Sansui R-50 loud popping speakers when turning on

[RodrigoPhid]

Guys, when I turn on this sansui, there's a boom in the dc voltage across the speakers, sometimes 5V, 8V, 12V. After one or two seconds powered, the dc voltage drops and becomes stable at 50mV per channel.

Already changed the filter caps on the power supply (2 x 6800uF 50V) and recapped the eletrolytics of the main amp section, but the problem persists. The bias adjustment is correct, according to the service manual. The output transistors are new and ok, as the diffs.

R50 it's working perfectly, except for this dc voltage issue on the speakers. The problem appeared quite recently, I have this amp since 2018.

Does anyone have a clue for this? Thanks.

 

[Hyperion]

This model uses the 2SA798 dual transistor as the input differential transistors, this transistor type often gives trouble, and is known to fail with age, it is possibly the source of the problem.

You say:-

The output transistors are new and ok, as the diffs

Does this mean you have replaced the 2SA798's ? - they haven't been made for a long time, so any you have bought recently are likely to be counterfeit. We usually replace these with a matched pair of KSA992's - available from 'Mouser' or 'Digikey' (for example), they need to be matched as there is no DC offset adjustment trimmer with this model.

As a temporary work-around set the speaker switch to 'Off' before powering on, then to 'On' when the voltage has stabilised.

Good luck.

 

[RodrigoPhid]

Thanks for reply, Hyperion.

Before I replaced the diffs, I was using a good and matched pair of these a992 substitutes. The offset was a little bit better, but the problem was the same. So, I think the diffs are not the cause of the problem.

 

[RodrigoPhid]

It seems that the negative feedback is the path of the problem.

If I set the bass tone control on the max, and then I turn on the amp, the voltage starts from about -9V, -5V and slowly backs to normal. When the tone control is defeated, then I turn on the amp, the voltage is less, about 1V, and quickly stabilises.

And I noticed a thing about the two main filter caps. The cap of + rail drops to almost 0V, while the cap of - rail drops always to -6.5V. Can this diference between negative and positive rails cause this loud pop?