One that I had in recently was a popular LG DC593W combo unit that had died during a power surge. Fuse F101 was open and after replacing it, you could hear a faint pulsating noise and the two front red LEDs were flashing.
I checked the seven main supply rails with an ohmmeter but couldn’t find any short circuits. However, there were only very low pulsating voltages on them when the unit was switched on.
The main problem with these units is access or rather, the lack of it. To get to the main board where the power supply is, you have to remove the front panels and the VCR deck and board. It is almost impossible to run the set outside its cabinet.
It was fairly obvious that the power supply was oscillating, so I started by replacing C107 (1μF 50V) and C108 (33μF 25V). I also mounted IC101 in an IC socket so that I could swap it later if necessary without removing the chassis.
This restored the power to the set but I wasn’t out of the woods yet. The display came on and the DVD did a few turns, as did the VCR, but a few bizarre symptoms remained.
First, all the segments of the display were glowing at once. Second, although the remote was working, when I switched it from DVD to VCR, the LEDs changed but the AV output remained in the DVD position. However, if I tuned the TV into the RF output (Ch68), I could choose between the DVD, the VCR and the TV stations but there was excessive RF interference patterning.
This red herring disappeared when I removed the aerial, which told me that I was getting co-channel interference from Ch66 or another digital channel. As a result, I chose a lower channel number for the RF output (press Ch Up and Down together for four seconds with the VCR off).
Next, I checked all the voltage rails and these proved to a bit high compared to the values marked on the circuit. As a result, I spent a lot of time changing IC101 (the "Power Management" IC), the optocoupler and IC102 (KA431) in an attempt to fix this. In the end, it turned out to be the main filter electro (C103, 100μF 400V). This was slightly low in capacitance and changing it fixed the DVD/VCR switching on the AV output.
But that wasn’t the end of it – I still had the Digitron fluorescent display (IC501) glowing too brightly. The filaments seemed OK at 4V and the -29V rail measured -30V at the anode of D108. However, when I checked the voltage on pin 11 of the P6M01 plug and socket for the front panel, it was very low at about -17V and varying.
With the board out, I soon tracked the fault down to capacitor C122 which had come loose and broken a very fine copper track. Repairing this fault fixed the Digitron display but I wasn’t out of the woods yet.
The last fault was intermittent "no eject" from the DVD. This was caused by a slipping loading motor belt and replacing it fixed that problem. You can see why, generally speaking, these combos are not worth fixing, especially when new units can be had for less than $200.
One of the traps one can get into with this unit is the front control panel not working due to inadvertent use of the child lock on the remote control.
vk2fm@tpg.com.au [fred]
January 2007
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