Sausui 9090 Bias issue

gto127

Member
2007-05-09 2:51 am
Just recapped driver board on sansui 9090(non DB version). I am able to get 50ma on left channel but only 43 on right channel(pots are about as low as they go on both sides-10ohnm and 2.7). I had also replaced 4 shorted outputs on the side that is slightly low. I can find no resistor out of spec on either side. They are supposed to be within 1ma of 50ma. Any suggestions on what could cause this?
 

gto127

Member
2007-05-09 2:51 am
I reversed sides with the driver board & problems remains on that side so guess driver board is OK. Does this mean output transistors I bought off ebay are fake or bad? They certainly look like new old stock. Paid $30 for 4. Is there a modern equivalent? 2SC1116A were originals.
 

llwhtt

Member
2008-06-12 3:43 am
SoCal
Since the idle current is so close and you ran out of adjustment you can either increase R33/34 OR decrease R35/36. Basically the base of TR07/08 is too positive. For modern replacements, may or may not fix the problem, you can try MJ21194, they are cheap and available.

Craig
 

gto127

Member
2007-05-09 2:51 am
Here is pics. Measured .55 drop with diode test out of circuit.
 

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llwhtt

Member
2008-06-12 3:43 am
SoCal
My examples are at work but those look pretty damn good if you ask me. Thick mounting base, lots of little nicks in the base, small diameter glass seals around the pins, and they seem to measure OK. I'll comment about the lettering in the morning.

I would do the resistor change I mentioned, move up OR down to the next value depending on what you have on hand. I usually use a decade resistor so I end up with the pot in the middle of it's travel. I was working on a Marantz 2275 yesterday and they used a 2.2k pot where all the amp wanted was about 50 Ohms, very touchy towards the end of travel. Knowing it only needed 50 Ohms I would use a 100 Ohm pot, puts the wiper close to the middle and not touchy.

Craig