Realistic SA-100B Pre-Amp (maybe) Problems

Discussion in 'Solid State' started by scootsity5, Nov 13, 2012.

  1. scootsity5

    scootsity5 Active Member

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    I think that they are, they are cerrtainly the facing the same direction on the board. It definitely is a postive ground

    I threw my multimeter on in amps mode across the switch, no difference. I didn't have any ps cap in at the time, as I was trying different values, but the PS caps remove the slight hum, not the static.
     
    Last edited: Nov 17, 2012
  2. scootsity5

    scootsity5 Active Member

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    I'm thinking tomorrow, I'll replace the power cap (35V 1000uF) with a 50V 2200uF, C24 in the above schematic (25V 470uF) with a 50V 1000uF, and the output caps, C21/22 (16V 470uF) with 16V 1000uF. Unless you think the transformer really is bad, in which case I'll junk it and either get a new transformer, or just junk the build, because I think the trasnsformer for this build is ~$30.
     
  3. Binkman

    Binkman AK Subscriber Subscriber

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    Just caught the tail end but I'll read after this at the beginning.

    Do not replace the output caps with higher values.. higher voltage is okay but not uf values.
     
  4. scootsity5

    scootsity5 Active Member

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    Those were just what I had on hand, figured higher rating would just diminish the roll off. My biggest problem is the long static, which may well be a transformer issue.
     
  5. Blast

    Blast I Wanna Rock Subscriber

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    I took my SA-100B apart. I decided to throw a couple of binding posts on it:

    BEFORE:

    [​IMG]

    AFTER:

    [​IMG]

    I now have more just in binding posts than the amp is worth. Story of my life... :smoke: Sure will be easier to hook up speakers, though.

    Let's get started on your problem, scoot.
     
  6. Blast

    Blast I Wanna Rock Subscriber

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    I had to clean my switches. Wiggling the power switch would cause the pilot light to blink. Also, had flaky right channel due to dirty selector switch. Works good now.

    With a line voltage of 121vac I have -22.6vdc on C25 (main filter cap). Yes, NEGATIVE 22.6vdc. Most definitely a negative voltage amplifier.
     
    Last edited: Nov 18, 2012
  7. scootsity5

    scootsity5 Active Member

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    iI ceaned the switches. iI have 24vdc on c25l
     
  8. scootsity5

    scootsity5 Active Member

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    There are the two red wires, coming from the transformer, what ac voltage do you get across them, and from each to ground? Also, is the ground at +24v with respect to the fuse?
     
  9. Blast

    Blast I Wanna Rock Subscriber

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    With no input signal I have 35.8vac on the secondary of the transformer (measured at the cathodes of each diode). Moving either probe from one of the cathodes to + side of C25 I have 17.9vac. I would call both identical.

    Yes, ground is + side of C25 (or you can use the - of speaker terminals).

    Let me ask this. Are you using the frame as ground? Or, the + side of C25? For these tests I would use the latter; don't measure to the frame of the amp.
     
  10. scootsity5

    scootsity5 Active Member

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    I measured from both the frame, and the + side of C25, both are at ~24V. Measuring from the transformer to the ground, I'm getting 24V on one side and 15 on the other. So, this is a transformer issue. I'm thinking this may not be fixable.
     
  11. Blast

    Blast I Wanna Rock Subscriber

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    Did you disconnect the transformer from the diodes to do that check?
     
  12. scootsity5

    scootsity5 Active Member

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    nope. actually, that's a good idea, I'm away from the amp right now, ill try that in a few
     
  13. scootsity5

    scootsity5 Active Member

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    I was thinking about it. You made an excellent point. That said, I don't want to remove the transformer from the board, it's really far back in there, and the solder globs are massive. Instead, I'm just going to go the other route, and replace the diodes. I've never seen a diode like that before, it's disc shaped. What diode should I replace them with? I'm also thinking that if I test it with the power switch off, such that the circuit is open and just check voltages between the switch V- and the V+, that should avoid the diodes and tell me if it's the transfo or the diodes. I'm thinking it's got to be the diodes, because what can possibly go wrong on a transformer, other than getting wet?
     
