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SX-990 Restoration

Discussion in 'Pioneer Audio' started by Ross Henning, Apr 15, 2018.

  1. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    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 AK Subscriber Subscriber

    Messages:
    106
    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 AK Subscriber Subscriber

    Messages:
    106
    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 AK Subscriber Subscriber

    Messages:
    106
    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?
     
  6. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    Major progress tonight on getting the muting and MPX to work. I replaced a factory resistor that didn't match the value on the schematic. This resistor grounds the voltage that feeds the muting/mpx circuit, and the value in-place was less than 1/2 the resistance specified on the schematic. I also changed out one of the diodes that rectifies the A/C to feed the muting and mpx circuit with DC. After doing that, and another alignment of the IF section, I got enough voltage on the MUT pin on the IF board to activate the muting and MPX.

    I have a question for my Pioneer restoration friends out there though. Does anyone have a source/part number for an alignment tool that will fit the Pioneer coils and allow access to the bottom core from the top of the board (through the first core into the second without disturbing the top). I have one that is sooooo close, but no cigar. It's just the slightest bit too small and slips. I've been searching high and low online, but can't seem to find one that looks right.
     

     

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  7. Watthour

    Watthour Electron Rancher - JS3600

    You might consider altering a standard GC alignment tool by shortening the hex on the undercut end of the tool. I ran into this on an old Sansui where there was not enough depth to slide the hex through the upper core and then orient it to mesh with the lower core without turning the upper. The unfortunate part is that the undercut tools are already somewhat weaker than the stepped end of the tool, and sticky cores can become a major challenge.
     
  8. Ross Henning

    Ross Henning AK Subscriber Subscriber

    Messages:
    106
    Location:
    Pinehurst, TX (Houston Area)
    I'm getting really close to having the SX-990 completed. Now that I've got the FM section all tuned up and have my replacement tuning and signal meters mounted, I've noticed that the signal meter seems to be way off reality. It reads about 4.5 on a signal that should produce about a 2 on it's scale. As best I can tell, there's no adjustment for the signal meter on the IF board where the meter is fed from. AM side seems about right. Should I expect this meter to be at all accurate? If so, should I simply place a resistor in-line on the wire that feeds the meter, or is there an adjustment somewhere I'm not aware of?

    UPDATE: Okay, I answered one of my own questions. The resistor doesn't do the trick. There's no linearity to what's coming out of the IF section. If it picks up a station, it's about 4 on the scale. A stronger station goes to 4.5. No station is about a 2. When the power is off, the meter goes to zero. I tried a resistor, and then fed my RF generator into it. I could back way off of the signal coming out of the generator and the meter would stay up close to 2. One more step down and the meter falls to 1-ish. Without the resistor, these numbers are 4 and 2. Based on the fact that the AM section pushes the meter at about the right amount, I'm guessing that the FM section is not driving the meter correctly. Any ideas?
     
    Last edited: Apr 30, 2018
  9. Tripqzon

    Tripqzon AK Subscriber Subscriber

    Messages:
    3,260
    Location:
    Maine
    My 990 has the same issue. The meter is at 4 for any station it is tuned to. Doesn't matter if it is a strong or weak signal level. I have found no way to fix this. Does anyone know if this is just the nature of this beast?
     
  10. Watthour

    Watthour Electron Rancher - JS3600

    Theoretically, R11 could be replaced with a 6.8K fixed and 10K trimmer in parallel, allowing adjustment of the meter driver from the FM IF board. Adjustment would be limited, however. Changing C17, C18, and D4 may also help restore operation without affecting the IF section performance or tuning.

    In fact, you may have one of those instances where recapping the IF board may be beneficial overall, but of course, that would also require alignment after the replacements are installed. Leakage of any of the caps in the circuit can degrade performance, but it the audio program sounds normal, there may not be significant degradation or leakage.

    It may be good to replace the IF stage drivers for that section if there is any suspicion the originals may be weakening, but in doing so an alignment would be necessary. The original TA7060Ps are not out there in abundance, but can be replaced with NTE1104s if they are determined to be degraded. Q3 and Q4 would be the initial stages, but don't proceed with any replacement of these SIPs without testing the circuit performance FIRST.
     

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