Dead AM section in Onkyo TX-51

[EngineerNate]

I've just finished working on an Onkyo TX-51 I purchased to get a friend a functioning vinyl playback system. She mentioned that she listened to the radio sometimes so I did some work to get the FM tuner working. The trimmer coils for distortion and IM were damaged so I ordered a near-identical board from ebay and cannibalized it's parts to fix the one in her receiver. I don't t have a signal gen or distortion analyzer so I just left everything alone aside from the IM trimmer and I simply adjusted that until the auto tuning function worked properly. For now that's good enough.

I noticed that both before and after the repair the AM section is dead silent. I thought it was just because it didn't have any antenna. My first repair attempt was to swap the entire "new" board in, but unfortunately the board was from the TX-61 and not the 51 and had some functionality that wasn't copacetic with the TX-51 so I had to pull it. During that testing though I noticed the Am section was working, though very staticy. All bands gave white noise and some had weak signal (no antenna, so expected).

Assuming there's not some muting circuit I'm not aware of, if it's dead silent like this I'm thinking there's an open cap/resistor in the signal path, the AM chip (LA1240) isn't getting power or is dead, or one of the other active components in the AM signal path is toast. The digital tuner readout works correctly so I think the oscillator board is fine. Am I on the right track here?

In all honesty I probably won't put more work into this piece because my friend doesn't listen to AM, but I'd like to at least know what the path forward would be if I were going to troubleshoot it. Tuners are a different beast.

Cheers,
Nathan

 

[chazix]

I thought it was just because it didn't have any antenna

I'm not clear on whether you have tried connecting an AM antenna. This receiver will need one in order to get much more than silence out of the AM section. It is designed to work with a wire loop antenna for AM. (You may have encountered other, generally older, receivers or tuners where there was an AM bar antenna built-in. That's a different kettle of fish.)

Sorry if I'm telling you something you already know,

chazix

 

[EngineerNate]

I'm not clear on whether you have tried connecting an AM antenna. This receiver will need one in order to get much more than silence out of the AM section. It is designed to work with a wire loop antenna for AM. (You may have encountered other, generally older, receivers or tuners where there was an AM bar antenna built-in. That's a different kettle of fish.)

Sorry if I'm telling you something you already know,

chazix

I haven't connected an antenna yet. I figured I needed one but when I swapped boards I at least got static. It seems strange to me that it's dead silent. I do have a cheap loop antenna coming from amazon.

 

[EngineerNate]

I'm not clear on whether you have tried connecting an AM antenna. This receiver will need one in order to get much more than silence out of the AM section. It is designed to work with a wire loop antenna for AM. (You may have encountered other, generally older, receivers or tuners where there was an AM bar antenna built-in. That's a different kettle of fish.)

Sorry if I'm telling you something you already know,

chazix

I'm an idiot and connecting an antenna had it picking up a slew of stations.

Had to press the mono button to get equal sound in both channels but that's not a problem in my book for talk radio...

Cheers!
Nathan

 

[scoloheros]

I have this same model, never have plugged it in. What do you think of it? I read it cost $400 new.

 

[EngineerNate]

It's a nice sounding little receiver. I'm not a big fan of units that use STKs simply because they're hard to track down and impossible to substitute for but it's nicely put together and has MM/MC capability which is somewhat rare at the prices these go for.

 

[dr*audio]

All AM tuners require an antenna to get any reception.