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

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

  1. scootsity5

    scootsity5 Active Member

    Hello all,
    I am setting up a desk nearfield system. I got some Mimimus 7s, and I picked up an SA-100B to drive them. The SA-100B is not working. It has two ins, an RIAA phono, and a regular one labelled "Tuner". I can get sound when I play through the RIAA in, but the Tuner in is silent, I just get a bit of static. Before playing anything, I cleaned all ins/outs and pots with deoxit. The switches are a bit harder to get to, so I didn't clean them. Given that I can get sound from the phone, and not the Tuner in, I'm assuming the issue is somewhere either in the preamp or the switch (certainly hoping for the switch). Anyone have a service manual or schematic for the SA-100B, there is none on either the AudioKarma Database, nor on Hi-Fi Engine. Any help would be appreciated. I'm finishing up graduate school over the next two weeks, so this may be a bit of a backburner type of thing until then, but I would love any input I can find.

    I did notice, that when I played the audio through the Phono in, that it was pretty quiet, I'm wondering if the pre isn't working, and I was just hearing the transformers amplify the signal.
     
  2. scootsity5

    scootsity5 Active Member

    Okay, update time. Turns out the switch was just oxidized, some deoxit cured that. As I did that, the wires fell off the power switch. Resoldered those. Did a quick test, seemed to be working all right. Recased it, put it in, and my god that's a lot of noise! I'm pretty sure there's a ground loop somewhere, because the ground channel seems to not be at 0V, closer to about 2VDC, which is odd. I'll try to figure out why/where later this week.
     
  3. typesix

    typesix Active Member

    The phono input is not RIAA, it is the same as the tuner input. This amp is not designed for magnetic cartridges.
     
  4. scootsity5

    scootsity5 Active Member

    what are the two transformers in there for? when iI pluged it in to the phono, it sounded very dark. are you sure? iI know tat was true for the sa 10
     
  5. typesix

    typesix Active Member

    I have one that's in storage. When it was in use, a Voice of Music turntable with ceramic cartridge fed into the phono input. Plus, the owner's manual states a ceramic or crystal cartridge to be used. You can also go to look it up on http://radioshackcatalogs.com around 1970ish.
     
  6. scootsity5

    scootsity5 Active Member

    Sorry, I didn't mean to doubt you, I just figured the transformers stepped up the phono. Do you by any chance have the user and/or service manual?

    I've been thinking about the amp a bit, and have come to the conclusion that there probably is not a ground loop as the static doesn't go away or change when I touch the chassis or a ground point. I'm thinking instead, that some of the old copper wire, and really old solder are oxidizing, and parts look cold soldered. I'm going to resolder everything when I get a chance, and hope that fixes it. I'll also replace all of the wire. I may recap it while I'm at it, but I'm not sure yet.

    How does yours sound? It's a pretty simple design, I'm pretty optimistic about the sound quality. Mine is supposedly a later model, the transistors are horizontally mounted in a sandwich of Al on the back plate, I've seen others with more traditional, To-220 style transistors and sinks.
     
  7. Blast

    Blast I Wanna Rock Subscriber

    The transformers couple the outputs. They don't have anything to do with the pre-amp. Here's the schematic:

    [​IMG]

    Brian
     
    Last edited: Nov 18, 2012
  8. typesix

    typesix Active Member

    Mine has an annoying hiss. I was only a kid when my family bought it and it always hissed. Other than that, it's sounds fine. Others have stated they do not have the hiss. There is a permanent loudness compensation built in for bass.
    The user manual basically has hookup and use info, not much to read. The circuit diagram provided by Blast is also in the manual. Specs listed are:
    5 watts/channel @ 8 ohms
    1% distortion or less
    50 - 10,000 Hz response within 2 dB
    Loudness compensation is 10 db @ 50 Hz @ - 40 dB volume
    Hum is 60 dB below rated output
     
  9. scootsity5

    scootsity5 Active Member

    I'm having the same hiss (actually, I have no idea if it's the same, it's a loud white-noise, that is independent of volume). I thought grounding loop, but I'm thinking that's not it. I cannot figure it out. That schematic should help considerably. Thanks!
     
