complementary pre drivers, which are the TO-5 (small round can) parts next to the TO-66 (bigger metal cans plus flange) NPN drivers operate at almost a constant temperature. They, therefore, are virtually useless for providing thermal compensation, which is the whole point. The best place for them is to be mounted on the output transistor heat sinks. Not too long ago, I rebuilt a 5000 using the original boards (as opposed to dumping them and replacing them with my update boards) specifically so I would have a "stock" 5000 to compare my modified ones to. On that receiver, I mounted the bias diodes on the TO-66 transistors as a compromise, using a bit of thin aluminum sheet bent into a loop and with a hole to run a transistor mounting screw through . The TO-66 drivers do heat up some when you drive the amplifier, and cool down when it is idle. That location appears to work acceptably well, as there have been no thermal runaway issues with that receiver.
I switched to using 1N4000 series rectifiers nearly 40 years ago for replacement bias diodes, 2 of them for each original diode package, and have never found that to be a problem. I tested 1N4148 diodes and found them to be too sensitive. The use of 1N4148, or 1N914, diodes will certainly prevent thermal run away, Unfortunately, as those diodes heat up they actually would cause the outputs to become under biased, which results in elevated crossover distortion.
TG
Krikke,
It appears that, for whatever reason, the clips that once held the bias diodes onto the complementary pre driver transistors were removed. Surprisingly , this is not necessarily a bad thing. I say that because placing those diodes on those particular transistors was the worst place Sansui could have put them, virtually in the entire receiver.
The complementary pre drivers, which are the TO-5 (small round can) parts next to the TO-66 (bigger metal cans plus flange) NPN drivers operate at almost a constant temperature. They, therefore, are virtually useless for providing thermal compensation, which is the whole point. The best place for them is to be mounted on the output transistor heat sinks. Not too long ago, I rebuilt a 5000 using the original boards (as opposed to dumping them and replacing them with my update boards) specifically so I would have a "stock" 5000 to compare my modified ones to. On that receiver, I mounted the bias diodes on the TO-66 transistors as a compromise, using a bit of thin aluminum sheet bent into a loop and with a hole to run a transistor mounting screw through . The TO-66 drivers do heat up some when you drive the amplifier, and cool down when it is idle. That location appears to work acceptably well, as there have been no thermal runaway issues with that receiver.
I switched to using 1N4000 series rectifiers nearly 40 years ago for replacement bias diodes, 2 of them for each original diode package, and have never found that to be a problem. I tested 1N4148 diodes and found them to be too sensitive. The use of 1N4148, or 1N914, diodes will certainly prevent thermal run away, Unfortunately, as those diodes heat up they actually would cause the outputs to become under biased, which results in elevated crossover distortion.
TG
