PDA

View Full Version : JBL/Urei 6215 Amplifier Bias Adjustment Help Needed



kazarkazam
06-14-2018, 07:56 AM
I have a JBL/Urei 6215 amplifier that has a bad STK2240 output chip that I am going to replace. I have a copy of the owners quasi service manual, but I cannot find any information on how to bias the amplifier. It looks like there is a bias pot for each channel, but again no information on where to connect a meter, what the reading should be etc.

Below is a link to the manual that I have:
http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/JBL-6215%20manual.pdf

Any help would be greatly appreciated.

Thanks

robertg
06-14-2018, 04:19 PM
I can't help with your question, but I was told there are lots of fake STK chips for sale. My tech tried two different suppliers of STK chips in a couple of 6215s, and he said they were all fakes.

Ian Mackenzie
06-14-2018, 06:32 PM
Try diyaudio.com forums

grey
06-18-2018, 10:15 AM
I have a JBL/Urei 6215 amplifier that has a bad STK2240 output chip that I am going to replace. I have a copy of the owners quasi service manual, but I cannot find any information on how to bias the amplifier. It looks like there is a bias pot for each channel, but again no information on where to connect a meter, what the reading should be etc.

Below is a link to the manual that I have:
http://www.jblproservice.com/pdf/Vintage%20JBL-UREI%20Electronics/JBL-6215%20manual.pdf

Any help would be greatly appreciated.

Thanks

It appears that the pots offer adjustment to the DC offset circuit. See section 4.6 in the manual for a description of Q1 and Q2.

For the amplifiers that I have working, I was able to adjust one channel that had a small offset back to zero.
It is my understanding that most stk modules are biased internally. Let me know if you find anything different as I didn't find a clear and complete schematic for the stk-2240.

If you can figure out the test points, the attached file has a column for typical/max quiescent.