  14. Blast

    Blast I Wanna Rock Subscriber

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    I'm confused. Isn't it the transformer connection to the circuit board the two blue wires seen in the upper left of this photo?

    [​IMG]

    In my amp said blue wires are attached to the same pads that connect to the cathodes of the diodes. So if your board is similar won't you have to desolder these pads to remove the diodes anyway? Do as you wish but, can you not simply heat the solder up and lift the wire off the pad? No "desoldering" would be necessary.

    I don't understand what you are saying about the test with the power switch off, V- and V+, etc. Sorry....

    If you don't want to disconnect anything use the diode test on your DMM and test each diode. For instance, on my Fluke 12B set to diode test, on my amp I read .544 one way and, reversing the leads, open ("OL") the other way. Which tells me the characteristics of the diodes are similar to each other. One of your diodes could be leaky and this test would tell you so.

    As to the transformer it could very well have a "semi-shorted" winding in the secondary. While it's possible it's not probable, in my book anyway.

    If you still want to "shoot from the hip" go ahead with the diode replacement. As to what type of diodes I would think that any 1N400x would work.
     
  15. scootsity5

    scootsity5 Active Member

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    In thinking about it, everything you say makes sense. Well, my wires are red, but whatever. I think my diodes both read 5V in one direction and open in the other, I'll check again in a few minutes. I don't have anything that fancy (in the way of multimeters), but it's should certainly do the trick. It's an extech EX330

    I don't know why I was thinking the transformer was mounted differently, my mistake.

    I'm trying to finish my MS thesis tonight, so no promises that I will get a chance to test everything tonight, but I'll try (sorry, you've been so helpful, I don't want to drag this out...but my thesis has to come first).

    In my defense, for anything silly I say, I've been running on about an hour of sleep per night (average) for roughly a month now... but I'm in the final stretch!


    EDIT: Diodes tested: Both reading 0.51V, well, one is 0.511, the other 0.509, that isn't enough of a difference to matter, right?
     
    Last edited: Nov 18, 2012
  16. Blast

    Blast I Wanna Rock Subscriber

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    No. 0.511 and 0.509 are within reason. However, please confirm that you get get an "open" or OL in the other direction.

    EDIT: I'm sorry, you did say "open" in your first sentence.

    And if you do, then I'd call the diodes good. The next test is to check AC volts of each side of the transformer relative to the center tap. We need to find the voltage imbalance that you told me existed in post #19, above.

    I'm glad you're on this because I'm on the edge of my seat here! :yikes:
     
  17. scootsity5

    scootsity5 Active Member

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    The reverse is open.

    I'd hate to make you hold out any longer, but I can't pull out the transformer leads just yet. I need to get a few pages written. I'll try later on tonight, no promises.
    Sorry, this was probably a really bad time to start this project!
     
  18. Blast

    Blast I Wanna Rock Subscriber

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    OK. And, no problem. :D I'm just giving you a hard time!

    I appreciate you inspiring me to open mine up and put the binding posts in it! :banana:
     
  19. scootsity5

    scootsity5 Active Member

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    I found an STA-11 on craigslist today, I've been staring at these minimus 7 speakers so long, I just couldn't stand it. So, I got the STA-11, it's all right, lacks the character of the SA-100, and it's not quite as clear as my Yamaha or Sansui, but it will do until we get this sorted.

    Sure beats the hell out of the creative computer speakers I've been using for years. It's funny, I have a great set up in my living room, but these days I spend all day at the computer, and my nearfield setup was crap. This at least brings me a step closer. I do have a phenomenal headphone set up here, though.

    Anyway, updates when I can. I'm really, really hoping to finish my thesis up tonight, but well, I said that a few days ago, too.

    The SA-100 is really neat, because I noticed while I was working in it, that everything in there either still is or was previously stocked at radio shack. When it was released, I bet you could build a point-to-point version of the SA-100 out of any radio shack store.
     
  20. Blast

    Blast I Wanna Rock Subscriber

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    I hope the SA-100B doesn't know you're cheating on it. :tears:
     

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