Sansui AU-217 Restoration

[Marcel27]

These are the emitter bypass capacitors for the predriver transistors (TR07-TR10). They isolate the transistor's bias from the music signal; to extend their effectiveness within the audible passband, they should be increased to 470uF from the stock value of 100uF.

Thanks for the information Leestereo. What is the effect of the higher value in the sound image? More bass? More gain/volume? Better details? When it comes to the "signal" path, I like to keep it as close to the original sound image of the AU-217. When it comes to buffers for DC power I have no problem to increase the capacity.

At the moment I'm busy with the replacement of the two 2SA798's. I have bought 20 trans and the next list is the Hfe measurement. I am curious to know how many millivolts the output has to process when there is a difference of 1 to 2 Hfe points in the differential gain. Hfe measured with UNI-T UT603.

 

[Marcel27]

2SA798 change done! The output offset was about 0 mV. It changed to +/- 3~4 mV for both channels with the individual but matched trans. In the old situation with the A798 there was a little distortion with heavy bass drums Glauco di Mambro - Me Sentu Lentu (PAAX tulum remix) and with the new trans also, but this is due to input overload. AU-217 AUX input sensitivity is 150 mV. I cannot hear any difference between old and new situation.

 

[Marcel27]

2SA798 change done! The output offset was about 0 mV. It changed to +/- 3~4 mV for both channels with the individual but matched trans. In the old situation with the A798 there was a little distortion with heavy bass drums Glauco di Mambro - Me Sentu Lentu (PAAX tulum remix) and with the new trans also, but this is due to input overload. AU-217 AUX input sensitivity is 150 mV. I cannot hear any difference between old and new situation.

After running a few hours with new components the offset became 0,000 ~ 0.001 mV. for both channels. Quite remarkable for individual trans.

I left the drivers 2sd358 / 2sb528 original. I bought replacements, 2sc2238 / 2sa968 but I do not know if these can be used. I read somewhere these trans were replacements. Maybe someone can give me advice.

The sound is remarkable, the amp opened up, the dull sound disappeared with the new parts. I find this amp quite analytic in sound, a little too analytic to my ears.