There is not a lot out there on this pre/dac, and all the secrecy around the schematic.. I mean really.
So, in an effort to help anyone else, I have opened up my unit, as it has a DC output of around 7.5V on one channel. The other is fine.
You’d think I can do the comparison between channels and sort it out..
With all the feedback and servo paths, coupled with the multi layer board, its a tough ask.
Hoping the collective will be able to make sense on where to start looking.. I don’t want to throw parts at this, as that’s not my style to fix things.
Attached is the internal view, with the covers off the supposed hybrids.. not hybrids at all.. its just a plastic cover on surface mount opamps etc..
I have taken measurements and recorded them on the attached. I haven’t bothered putting the rail voltages of the opamps as they are all good.
Just need to know where to start and what is causing the voltage buildup..
All the passive components measure fine, left to right channels.. that just leaves the active opamps and transistors.
The transistors measure fine (statically), as you can see, they are not biased correctly..
Ask me any other question that would the discussion along.
So, in an effort to help anyone else, I have opened up my unit, as it has a DC output of around 7.5V on one channel. The other is fine.
You’d think I can do the comparison between channels and sort it out..
With all the feedback and servo paths, coupled with the multi layer board, its a tough ask.
Hoping the collective will be able to make sense on where to start looking.. I don’t want to throw parts at this, as that’s not my style to fix things.
Attached is the internal view, with the covers off the supposed hybrids.. not hybrids at all.. its just a plastic cover on surface mount opamps etc..
I have taken measurements and recorded them on the attached. I haven’t bothered putting the rail voltages of the opamps as they are all good.
Just need to know where to start and what is causing the voltage buildup..
All the passive components measure fine, left to right channels.. that just leaves the active opamps and transistors.
The transistors measure fine (statically), as you can see, they are not biased correctly..
Ask me any other question that would the discussion along.
Attachments
You might note voltages on the working channel for reference.
Looks like opamp on far left is stuck on plus rail—- 13.6V and unequal voltages on its inputs. Failed opamp? What are same points on the other channel? Where do those inputs originate? I assume 13.6V is on pin 1?
An active experiment I often perform to test an opamp section is short output to inverting input, usually using a scope probe tip to bridge the . adjacent pins together momentarily. This converts the stage into a unity-gain follower and the output should replicate the voltage on the non-inverting input. But this is risky unless you have a schematic and are confident the test won’t cause collateral damage.
I detest/dread troubleshooting without a schematic. Good luck!
BTW, I applaud your reluctance to swap parts or remove parts unless it’s unavoidable. My creed is to troubleshoot the circuit and its symptoms.
Looks like opamp on far left is stuck on plus rail—- 13.6V and unequal voltages on its inputs. Failed opamp? What are same points on the other channel? Where do those inputs originate? I assume 13.6V is on pin 1?
An active experiment I often perform to test an opamp section is short output to inverting input, usually using a scope probe tip to bridge the . adjacent pins together momentarily. This converts the stage into a unity-gain follower and the output should replicate the voltage on the non-inverting input. But this is risky unless you have a schematic and are confident the test won’t cause collateral damage.
I detest/dread troubleshooting without a schematic. Good luck!
BTW, I applaud your reluctance to swap parts or remove parts unless it’s unavoidable. My creed is to troubleshoot the circuit and its symptoms.
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