Today I decided to look at a little CRT TV a friend gave me as non working, he said it would power on then power off. When I tried, it didn't do anything.
320V was present at the primary capacitor, and I saw nothing obviously bad in the power supply. Then I checked underneath and spotted two power resistors with broken solder joints (R813, R814). (Also see bodge diode under IC201
)
I re-soldered them and the rest of the primary side components as some joints looked a little off, and the TV came on just fine - but then I discovered the front panel buttons were not operating properly.
The design is very simple, they use a resistor ladder with a button at each 'rung' so the IC is just sensing different voltage levels on a single input (Pin 3).
At first I thought to blame the resistors but that would be unlikely and I thought I better check the simple thing first: Are the buttons working properly? - and guess what - they aren't!
They are standard right-angle PCB mount tactile switches but the on resistance is all over the place. One of them wouldn't consistently go below 150 Ohms. I guess they were either made of crap or have corroded internally, probably both.
I also notice they seem to really like axial leaded ceramics! Not sure what the special treatment of C514 on the neck board is for, though.
But what I really want to know, what is up with the general purpose, 85°C rated capacitors everywhere, even on the SMPS outputs. Is this normal for CRT TVs in general or just the el-cheapo ones like this?
Edit: Found service guide\schematic etc here: http://monitor.espec.ws/section1/topic121112.html and attached files here in case anyone ever wants them.
320V was present at the primary capacitor, and I saw nothing obviously bad in the power supply. Then I checked underneath and spotted two power resistors with broken solder joints (R813, R814). (Also see bodge diode under IC201
)I re-soldered them and the rest of the primary side components as some joints looked a little off, and the TV came on just fine - but then I discovered the front panel buttons were not operating properly.
The design is very simple, they use a resistor ladder with a button at each 'rung' so the IC is just sensing different voltage levels on a single input (Pin 3).
At first I thought to blame the resistors but that would be unlikely and I thought I better check the simple thing first: Are the buttons working properly? - and guess what - they aren't!
They are standard right-angle PCB mount tactile switches but the on resistance is all over the place. One of them wouldn't consistently go below 150 Ohms. I guess they were either made of crap or have corroded internally, probably both.
I also notice they seem to really like axial leaded ceramics! Not sure what the special treatment of C514 on the neck board is for, though.
But what I really want to know, what is up with the general purpose, 85°C rated capacitors everywhere, even on the SMPS outputs. Is this normal for CRT TVs in general or just the el-cheapo ones like this?
Edit: Found service guide\schematic etc here: http://monitor.espec.ws/section1/topic121112.html and attached files here in case anyone ever wants them.
Attached Files
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