Sansui 551 bias not coming down.

anderslober

Yikes...not again!
Hello Forum.........working on this nice little unit. It came to me with a blown left channel(one 2sd315 and a driver were OL). I replaced all driver tr's and diodes on both channels. DBT'ed...all ok. I then powered up on normal power....all ok.

I then wanted to set the bias....on the right channel it dialed in easy(20mA). On the left channel, I can't get it below 36mA........the vr9 is turned all the way down.

Should I check the emitter resistors for the left channel? What else could cause the bias to be a bit to high?....I checked the emitter resistors...they are ok.

Should I worry about the (almost twice as high) 36mA bias?

My voltages on the drivers and main outputs are fine.
 
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Hello Forum.........working on this nice little unit. It came to me with a blown left channel(one 2sd315 and a driver were OL). I replaced all driver tr's and diodes on both channels. DBT'ed...all ok. I then powered up on normal power....all ok.

I then wanted to set the bias....on the right channel it dialed in easy(20mA). On the left channel, I can't get it below 36mA........the vr9 is turned all the way down.

Should I check the emitter resistors for the left channel? What else could cause the bias to be a bit to high?....I checked the emitter resistors...they are ok.

Should I worry about the (almost twice as high) 36mA bias?

My voltages on the drivers and main outputs are fine.

Hi Mr Anders
U gotten a fine Sansui amp there:thmbsp:
How things recently?

Meanwhile i still on the restoring mode for the Kenwood 500 supreme, power side is pretty done awaiting some parts for the pre stages n certain parts oly
 
John.......the bias pot works fine. I can easily adjust it higher than the 36mA. So, the pot is ok.
I will check tr07.........by the way.... The SM from hifiengine has quite a few differences compared to my unit. All the tr's on the amps were different from the SM, except the main outputs, 2sd315's. The labelling of the tr's is significantly different...for example tr07 is actually tr09 in my unit. My drivers were 2sa697 / 2sc1211 .....I put in ksa1220 / ksc2690.
All the tr's on both channels have been changed to new. The right channel adjusts bias fine to 20mA. I measured many voltages on both amps and they are very close.

The amp does not get hot or anything. I think I will hook up some speakers and let it run......
 
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I will try a measurement across the emitter resistors. Using Ohm's law.......I will measure about 18-19mV.
V= r x I......(2x 0.47 ohm)..0.94 ohm x 20mA = 18.8mV.

Is the math correct? Confirm? Thanks.

Any other ideas are welcome.
 
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Well...I tried the aforementioned procedure....measuring voltage across the emitter resistors. I was easily able to dial in 18-19mV on both channels!

Should I assume all is ok?
 
Hi,

I was going to suggest using the emitter resistor measurement as a cross check.

It appears that the left channel still has a problem - as you cannot get the current as measured via the fuse method below 36ma - meaning that there is an additional current path than that that is through the output transistors. The fuse method measures the total current through the outputs and the driver transistors.

Could you measure the voltage across each emitter resistor?

Cheers

John
 
Ok John......sounds reasonable......what could cause the extra current? There isn't much in there to check.......and I have voltage-checked quite a few places....and all are very similar/close.........

I'll poke atound a bit more......

The amp works fine though......
 
Capacitor back to front?

Have a look at the schematic and investigate the other current paths (other than OP transistors) which are present when measuring the bias.
 
Confirm balanced PS voltages on that channel.... If yes... betting a leaker transistor or diode in that path.....
 
I was thinking a cap backwards to John. Anders, Check C57 (C58) 4.7uf polarity, if this is reverse and has shorted if would take an extra 25ma in the fuse current bias measurement.
 
Ahh yes 47uf, misread the scat.

Did you get a chance to measure the voltage drop across either emitter resistor on the suspect channel?
 
Hello Forum.........working on this nice little unit. It came to me with a blown left channel(one 2sd315 and a driver were OL). I replaced all driver tr's and diodes on both channels. DBT'ed...all ok. I then powered up on normal power....all ok.

I then wanted to set the bias....on the right channel it dialed in easy(20mA). On the left channel, I can't get it below 36mA........the vr9 is turned all the way down.

Should I check the emitter resistors for the left channel? What else could cause the bias to be a bit to high?....I checked the emitter resistors...they are ok.

Should I worry about the (almost twice as high) 36mA bias?

My voltages on the drivers and main outputs are fine.
I actually have the same problem. Did you ever figure out a solution to this?
 
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