Mitsubishi CS-1939 Tuner control Help

[Dan Schafer]

Hi All
Working on a Mitsubishi model CS-1939 circa 1986 color TV. I get only snow (has good raster) and no LED display and none of the buttons on the front keyboard do anything. The voltages to the tuner control are low and measure Ok when disconnected. I guess something in that circuit is loading it down, the sound level goes up and down every few seconds also (just snow in the audio), but audio amp seems ok. There is a 28 pin microprocessor upd-1709CT which seems to not have an NTE cross, & LA7910LED drive, LA7911 band switch low pass filter and a 2 character LED display. Does anyone have a troubleshooting suggestion before I send it off to a landfill? I'm at a roadblock. I like appearance of the blue phospher on the screen of the CRT when off...diden't they call those rare earth tubes?
Thanks
Danny in Cleveland

[andy]

If it's pulling down the power supply (I assume 5v for the micro) then you need to start diconnecting things one at a time until you find the component that's loading it down. You might as well start by lifting the Vcc pins to the micro since the show's over if it's bad.

How low do the voltages go? It's also possible that the power supply has failed and can't supply the micro with enough power, but it's fine with no load.

[Dan Schafer]

Thanks for the reply...I get apx. 1.5 VDC for the 4.9V source...I do think the micro is bad. Is there a way to bypass the tuner control and just hook up the output of a cable box directly to the IF input and make it a monitor?
Thanks
Dan

[andy]

There's really no way to do this unless there's a video input because the tuner control needs to get data from the micro.

[rcaman]

have you unloaded the 5 volts to the tuner. i use solderwick then see if your 5 volts come up. also a bad 5 volt regulator i.c. or a electrolytic cap in the 5 volt supply can be bad and the voltage come up when you unload the supply have seen it many times. the regulator is bad but it can regulate the 5 volt line without a load. also check what voltage you have going into the regulator it may be low it should be around 9 volts or so going into the regulator. this will be a 3 leg stand up i.c and may have a heatsink on it. mitsu. has had a ton of capacitor problems you can also look at the negetive leg of the caps and see if they are green or turning or have turned dark. another way is to put your soldering iron on the board side negetive lead of cap and add a drop of solder and it it stinks it is bad. some people say it smells like fish but to me it smells like a dirty public bathroom.(piss) i reoaired a mitsu. big screen a few months back and i replaced about 70 capacitors in it. reg ic
pin1 +9v>in____pin3 5v>out
pin2 ground

[Dan Schafer]

I scraped the set...too complacated a repair and have way too much time in it. And the thing is very old. I did get the B+ back and the LED and push buttons operate but I had varying signal and noise. lots of retrace lines intermient signal an color, not watchable. I saved a lot of parts though. A bad cap probably but it is now on my treelawn for the garbage man.
Thanks Anyway all
Danny in Cleveland