Author Topic: Onkyo A-9355 DC Offset measurement  (Read 313 times)

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Offline stojke

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Onkyo A-9355 DC Offset measurement
« on: June 18, 2018, 10:31:14 pm »
Hello, I´m having a hard time trying to figure out how to measure DC Offset on an Linear Class D amplifier. Usually it is indicated to measure on certain test points between two transistors on class A/B and it is usually in 10~25mV range.
From this device Service Manual I understood the following:

Disconnect speakers, Volume to minimum, no input, Disable Speaker terminal B and enable terminal A, measure at terminal A RED (+)  and BLACK (-) and adjust using  R5022 and R5021 untill it is either 0V or in tolerance of +/-10mV.

What i get as reading is 220mV for LEFT and ~170mV for RIGHT (Where right starts at 0 and climbs up on its own).
When trimming i get no difference in values, not down nor up, even when i close or open the trimmers all the way.
(EDIT - Measures the same with Speakers connected)

Can anybody guess what the problem might be, could it be bad trimmers?
« Last Edit: June 18, 2018, 10:38:14 pm by stojke »
 

Offline stojke

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Re: Onkyo A-9355 DC Offset measurement
« Reply #1 on: June 19, 2018, 08:24:59 pm »
Bad pots. Managed to get it , measured, down to 0.5mV after an 1h of playback.
 

Offline cvanc

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Re: Onkyo A-9355 DC Offset measurement
« Reply #2 on: June 20, 2018, 12:14:54 am »
Interesting, did you have to replace the pots or did you just 'exercise' them to clean up the behavior?
 

Offline stojke

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Re: Onkyo A-9355 DC Offset measurement
« Reply #3 on: June 22, 2018, 05:10:43 pm »
I had to replace them as they were faulty. They increased and decreased the value on their own. On closer look they look melted, like if they were getting hot for some reason, one is also a bit busted.
It is probably the one who repaired before me that damaged them because all the transformer wires were either scratched or cut. The person who worked on it also completely cut the CPU/Relay power cable.
 

Offline drussell

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Re: Onkyo A-9355 DC Offset measurement
« Reply #4 on: June 23, 2018, 01:03:30 am »
:palm:

I'm glad you managed to get it working after someone else's butchery...  :)
 


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