I just repaired a power amplifier in a DRA-825R and upon further inspection I saw a defective trimmer pot. The yellow rotary slider for adjustment was missing. I have now exchanged it for a new one, but I can't find anything in the manual that indicates a setting on the VR506.
I thought it was for the DC balance, but a DC meter connected to the outputs shows no change when turning the trimmer pot.
Does anyone have any idea what value needs to be set?
Greetz,
Elektroniklurch
I thought it was for the DC balance, but a DC meter connected to the outputs shows no change when turning the trimmer pot.
Does anyone have any idea what value needs to be set?
Greetz,
Elektroniklurch
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Hi,
Can you post the entire service manual?
The resolution of the png is marginal, but it appears that the pot joins C550 and C552 and that the wiper is not connected to the emitter resistors. Assuming this is correct, the pot would not have any effect on offset voltage. My guess is that it's intended as a trim to minimize crossover distortion.
I can't spot any offset trim in the png and believe the design relies upon low offset error in the opamp and unity gain in the feedback at DC to ensure low offset at the amp output. Is it acceptably low?
Can you post the entire service manual?
The resolution of the png is marginal, but it appears that the pot joins C550 and C552 and that the wiper is not connected to the emitter resistors. Assuming this is correct, the pot would not have any effect on offset voltage. My guess is that it's intended as a trim to minimize crossover distortion.
I can't spot any offset trim in the png and believe the design relies upon low offset error in the opamp and unity gain in the feedback at DC to ensure low offset at the amp output. Is it acceptably low?
Hi BSST,
thanks for your effort in helping. I'll send you the link to the entire manual.
If it is for the crossover distortion, can I look at the distortion under load with the oscilloscope and reduce it using the potentiometer!?
But I'll wait for your assessment after looking through the entire manual.
Thanks again and have a nice day
https://www.eserviceinfo.com/downloadsm/59748/Denon_DRA-825R/.html
thanks for your effort in helping. I'll send you the link to the entire manual.
If it is for the crossover distortion, can I look at the distortion under load with the oscilloscope and reduce it using the potentiometer!?
But I'll wait for your assessment after looking through the entire manual.
Thanks again and have a nice day
https://www.eserviceinfo.com/downloadsm/59748/Denon_DRA-825R/.html
Unless you can attach the manual here, I won't be able to help you with even a look.
The link you provided wanted my credit card to enroll, and I don't do that. A second attempt, within the same website, to get free download led me to a very scary place, with all manner of dire warnings to not restart my computer and phone numbers to call for help. Rebooting my computer is precisely what I did, and I feel better now.
None of the two or three sites I trust had the service manual. I followed a Google search to noinfo.com and its appearance was so similar to the to the original link that I backed out immediately.
The link you provided wanted my credit card to enroll, and I don't do that. A second attempt, within the same website, to get free download led me to a very scary place, with all manner of dire warnings to not restart my computer and phone numbers to call for help. Rebooting my computer is precisely what I did, and I feel better now.
None of the two or three sites I trust had the service manual. I followed a Google search to noinfo.com and its appearance was so similar to the to the original link that I backed out immediately.
I have been working with eserviceinfo for years, you are not obliged to give money every time, I give once or twice a year as a matter of principle.
@huggygood Thank you for the manual!
My German is completely inadequate. I could have missed it, but I couldn't find any reference to VR505 or VR506 aside from the schematics. I still suspect they are intended as trims for distortion. A distortion analyzer would be best if available.
You might be able to see with a scope, but I'm doubtful. Try with a 4 ohm load, a few volts at output, maybe at 20kHz where loop gain will be lower. Try with pot at extreme ends to look for hint of distortion and adjust drive amplitude for best visibility of distortion. Then trim VR505/VR506 for null. Again, I'm skeptical.
Good luck!
My German is completely inadequate. I could have missed it, but I couldn't find any reference to VR505 or VR506 aside from the schematics. I still suspect they are intended as trims for distortion. A distortion analyzer would be best if available.
You might be able to see with a scope, but I'm doubtful. Try with a 4 ohm load, a few volts at output, maybe at 20kHz where loop gain will be lower. Try with pot at extreme ends to look for hint of distortion and adjust drive amplitude for best visibility of distortion. Then trim VR505/VR506 for null. Again, I'm skeptical.
Good luck!
Hello,
and a big thank you to both of you
@BSST
As huggygood already mentioned, everything is ok with this site, except for the note/request for a small donation from time to time.
I can't understand why these messages appeared on the PC and I definitely didn't want to cause any trouble!!!
As described (unfortunately only with an oscilloscope), I attached a load resistor and tested it at 1KHz. There is a slight change in the distortion behavior, at the peaks on the sine. However, in a minimal area. When I turn the volume control to "3 o'clock pm", there is hardly anything that can be adjusted (probably because of the general clipping). I have now set the subjectively best value with increased volume and let it "go right" for 3 hours. I have the impression that the device runs great and sounds really good.
I'll test it for a few more hours today and then decide that everything is fine; )
Thanks for the help in deciding how to turn the pot
and a big thank you to both of you
@BSST
As huggygood already mentioned, everything is ok with this site, except for the note/request for a small donation from time to time.
I can't understand why these messages appeared on the PC and I definitely didn't want to cause any trouble!!!
As described (unfortunately only with an oscilloscope), I attached a load resistor and tested it at 1KHz. There is a slight change in the distortion behavior, at the peaks on the sine. However, in a minimal area. When I turn the volume control to "3 o'clock pm", there is hardly anything that can be adjusted (probably because of the general clipping). I have now set the subjectively best value with increased volume and let it "go right" for 3 hours. I have the impression that the device runs great and sounds really good.
I'll test it for a few more hours today and then decide that everything is fine; )
Thanks for the help in deciding how to turn the pot