    Last edited: Nov 15, 2012
  10. scootsity5

    scootsity5 Active Member

    My next go is going to be just reflowing all of the solder (some look cold soldered) and throw in a bit more solder. If that doesn't work, then I guess replace the copious amounts of wire in it. After that? Recap, and after that? Reclaim it for parts.
     
  11. scootsity5

    scootsity5 Active Member

    Scratch that. New plan.
    The power supply on here is just a pair of diodes, with a cap between them (1F), there isn't even a rectifier. I'm thinking that is probably pretty noisy to begin with. If that cap started to drift below 1F, the chances of it getting noisier and noisier are pretty high. I'm going to get some scrap board and build a bank of caps, probably a 2F, 1F, and something small to bypass them. Solder that in, and see if it isn't quieter.
     
  12. Blast

    Blast I Wanna Rock Subscriber

    I remember the hiss from the day I bought my SA-100B in 1970. Didn't care if it hissed and didn't know or care much about transistors back then. But, as the years went on I thought maybe the hiss from due to the quality of the transistors.

    But, you say "loud" and I wouldn't have used that word to describe it. Is the hiss equal in both channels? Or, more to one side? If it's equal in both it could very well be the power supply (a pair of diodes ARE the rectifier). I would be interested in your results as you attempt to reduce the hiss.

    If the hiss is more prominent in one channel I might suspect a transistor of breaking down.

    :yikes: Speaking of a pair of diodes, are they drawn in backwards? I never noticed that before!
     
  13. scootsity5

    scootsity5 Active Member

    its in both. I've built a few power supplies, I'll do a few things to quiet it down, and see how that goes. I'll keep you posted. iI want low esr, cheap caps, so iI may need to order them. It may be a little while.
     
  14. scootsity5

    scootsity5 Active Member

    If the noise is really all from the PS, a simple option would be to use an LM317 with a big heat sink (there's room in the case) and get the supply really flat.
     
  15. typesix

    typesix Active Member

    I'm interested in your results as well, as I would like to use this cute little amp without the annoying hiss.
     
  16. Blast

    Blast I Wanna Rock Subscriber

    Interesting idea. Notice how the transformer stays on all the time? :D I'm sure it doesn't draw much but, no "Eco Mode" there. The SA-175 was the same way.

    And, cute it is. Too many memories of busting my butt cutting lawns in 1970 to build my stereo and record collection to let my 100 go!

    Speaking of 1970, here's a shot of the stereo in my bedroom. Note the Rat Shack Baffle-Pak speakers in their, ummm... baffle-paks. 14 years old then and I knew everything! :D

    [​IMG]
     
  17. scootsity5

    scootsity5 Active Member

    Well, I tried just bypassing the big cap that was in there with a new one. They clearly charged because I just set them down and one discharged quite violently. Anyway, it quieted a very small bit of the noise, but not anywhere near all of it. It may quiet your hissing. I need to look elsewhere, I'm thinking the diodes next.
     
  18. Blast

    Blast I Wanna Rock Subscriber

  19. scootsity5

    scootsity5 Active Member

    I'm having all kinds of problems. cleaned all of the pots, switches, and connectors. I'm getting some really, really loud static. My first thought was a bad power cap, I replaced that, and it got rid of the very tiniest bit of static, but the overpowering sound of static is still there. My DC offset is around .5VDC, and I've got no idea why. Also, the split transformer isn't split evenly. The transformer has a coil going to the light bulb, AC, then a center tapped coils going through a pair of diodes to rectify with a 1000uF transformer between the V+ and ground. The ground is off of the center tap. I expected the ground to be at 0 V, but it appears to be at 24V. The VAC between the two ends (from which he ground is center-tapped) is 38V, split with 24VAC on one side and 15VAC on the other. Shouldn't the ground be centered between the two? I'm very confused, and seriously considering junking this sucker.
     
    Last edited: Nov 17, 2012
  20. Blast

    Blast I Wanna Rock Subscriber

    The power switch connects the transformer secondary center tap to ground. Pretty sure it has to be truly in the center based on the schematic, above.

    I wonder if you have a flaky power switch (not the side with the pilot light) that is putting noise in the B+. Try jumping it.

    Are the diodes truly drawn correctly and that's a positive ground (I was just reading another post about positive ground)?

    In any regard, I'll get into the problem with you. But, I can't start until tomorrow morning, maybe around noon.
     

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