Substituting the SFC6120 double transistor in the Tandberg TR-1055

Tandberg TR-1055

Tandberg TR-1055

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.

The SFX6120 double transistor

The SFC6120 double transistor in the negative feedback circuit

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.

Transistor 1 installed

Transistor 1 installed

Both replacement transistors installed

Both replacement transistors installed

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.

R712, the blue resistor, determines the gain of the amplifier module

R712, the blue resistor, determines the gain of the amplifier module

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!

The finished pair of output modules

The finished pair of output modules. Note the thermal goop on the KSC1845s. The two modules have been (partially) recapped at different occasions.

An untouched Tandberg TR-1055 output module

An untouched Tandberg TR-1055 output module (For reference)

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4 thoughts on “Substituting the SFC6120 double transistor in the Tandberg TR-1055

  1. Hello, an unrelated question. What king of tool do you use to measure ESR? Please fill me in with details, I would love to have one of my own. Feel free to delete this after you answer. Sincerely, G.

    • I use a prototype ESR/L/C/R meter that a friend of mine has designed and produced. It is not available for sale yet as it is unfinished and unreliable. It might be released as an open-source hardware kit some time in the future, but don’t hold your breath.

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