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Pioneer receiver model SX636 amplifier bias question

I replaced two noisey intermittent preamp transistors in one channel of this unit with NTE324's. These are supposed to be a direct replacement for the originals. The unit seems to work well. All transistors in both channels under quiescent conditions seem to run cool enough and there is no noticeable distortion when driving the amps at sufficient volume. However I have noticed that after a few minutes at sustained loud volume the complementary output transistors on the repaired channel seem to be running noticeably warmer than the other. They are nowhere near "alarmingly" warm but just enough to let me know that there is a difference. I have taken some comparative voltage measurements between both channels and as expected have noted some differences. I have attempted to tweak the bias pot on the repaired channel to even things out between the two channels but it seemes like for instance if I adjust the pot so that the emitter on one of the outputs matches the same transistor on the the other channel it throws sonmething else off. Should I just go for 0volts at the midpoint of the two outputs at the big inductor or might there be more to it than that? Without the service manual I don't know where to make the measuremenrt or what to adjust it for. This appears to be a direct coupled amp with 4A fuses to the speakers and without a capacitor on the output. I've seen amplifiers of this type destroy speakers before so I don't want to make a mistake with this. Can anyone advise me further. Thanks for any assistance. Lenny Stein, Barlen Electronics.

Reply to
captainvideo462002
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Measure across the emitter resistors with no load. Adjust for the same as the other channel. If the heating difference re-appears with a load, there's a dc offset, maybe a small one but it can make a difference in effective bias.

Mark Z.

Reply to
Mark D. Zacharias

Wow, an SX636...long time since I've seen one of those!

I may be wrong but I seem to remember that the audio output sections of these old receivers actually had *two* adjustments per channel: bias and DC offset. The DC offset seems to be what you are talking about, not the bias.

The DC offset is supposed to give you 0 volts at the output, best measured with no audio signal of course. So you hook a voltmeter to the output with no signal, and adjust for 0 volts. You'll know you're adjusting the right pot if you can swing the voltage to either side of zero (+ or -).

The bias is something different: it sets the quiescent current flow through the output transistors (the amount of current flowing through the transistors when there's no signal). It is there in order for you to get the transistors working on the most linear part of their amplification curve (thereby giving you the least distortion.) To do that, the bias is actually turning the transistors slightly on (causing them to conduct).

You stated you've got one side running warmer when you play the unit, and you're adjusting bias to fix it. But if you want to see if bias is really your problem, what you want to do is turn the unit on from cold with *no* signal, and just let it "idle" for an hour or so. Then check the temps. If one side is hotter than the other, that is a good indication of mismatched bias levels.

Usually there are test points you attach a voltmeter to and adjust the bias for a specified reading. Can't do that without a manual, though. One thing we noticed: since we always had these things on variacs when working on them, and since the variacs had current meters, we could see that almost always, when adjusting the bias to spec, the adjustment put you right on the "knee", where fine adjustments of the bias pot caused the current being drawn to jump up dramatically. Given that you don't have the spec, if you had a way to watch the current being drawn, that'd probably put you in the ballpark.

Cheers.

Reply to
Mr. Land

Hi Lenny,

The service manual says:

Hook up an 8 ohm dummy load to "A" speakers. Set function switch to AUX with no input. Let the set warm up for 10 minutes. Connect dc voltmeter to pins

21 and 17. Adjust VR1 for 20mv. For the other channel, use pins 8 and 11 and adjust VR2.

Hope this helps.

--
David Farber
David Farber's Service Center
L.A., CA
Reply to
David Farber

Thanks very much for the help. It went out of here the other day. best regards, Lenny Stein.

Reply to
captainvideo462002

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