Sansui au-999 unable to bias.

anderslober

anderslober

Yikes...not again!
Hi forum. Workin’ on a nice au999. Unit came to me with f003/f004 blown. I took the unit apart, cleaned it(very dirty) and measured all main outputs and drivers. All were good. I recapped the unit…double checked all work. I only changed 2 tr’s on f1228 board….2sc458 ‘s….put in ksc1845. I have not changed any ‘can tr’s’…all were good.
I dbt’ed the unit….with meters hooked up at emitter resistors, r853 and r854…..unit came on….no ‘bright bulb’….tried to get 13mv spec….5.5mv L and 1.0mv R. Unable to dial it higher…..ideas welcome.
Nothing burnin’ or blowin’ up….
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I removed the fuses f003/004 and measured bias in mA. Able to get about 5mA both sides. See picture. Could the dbt be causing the bias not to go up?
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I have music in both channels…..but both equally scratchy. Still on dbt. All switches have been cleaned.
Ideas welcome.
 
I would turn the bias down to minimum, remove the DBT and see if you can set the bias.

If not then I would investigate the bias control transistor/diodes (?), and the gain of the driver transistors which might have drifted low with age.

And note that despite testing 'good' some transistor testers are too forgiving, and replacing suspect (but 'good') transistors can resolve issues.

And while you have little to no control of the bias, keep using the DBT, as I believe you should be able to set bias on DBT.
 
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Yes Sir…I did do that. No worries…anyway….I took both au999 amp boards out and poked around with my trusty diy signal tracer. An 2sa640 on the right f1209 board was bad. I put in ksa992…..playing on both channels now. Au999 Preamp section going to amp section of a Pioneer a450r. Will test a bit furher…….and see…
I was tricked(again) by Sansui ‘negative feedback’ system…..
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Check the 1/2 watt resistor next to C811. Mine had been replaced by a previous tech, and showed signs of overheating. On the other board it measured out of spec. Mine had flying saucers not the cans, so those were replaced.
 
I redid both amp boards. New tr’s/caps….bias is sensitive, but easily dialed in now. 28mA or so….
2sc875 to ksc2383.
2sa532 to ksa1013.
2sa640 to ksa992.

Time for sound check…..
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I have sound on phono 1/2, but with hum…….all other inputs no hum. I am hoping this is shielding/grounding…….ideas welcome.
The f1228 head amp(phono) board has been recapped and new tr’s….ksc1845 and ksa992. Double-checked for orientation. I have also cleaned the slot for the board.
When I put my grubby hands on the chassis, the hum nearly disappears. Normal for sn au999?
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anderslober said:
When I put my grubby hands on the chassis, the hum nearly disappears. Normal for an au999?
Click to expand...
Most likely because you have all its clothes off. ;)

See if it improves with RCA shorting jacks plugged in to the inputs?

Check the smoothing (regulation) of TR001 on F-1230, for the B7 & B8 voltages?
 
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Here’s a new one…(to me anyway)…...the ipad charger was sending hum to phono inputs….as soon as I disconnected the charger…..no more hum! Cool!
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Wondering why you have electrolytic caps in place of the green mylars on F1228? Also, on my F1230 there was an open carbon resistor that appeared ok, it effected the regulated output voltage.
 
On this unit there was 2 1uf/50v caps on f1228…..so, I put in new 1uf/50v caps…..no mylars at those 2 top left/right positions. The scat says 0.47/50v mylar, but I elected to put in what was original. This unit hadnt been worked on at all.
 
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