Help Repair Marantz 2285B - Bias Adjust

Halisson

Member
iw9Zo
Hello friends, greetings from Brazil.
I'm doing the repair on a Marantz 2285b and I'm having a hard time getting the BIAS adjustment.
DC OFFSET + - = 0vdc.
Reading based on q710 ranges from 0.7vdc to 2.1vdc, stirring in the BIAS trimpot
Base of q709 does not vary anything, staying at -0,800vdc.
Replaced q704 and q705 compatible.
Base q704 = +45, c-46 and +46 (vdc)
Base q705 = +45, c-45 and +46 (vdc)
Diodes d716 and d717 are ok, I even did the test without them in the circuit has not changed.
Compared the value of the sensor of the heat resistors, the channel L and R are equal.
Base of q709 should be with -1.1vdc this with -0.800vdc, on which channel it works is with -1.300vdc, the emitter of the same transistor is with -0.400vdc.
Resistor 709 OK.

The test point j742 and j741 not the value does not change the change in the trimpot. And the voltage between the base and the emitter of the power transistors does not change. (0.420vdc Same value always, another channel, this OK is + - 0.600vdc)
If I inject sine wave the output volume I read in the sound output 25vrms - 1khz, I can read in the test point 0.70vdc, but with the volume without input signal, the 0vdc offset in the output sound, however, I can not set None on The BIAS trimpot.

The components tested outside the circuit are marked in the following image:

http://imgur.com/a/iw9Zo

Obrigado por qualquer ajuda!
 
No response to bias trimmer may be due to no current flow, so no voltage drop.
Measure dc voltages at TR707, b, c, e. While it's tested ok, consider replacing TR707.
 
No response to bias trimmer may be due to no current flow, so no voltage drop.
Measure dc voltages at TR707, b, c, e. While it's tested ok, consider replacing TR707.
Hello mbz!
After the tests confirmed that the q707 was ok, I even changed it by any possibility of doubts.
I still would not have done the ghost load test on the output along with the oscilloscope to monitor the half-cycles and, to my surprise, I warned of a problem in one of the half-cycles. (My fault, before not having been the test with phantom load @ 4ohms), only oscilloscope whitout ghost charge.
Analyzing the 2sc1913 (Q710) out of board was ok in hfe, but with slightly black legs.
I decided to replace it anyway and immediately the semi-cycle normalized and everything returned to normal, including the BIAS configuration.
Thanks again for your help.
 
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