Sansui 3900Z Bias adjust issue...Help !

johndoe10

johndoe10

Active Member
Greetings, I have a Sansui 3900z that just had the output transistors replaced, and I need to know how to properly adjust the bias for them. Any help would be greatly appreciated.
Thanks, JD
 
Thank you for the response, I did try the way it says in the manual, by turning the bias pots counter clockwise, putting the probes from my meter one on Q13 and the other on Q14 emitters. My problem is that when I turn the bias pots up I get no mV reading (nothing but zeros) until I turn up the volume control with signal applied, then I get a unstable reading that changes with the volume control. When I turn the vol. up mV go up, when turned down mV go down. Any Ideas ?
Thanks, JD
 
johndoe10 said:
Thank you for the response, I did try the way it says in the manual, by turning the bias pots counter clockwise, putting the probes from my meter one on Q13 and the other on Q14 emitters. My problem is that when I turn the bias pots up I get no mV reading (nothing but zeros) until I turn up the volume control with signal applied, then I get a unstable reading that changes with the volume control. When I turn the vol. up mV go up, when turned down mV go down. Any Ideas ?
Thanks, JD
Click to expand...

Did you check your emitter resistors after replacing the output transistors ? KR26 a dual .33 Ω 5watt ?
 
Last edited:
johndoe10 said:
Thanks for the reply, Just to making sure which ones would they be?
Thanks, JD
Click to expand...

KR26 a dual .33 Ω 5watt ?
Click to expand...

Probably white ceramic square nobles. If one of the .33's in the dual package is open you won't get any bias current reading.And they typically get burnt up if the outputs are shorted.
 
Last edited:
No outputs were shorted, I had one open 2SA1102 in each channel. The 2SC2577's were Ok but, I replaced those too.
JD
 
Need to check KR28 and KR29 4.7Ω 1/4 watt fusibles.

Are you getting any sound at all ?
 
Seems to be playing Ok as far as I can tell, I didn't want to run it very long due to this bias situation.
JD
 
Do the output transistors get very warm ? Are you sure your connecting your test probes to the emitters ?
 
The heat sink seemed to stay cool but, I haven't ran it very long. I double checked twice to make sure I had the probes on the emitters. I am at a loss here...Hmm
JD
 
I am afraid that if I turn the volume up past 4 or 5 it will definitely exceed the 10mV at the emitters and cause damage to them.
 
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