for the sake of reference for future repairs, i had a KA3500 recently going on a full recap and restoration. sound was very poor, unstable and unpleasant.
Once all done it had a random issue of terrible distortion once hot.
Bias went to 0,0000mv (ie no bias at all). Sometimes it was good, sometimes not.
I had a thermal test done on all components while it was doing the issue and quickly found that the P channel output (the light green or blue ones) are defective, the B-E junction become resistive on a curve tracer. HFE falls to near null. Once heated up very much the B-E junction is totaly erratical. I guess it is quite normal for 40+ years transistor....or maybe humidity went it who knows.
I had the poor original 2SD587 replaced by 588 (as well as its matching 618) with e-bay knock offs.
With new caps all around this litle amp sounds very great.
fwiw Q25 is deleted "factory" in mine.
HTH
Once all done it had a random issue of terrible distortion once hot.
Bias went to 0,0000mv (ie no bias at all). Sometimes it was good, sometimes not.
I had a thermal test done on all components while it was doing the issue and quickly found that the P channel output (the light green or blue ones) are defective, the B-E junction become resistive on a curve tracer. HFE falls to near null. Once heated up very much the B-E junction is totaly erratical. I guess it is quite normal for 40+ years transistor....or maybe humidity went it who knows.
I had the poor original 2SD587 replaced by 588 (as well as its matching 618) with e-bay knock offs.
With new caps all around this litle amp sounds very great.
fwiw Q25 is deleted "factory" in mine.
HTH
I repaired one last year - this one produced a load hum at switch on. I replaced all the electrolytic capacitors apart from those in the main power supply and re-made some solder joints that looked suspect. The volume control also needed a squirt with a cleaning agent.
I agree with your sentiments about the outcome in terms of the sound.
I agree with your sentiments about the outcome in terms of the sound.
Connect a dummy resistor and play it at full power, so that when your fake transistors blow up and presents rail voltage to the output, it happens with dummy resistors rather than your speakers. This certainly isn't safe to use.
The main problem was a malfunction in delay circuit to prevent turn on or turn off noises. This was of a type which did not rely on a relay.
The amplifier worked normally once all the capacitors were charged so the temporary dc problem at switch on disappeared. There is a speaker switch that could have been used to skip this - not something to rely on in the long term.
The amplifier belonged to my brother and it was not fit for further use until the fault had been found and rectified. It was safe to use head phones which was a convenient means of monitoring the noise during that process.
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