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This nice-looking unit came into the shop with a noisy left channel as well as an intermittent DC offset. After seemingly repairing it by replacing the output transistors, the customer returned it complaining about the same issue arising after a few hours of use. After some further troubleshooting, the problem was found to be a Motorola-branded double transistor in the power amplifier.
Neither the part nor a datasheet for it was anywhere to be found, so a substitute had to be manufactured. I settled for a matched pair of the common KSC1845 to do the job. Gain matching is important, as an unmatched pair will result in a DC offset on the output of the amplifier.
Installing the transistors is easy, as the pin-out for the SFC6120 is printed on the circuit board.
However, my KSC1845s had roughly twice the gain of the SFC6120 (380 vs. 160), which resulted in a considerable increase of the amplifier’s gain. To counteract this, feedback resistor R712 was decreased from 10 kOhm down to 3,6 kOhm.
Since the modification altered the gain of the amplifier, I decided to perform it on both channels to ensure proper matching and guard against future SFC6120 failures. It is important to ensure thermal coupling between the two transistors, in order to guard against DC offset when the amplifier warms up. That’s probably why Tandberg decided to use a double transistor in the first place.
Somewhat unexpectedly, the THD+N of the amplifier decreased from 0,08 % at rated output into 4 Ohm, to a mere 0,033 % after the modification. (Measured with my HP 339A at 1 kHz)
After many hours of heavy load testing into 4 Ohm, I think this unit is ready to go back to the customer again – and hopefully not return!