Pioneer SX-1280 with Hum in the Audio

[QSilver]

One thing you could try is this. On most Pioneer tuners the IC's are fed power from the +13V through a resistor. I guess the reason here is that if something goes wrong, the resistor limits the current to an IC that may have gone faulty or shorted and won't let it pull the 13V regulator down.

You could follow through the schematic and check these voltages before and after the resistors. You will at least find if the chips are toast or if they might be ok. You might be lucky.

I once did a similar thing with a little SX-440 - accidentally shorted out the 13V regulator and gave everything around 25V and thought I'd killed it. Turned out it was fine in the end - I think I'd just damaged the regulator. It's not the same situation exactly but just shows sometimes you can be lucky.

 

[Brivan]

One thing you could try is this. On most Pioneer tuners the IC's are fed power from the +13V through a resistor. I guess the reason here is that if something goes wrong, the resistor limits the current to an IC that may have gone faulty or shorted and won't let it pull the 13V regulator down.

You could follow through the schematic and check these voltages before and after the resistors. You will at least find if the chips are toast or if they might be ok. You might be lucky.

I once did a similar thing with a little SX-440 - accidentally shorted out the 13V regulator and gave everything around 25V and thought I'd killed it. Turned out it was fine in the end - I think I'd just damaged the regulator. It's not the same situation exactly but just shows sometimes you can be lucky.

Well, after replacing the tuner board three times, then replacing a couple IC's, I'm happy to say I have a normally-operating receiver. I have two spare tuner boards for an SX-980, both of which were installed in the SX-1280 (one after another), with the same bad results. I should note here that these spare boards were not previously tested by me, so I can't say if they're fully operational - the person I bought them from said they were, but you never know. With both boards, I got working AM radio, but FM was more or less silent, with no indication of signal reception at all, as well as the FM stereo light illuminating in every function except FM. After monkeying with those two boards, I did some more digging into the schematics and found the FM multipath function (which the SX-980 doesn't have) includes some different circuitry on the tuner board. Because of this, I'm nearly certain you can't swap tuner boards between an SX-980 and an SX-1080/1280.

I reinstalled the original tuner board and ended up replacing the HA1197 AM tuner IC, as well as the PA3001 FM tuner front end IC. That's all it took to get the tuner back to functionality. I'm glad the PA1002 survived - I have spares, but they're hard to find.