I had interesting day trying to debug this KOSS 27" set (Don't remember the model but I'll get it on monday and post new note about it.)
Well, Unit works fine aside from no audio till volume maxed out and you get pops/barks from the speakers with signal in.
1/2 hour later audio quickly comes up staring gutteral sounding gradually good.
Taking apart the chassis is interesting, you had to take the black box with the analog inputs off first then cabinet back off. Then have to take tinbox off the mainboard to see what the problem is. Mainboard only have the VGA and DVI inputs. Other inputs is through this black box.
Audio amp section is on mainboard itself and has audio power feed thru domed capxon that also had broke the connection inside the cap that leg wiggles freely. Duly replaced that. No change!
To troubleshoot more, I had to unbox the die-cast black box and get die-cast base off the analog daughterboard so I have more room to access around the daughterboard, I was in luck as you will see.
Heated the PWM audio amp section made audio come up, but freezing spray didn't quite gave the result. Went wide with the spray made audio go away. Narrowed to the -8V linear regulator (LM7908 type in SMD factor). with a cold swab dab the regulator's tab gave instant results. WHAT?! -8V needed in this chassis? That smacks of bad design. I have not found the source of circuit that converted positive voltage to negative voltage as this PSU does not have -V supply (all 5V and 12V only, 24V for lamps only).
I knew from my previous study in the Art of Electronics book there is many ways to generate negative voltage supply. So no worries to locate the source of this dang thing on this mainboard because I needed to see where this is coming from.
Stacking the boards 3 deep is sloppy. Mainboard then this daughterboard with tiny sub-daughter board on it. Increases the cost and more failure points, and quality of PQ is terrible for this even mainboard does have 2 big chipsets as that two should implies video processing grunt but this was not the case as I sadly discovered. Uses two Genesis chipsets (one BGA, one gullwing 208 pin type).
I had so much issues with lack of attention to reliablity and servicablity in generic Plasma & LCD TVs is appalling.
Cheers, Wizard
Well, Unit works fine aside from no audio till volume maxed out and you get pops/barks from the speakers with signal in.
1/2 hour later audio quickly comes up staring gutteral sounding gradually good.
Taking apart the chassis is interesting, you had to take the black box with the analog inputs off first then cabinet back off. Then have to take tinbox off the mainboard to see what the problem is. Mainboard only have the VGA and DVI inputs. Other inputs is through this black box.
Audio amp section is on mainboard itself and has audio power feed thru domed capxon that also had broke the connection inside the cap that leg wiggles freely. Duly replaced that. No change!
To troubleshoot more, I had to unbox the die-cast black box and get die-cast base off the analog daughterboard so I have more room to access around the daughterboard, I was in luck as you will see.
Heated the PWM audio amp section made audio come up, but freezing spray didn't quite gave the result. Went wide with the spray made audio go away. Narrowed to the -8V linear regulator (LM7908 type in SMD factor). with a cold swab dab the regulator's tab gave instant results. WHAT?! -8V needed in this chassis? That smacks of bad design. I have not found the source of circuit that converted positive voltage to negative voltage as this PSU does not have -V supply (all 5V and 12V only, 24V for lamps only).
I knew from my previous study in the Art of Electronics book there is many ways to generate negative voltage supply. So no worries to locate the source of this dang thing on this mainboard because I needed to see where this is coming from.
Stacking the boards 3 deep is sloppy. Mainboard then this daughterboard with tiny sub-daughter board on it. Increases the cost and more failure points, and quality of PQ is terrible for this even mainboard does have 2 big chipsets as that two should implies video processing grunt but this was not the case as I sadly discovered. Uses two Genesis chipsets (one BGA, one gullwing 208 pin type).
I had so much issues with lack of attention to reliablity and servicablity in generic Plasma & LCD TVs is appalling.
Cheers, Wizard
