SX-990 Restoration

Discussion in 'Pioneer Audio' started by Ross Henning, Apr 15, 2018 at 9:33 PM.

  1. Ross Henning

    Ross Henning New Member

    Messages:
    17
    Location:
    Pinehurst, TX (Houston Area)
    Hi Guys,
    Well I'm starting a few Pioneer restorations now, including an SX-990. I have a really simple question that I'm actually a bit embarrassed to ask, but I figure it's better to be safe than sorry. I'm installing LEDs in place of the bulbs in this unit. I've ordered white LEDs that should be a direct replacement for the existing incandescent bulbs, but there were three places where I had to do something different. For the dial pointer, there is no socket, and I was able to fit a flat SMD mounted white LED in place of the old wheat bulb along with a resistor to drop the voltage. The other spots are behind the two meters, the sockets for the two bulbs were messed up, and actually shorting out. For those, I fashioned two small Vero boards with SMD LEDs mounted on the Vero along with resistors. The LEDs look great, but because AC fed the original bulbs, I can see the 60 cycle "flash". All of the lamps in the receiver are powered by an 8V secondary off of the main transformer. One side of the 8V AC is tied to chassis ground, and the other side runs through a fuse. I'd like to insert a rectifier and a filter cap to smooth out the power to the LEDs at this point before the power is routed to the selector switch. It's simple enough, but the one thing that really bugs me is that I'm going to have one side of the AC tied to the same ground that will also be the negative for the DC side. That just feels wrong to me, but when I think through it, it seems to me that it would work. I just wanted to run it by the experts before I build my little rectifier circuit. I'm used to the secondary coming off of the transformer with both legs tied to the rectifier and neither to ground.
     

     

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  2. Ross Henning

    Ross Henning New Member

    Messages:
    17
    Location:
    Pinehurst, TX (Houston Area)
    So I think I'm going to answer my own question here. I thought about it further and realized that this would actually be equivalent to tying one A/C lead on a bridge rectifier to the ground lead on the DC side. Obviously, that won't work, or we'd be buying 3 legged DC rectifiers. I think I'm pretty much out of luck as far as doing what I originally wanted. I think I'm going to have to swap out my LEDs with some that are made for this purpose. The must have a tiny built in rectifier circuit, because the ones I've seen do not flicker.
     
  3. Watthour

    Watthour Electron Rancher - JS3600

    It might be possible to rewire the 8V lamp circuit so that the wiring to the lamps is dedicated solely to the lamps. Install a bridge and filter cap and run wire to only the LEDs. Keep the rest of the 8VAC circuit intact since it also affects the stereo indicator and tuning meter lamps.
     
  4. Ross Henning

    Ross Henning New Member

    Messages:
    17
    Location:
    Pinehurst, TX (Houston Area)
    I certainly considered that, but unfortunately, the "ground" side of the lamps is all exposed metal and in places is attached directly to the chassis. It was just easier (although not as cheap) to buy the LEDs made for this purpose. Thanks for the suggestion though.
     
  5. Ross Henning

    Ross Henning New Member

    Messages:
    17
    Location:
    Pinehurst, TX (Houston Area)
    I have another question for my SX-990 that I'm hoping for input on. I have been able to tune FM, but I wasn't able to tune FM stereo, nor could I turn on the "Muting" switch and get anything. after a bunch of tracing, I figured out that on the IF unit board, there's a point that should produce a DC voltage by rectifying AC off of the 3rd amplification stage. I checked every part in that area, hoping that I had a bad diode or shorted resistor or something. As best I can tell, the TA7060P is bad and not producing enough output to drive the little twin diode rectifier to any output. If I jump a couple of pins on the muting board to provide DC voltage as would be received from this circuit, things work as expected. The weird thing to me though is that looking at the circuit, I would think that if the TA7060P were in fact bad, I shouldn't get FM reception at all, as I would think it would kill what's coming from the first two stages. Anyone have any thoughts/experience on this?
     

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