Atari jr has sound but no picture AND when channel selector switch is on the"other" channel

[stuartmcivor]

Atari Jr is rev F. TV is older flatscreen and known to work with other Atari jrs. I'm testing with a known working ET cartridge.

 

With this junior when I power on, with the ET cartridge inserted, the picture is scrambled black & white snow. I can hear the background ET music fine. I can play the game judging from the sounds. Now what makes this even stranger is that the TV is tuned to ch2 but the channel select switch is set to ch3. When the switch is on channel 2 there is different scrambled snow and NO sound. The cartridge slot is clean. In fact the unit worked normally until I "cleaned" it. All the solder joints look good. None of the chips or components get hot to the touch. The voltage regulator shows 9v and 5v on opposite outside legs.

 

Might anyone have some ideas where to start?  I was thinking to removing the channel select switch and cleaning it some more.

 

My end goal is to do an AV mod. I got all the components and have done it before to other Jrs with success.

[stuartmcivor]

Well I can't explain what went wrong but whatever it was doesn't matter anymore.

 

I removed, cleaned and put back the channel selector switch. This did not improve the scrambled snow picture. I had my doubts it would but before doing this when I wiggled with the switch the scrambled image would flicker.

 

I then spent the rest of the morning building and installing the A/V mod like I had planned before messing the jr up. Removed Q4, R56, R17 and C33. Pulled +5 from Q4, GRN from C33, video from R56, and audio from A3 near the top right of the bottom-most chip. Now it has good video and sound.

 

 

 

Maybe this information will help someone in the future with a similar issue.

[zylon]

your RF modulator was likely bad

[stuartmcivor]

Yes, you are probably right. It did work using the RF connection before I cleaned the board with isopropyl alcohol and a toothbrush. I have no idea what component was damaged during the cleaning. None look damaged and I wasn't rough with the brushing. I am happy it works after the A/V mod which I intended doing all along.