SCsoundguy
New Member
Hello,
This is my first time posting but I have been following much of the great advice here for a couple of years. I grew up in my dad's TV/Electronics repair shop and have recently gotten back into vintage equipment. I need some help with a 939 rebuild project for my son. I picked up the unit early this year from the Goodwill auction site. When I received it I did not spend any time on it, just made sure it played sound from the Aux and FM. All seemed to work okay so I started on my rebuild. I focused mostly on the signal path. I detoxed/cleaned all, replaced caps, transistors, and most diodes on the Control board, Eq board, power amp assy. rebuilt the protection board including relay, the power supply board, main caps and replaced the output transistors. I basically followed MFT rebuild plan. I did a DBT after each board and checked for sound from both channels. I have set the idle current and DC offset, both adjusted with not problems. However, now that my masterpice is done I have an issue. The left channel sounds slightly "muddy" for lack of a better term. This could have been there from the beginning but now I have a real problem to solve.
I have disconnected the pre-outs and fed in a signal to just the amp section. It sounds great. So, I have a pre-amp problem. I pulled out the O-scope and started injecting signals from 100HZ, 500, 1000, 5000, 10,000 and 15,000. Everything was checked with the tone switch off. I compared the signals from each channel and they fall right on top of each other. I don't see any distortion on the wave forms and amplitude of both channels is the same. So, I tell myself that I really don't have a problem. I connect the preamp and amp back together and it still sounds the same. I must me missing something but I am not sure what it is. I started comparing signals at the pre-amp output and worked back to the control board. In and out of the control board looks fine. Anyone have any ideas?
This is my first time posting but I have been following much of the great advice here for a couple of years. I grew up in my dad's TV/Electronics repair shop and have recently gotten back into vintage equipment. I need some help with a 939 rebuild project for my son. I picked up the unit early this year from the Goodwill auction site. When I received it I did not spend any time on it, just made sure it played sound from the Aux and FM. All seemed to work okay so I started on my rebuild. I focused mostly on the signal path. I detoxed/cleaned all, replaced caps, transistors, and most diodes on the Control board, Eq board, power amp assy. rebuilt the protection board including relay, the power supply board, main caps and replaced the output transistors. I basically followed MFT rebuild plan. I did a DBT after each board and checked for sound from both channels. I have set the idle current and DC offset, both adjusted with not problems. However, now that my masterpice is done I have an issue. The left channel sounds slightly "muddy" for lack of a better term. This could have been there from the beginning but now I have a real problem to solve.
I have disconnected the pre-outs and fed in a signal to just the amp section. It sounds great. So, I have a pre-amp problem. I pulled out the O-scope and started injecting signals from 100HZ, 500, 1000, 5000, 10,000 and 15,000. Everything was checked with the tone switch off. I compared the signals from each channel and they fall right on top of each other. I don't see any distortion on the wave forms and amplitude of both channels is the same. So, I tell myself that I really don't have a problem. I connect the preamp and amp back together and it still sounds the same. I must me missing something but I am not sure what it is. I started comparing signals at the pre-amp output and worked back to the control board. In and out of the control board looks fine. Anyone have any ideas?
