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Seeking advice on an SX-2500

Discussion in 'Pioneer Audio' started by PoDuck, Nov 6, 2018.

  1. PoDuck

    PoDuck New Member

    Messages:
    33
    A friend of mine brought me an SX-2500 that he had some issues with, and I am to a point where I need to see if it's worth fixing.

    It presented with no sound from the A channel, and very quiet sound from the B channel. I checked the fuses, and the fuse was blown for the A channel, so I checked for a short and found that one of the output transistors was blown short. On the B channel, I tested the output transistors, and one of them was blown as well. I then checked all the driver transistors, and two of them were blown. Because of this, I decided to just replace all four driver transistors, and all four output transistors.

    Now, after replacing the transistors, I tested the voltage going to them. I expected to see the same voltage going to both channels, but instead I found around 35 volts going to the output transistors on one side, and 97 volts going to the other side. Not having anything telling me what the voltage should be here, I can still assume that both voltages should be the same.

    I had also tried to get sound out of the speakers, and only got sound from the B speakers again, but it was still quiet.

    I hooked my oscilloscope up to the driver transistors, and noticed quite a bit of distortion of the type that suggests to me that possibly capacitors are bad somewhere. I also see no way to tune the power levels, so I am assuming that there is a bad component somewhere that is either adding resistance, or reducing resistance.

    Now, here's the problem I need advice on. After talking to this guy, he says that he paid someone else to recap this amplifier. I had noticed that there were signs of someone else having worked on this before, but I didn't see signs of a full recap. I only saw that a few caps were newer style caps, and most were the old smooth top Nippon Chemicon caps. This other guy, that charged him for a recap that was never done, was supposed to have attempted to repair the problem he brought it to me to fix, but wasn't able to do it. Now, given that I'm no real professional when it comes to repairing these old receivers, and I was able to diagnose some bad transistors, at the very least, I'm thinking this guy he went to didn't have a clue what he was doing. That makes me hesitant to try to go any further on diagnosing this thing, because I don't know what he tried to do.

    My first reaction at this point is to hand the thing back to him and tell him to cut his losses, because I don't want to go and figure out what this other guy did or didn't do. On the other hand, this could be something that can be easily fixed.

    Does anyone have any suggestions as to where to look, or whether or not to give up?
     

     

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  2. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    613
    Location:
    La Crosse, WI
    Not sure about where to look - those are some strange problems. However, the SX-2500 is both scarce and not valuable. Very few of these come up for sale. They were TOTL in the Pioneer catalogs 1970 - 1972, so they outlasted other receivers that usually were only cataloged for two years. The 2500 is also the highest power output cap coupled receiver Pioneer made. So, if you like that sound and want power, this is it. It also has the unique auto-tuning feature, if that turns you on. I happen to think it is pretty cool and fascinating. The tuner is excellent in these units.

    So, is it worth saving? From a quality receiver/feature standpoint, I say yes. From a value standpoint, probably not. You will likely put more time into it than they sell for.

    The one I have, I've put some time into, recapped the PS, Main Amp and Protect circuit as well as change all transistors in the main amp to eliminate static, but it still has a low background hum. However, even with that issue this is a really great sounding receiver if you like the cap-coupled sound. In my humble opinion, a very underrated receiver.
     
    PoDuck likes this.
  3. PoDuck

    PoDuck New Member

    Messages:
    33
    Background hum, in my experience, if things are properly grounded, and caps are good, is probably a biasing issue, and I don't see any bias adjustment on this thing, so that would suggest a resistor that has gone out of spec.
     
    KeithD likes this.
  4. KeithD

    KeithD AK Subscriber Subscriber

    Messages:
    613
    Location:
    La Crosse, WI
    Thanks for that thought. There are trimmers on the main amp board. Mark the Fixer has a procedure for setting the bias here:

    http://audiokarma.org/forums/index.php?threads/sx-2500-bias-adjustment.526411/

    I have not done that yet, probably should work on that next!
     

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