nickhead
Active Member
Hi all. I am moving this thread from the solid state forum to the tuner forum as it might be more appropriate here. I have experience working on older audio equipment, but generally don't work on these "newer" IC controlled machines. I got this one to give it a shot. The problem, as described to me when I got it, was that it would not lock on FM stations, even strong local ones. I found online that quite a few others have this same issue with this tuner, but no one seems to know the solution. While troubleshooting I found that the problem will go away in about 15 minutes and lock stations in stereo if it was left on. Nothing obvious stuck out as I looked it over.
Voltages all looked good with the exception of pin 9 of U104 it's a PLL FM MPX demodulator LA3400N that I can't find a datasheet on (a main reason why I don't like working on these machines with the mysterious little black boxes all over inside them). The schematic says it should be 0V, I see 3.25V. The only thing I can find on that pin is in the service manual and all it says is "Vcc on muting". I have 3.25V there all the time in every mode. Not sure if that's an issue or if the schematic is just wrong.
I also saw voltage missing at the collector of Q114. I reflowed him. No change. Removed him and he tested good. Re-installed and now have the correct voltage there. Didn't fix the problem though.
I've found that I can "fix" the problem for a few seconds by pressing the "memory" button. When it lights up and is waiting for you to press the button you want to store the station in everything works perfectly and sounds great. After it exits memory mode back to static and no lock. Weird...must be them mysterious little black boxes...
This tuner/amp is otherwise in great shape. Seems like it hasn't been abused. The normal heat related issues, like the fried underrated resistors, bad solder connections, etc. do not exist on this amp. Does anyone have any ideas on what could cause this issue and where to look to try and fix it?
Nick
Voltages all looked good with the exception of pin 9 of U104 it's a PLL FM MPX demodulator LA3400N that I can't find a datasheet on (a main reason why I don't like working on these machines with the mysterious little black boxes all over inside them). The schematic says it should be 0V, I see 3.25V. The only thing I can find on that pin is in the service manual and all it says is "Vcc on muting". I have 3.25V there all the time in every mode. Not sure if that's an issue or if the schematic is just wrong.
I also saw voltage missing at the collector of Q114. I reflowed him. No change. Removed him and he tested good. Re-installed and now have the correct voltage there. Didn't fix the problem though.
I've found that I can "fix" the problem for a few seconds by pressing the "memory" button. When it lights up and is waiting for you to press the button you want to store the station in everything works perfectly and sounds great. After it exits memory mode back to static and no lock. Weird...must be them mysterious little black boxes...
This tuner/amp is otherwise in great shape. Seems like it hasn't been abused. The normal heat related issues, like the fried underrated resistors, bad solder connections, etc. do not exist on this amp. Does anyone have any ideas on what could cause this issue and where to look to try and fix it?
Nick