I am working on this Akai AA-R20 and am getting clipping in both channels, I included a pic for reference (1Khz sine wave). I traced a signal to Q701 and Q702 which are 2SA798, the signal is good up to the first base and shows clipping on the common emitter and both collectors, no signal on the base.
The DC voltages are really close except that base #2 should have 0 volts but has 300mv.
I have seen these 2SA798 dual transistors go bad before but both channels have exactly the same readings and my mind will not accept that the 2SA798s in each channel went bad at the same time.
Any ideas what I should check?
Partial schematic, one channel shown
The DC voltages are really close except that base #2 should have 0 volts but has 300mv.
I have seen these 2SA798 dual transistors go bad before but both channels have exactly the same readings and my mind will not accept that the 2SA798s in each channel went bad at the same time.
Any ideas what I should check?
Partial schematic, one channel shown
Q719 and Q720 lack the REQUIRED current limiting base resistor (~100 Ohms) so I would not be surprised if they are in permanent current limit (shorted) state. Also check D707 and D700 for shorted state.
I am going to start pulling transistors and testing them. Q717 is common to both channels, I will start there and work my way out.
Q709 is a 2SC1627m it looks like 2SC2274 is a reasonable sub, anyone think differently? The voltage ratings are 60 and 50 as opposed to 80 but they should never see more than 35V unless something goes wrong.
2SC2274
2SC2274
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The driver needs to be rated for the total voltage, ie -35 to +35 = 70V. But any ~TO-92 is a bad choice for a driver transistor above about +/-15V. I would change them to TO-126 devices with a small heat sink, good for about 3W+, such as DB139 and DB140, or perhaps mje340, mje350.
Yes, and I just realized that 2SD716 and 2SB586 are only 6A parts! At +/-35V: I don't think so! Are you sure the supply isn't more like +/-20V? 35V into 4 Ohms is 8.75 Amps and you never want to see half the rated current, ie <3A. If you really have a +/-35V supply, then it needs a set of 15A+ parts like 2SC5200/2SA1943 power transistors, and TO-220 drivers like mje15028, mje15029 would be a good idea. Beyond +/-30V, it should actually have two sets of OPs per channel.
Back in 1970's, a lot of stereos died in short order because tube engineers did not understand transistor safe operating area. People used 2N3055 way past their limits, so makers sold them "H" parts which were more like 2N3773's rather than tell them how stupid they were. Today silicon is cheap and skimping just means repeated failures. Does it have a decent heat sink?
Back in 1970's, a lot of stereos died in short order because tube engineers did not understand transistor safe operating area. People used 2N3055 way past their limits, so makers sold them "H" parts which were more like 2N3773's rather than tell them how stupid they were. Today silicon is cheap and skimping just means repeated failures. Does it have a decent heat sink?
The schematic shows 33v on the collectors of the outputs and -33v on the PNP, that is what I am measuring. The voltages are in red, the red in parenthesis is this model.
Yes the heatsink pretty large.