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Promedia 2.1 low volume from sub


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I have the Promedia 2.1 (the one with the power switch and transformer) system for my PC.  I have had for some time now.  My absolute favorite speaker.  I had a couple of issues in the past that I repaired like Din plug and scratchy volume.  Several months ago, I was listening to some music at a high volume.  After 45 minutes, I noticed that only one speaker was working and could not hear the sub.  I checked the speakers and they were fine.  Opened up the sub and found a blown fuse on the HF amp.  Now HF speakers work fine but the sub is very low in volume even though the adjustment on the pod is all the way up.  I checked the sub and it is not blown.  Measures good and tested with another audio source.  Connected another sub in place and still had low volume.  When I looked at the LF amp, I could see that the board was very discolored due to heat.  I removed the board and found R16 and R17 to be loose.  I then cleaned up the solder pads and reinstalled the resistors.  I checked the fets and they seemed to be ok.  I checked all the other parts for bad joints.  Didn't notice any components to be damaged. 


I was wondering if there was a service manual for the original Promedia 2.1 (the one with the power switch and transformer)? 


Does anyone know why the audio would be very low?



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Thanks Justus for the link.  I found that earlier but I was wondering if there were any service manuals that gave a little more information on values, test points and voltages.


I did some more troubleshooting and here is what I found.  I wondered if the supply was good.  Measured voltages on the transformer and they check out.  I checked the bridge rectifier and it seems good.  The input voltage to the rectifier is 78VAC and output is 103VDC.  I expected to see 78VDC but I assume the filter caps might be adding a little.  The voltage going into the LF amp is the same 103VDC. 


I inspected the LF amp a little closer and I think I may have found the colpret.  Looking at the burn pattern, it seems that D03 and D04 might be bad.  Those are 15V zener diodes.  I will grab a couple of those as well as a couple TL074's in case the opamp is out.


I'll update this post if I find anything out.



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