Sony STR-3800L Stereo Receiver Help

Discussion in 'The Projects Forum' started by mroneshotaway, Aug 10, 2015.

  1. mroneshotaway

    Thread Starter New Member

    Aug 10, 2015
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    Hope every one is doing good :).
    Any way, I got my hands on an old Sony AMP, that's not super impressive with stats, but I would really like to get it back into working condition.
    When I've connected it to power, the bulb lid up and it seemed to work. As soon as I connected it to a speaker, it started producing very large amounts of noise, and with a kind of loop that makes its amplification larger over time (matter of secondes).

    I'm a really big noob, so I've tried Googling some things. Some posts suggested checking the transistors, so I did. The NPN once read around 0.53V B-C, B-E and the PNP's read around 0.6V too. The resistance for all transistors from E-C was okay, well at least it gave a large resistance and not something that is striving for a zero value. I've tried checking the speakers connections, and I've found out that the black connection, the minus one, at the B speakers isn't connected together. Maybe it's just an old wire, but what it's really that, how do I diagnose the problems?

    Therse a dual C filter cap, that means it's a cap based on two capacitors that have the same minus lead (Parallel?). Do I need to change that big cap? and can I change it with two of the same value capacitor? I've tried searching for the dual c cap, but no luck...
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  2. TheButtonThief

    Active Member

    Feb 26, 2011
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    Behind all the noise, can you hear any modulated sound?

    I can see it's an AM/FM radio receiver, does that mean that you're using the radio to play music or are you connecting your own audio source and using the unit to amplify it?

    Are you using the original speakers?

    Does the volume control have any effect on the level of noise?

    Do the speakers connect directly to the receiver or do you have to hook it up to an amp?
     
  3. mroneshotaway

    Thread Starter New Member

    Aug 10, 2015
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    No, I'm connecting my turntable to the phono connections. Pretty sure that last time I've connected my mixer to one of the tape inputs. I'm using my crappy cheap Sony speakers, they didn't come with the amp. I'm pretty sure not, but they must add noise because the pots are scratchy from all these years. Speakers go to the amp.

    I wanted to point out that from simple multimeter test I found out that the Minus (black connection) of the B speaker set showed no connection what so ever, something weird...
     
  4. TheButtonThief

    Active Member

    Feb 26, 2011
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    So is there any modulated sound (music) behind all the noise?

    When you turn the volume knob, is there any change in the noise?

    Also...

    What's the DC resistance of your speaker coils?

    Can you define "simple multimeter test"? Were you testing continuity or for a voltage present? And when you say "no connection", connection to what? Between what points exactly were you probing during your test?
     
  5. mroneshotaway

    Thread Starter New Member

    Aug 10, 2015
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    The rated impedance on the speakers is 6 Ohm's. Oh, I should've been more accurate. I used the option that tells you if you have a connection between the two tested point, the option that makes a buzzing sound when there is conductivity(That's a positive answer to your question, in a sum...)

    You think it's a good idea to try and connect the equipment again and try the knob test?
     
  6. TheButtonThief

    Active Member

    Feb 26, 2011
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    That would be a continuity test, the metre gives an audible indication that the resistance between the probes is less than about 50 Ohms (on some metres, it can be as much as 200 Ohms). Where were you probing, between what points? You mentioned that one was a black wire on the speaker, where were did you test with the other probe?

    Again, (I ask for a 3rd time because it's pretty important and so far you haven't answered) was there any music at all?
     
  7. mroneshotaway

    Thread Starter New Member

    Aug 10, 2015
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    Okay, I've connected every thing and came with some results. I've connected my turntable straight to the phono input and the amp worked just fine. There was some noise when moving the vol pot, but generally it was okay. FM radio worked too. The main problem starts when I connect my turntable to my mixer and to the tape input on the amp. As soon as I turn the amp in that config, no music is coming out, and blasting strong noise shooting out the speakers. moving the pots makes it worse, because they're super scratchy! And again, no music in the tape 1 and 2, just noise.

    I've included a photo of a random old amp from Google images, hope that I can explain it well.
    [​IMG]
    Look at the speaker connctions, you have left and right, and in between you have two common minus leads. I have two configs, one for a set of two speakers called A and another one called B, so that I can choose witch speakers I would like to hear my music from. When I try the continuity test on the two speaker connections in the speaker set A, I get a buzzing sound when putting the two leads in each one of the common minus connections (One for right and one for left), hence every thing is good. Now, in most amp you have two speakers set, like i've explained earlier - set A and set B - but when I do that same test, the common minus connections in speaker B down't have a connection, witch should be alarming, or i'm just a really derpy noob.

    I think that we should maybe put out attention to the tape connections and that part of the amp, correct me if I'm wrong.

    Hope to hear from you soon!
     
  8. TheButtonThief

    Active Member

    Feb 26, 2011
    219
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    Ok, try flooding the pots with a generous amount of switch cleaner and we'll go from their...

    I like the WD40 stuff: http://www.amazon.co.uk/WD-40-Specialist-Drying-Contact-Cleaner-x/dp/B006UCJ5WQ

    Get the straw right in their, press hard to release loads of it and don't be afraid of how wet things are getting, keep turning the knob through its entire range of travel while you do this. Do the same with the buttons and the connectors (power, photo ect...)
     
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  9. mroneshotaway

    Thread Starter New Member

    Aug 10, 2015
    5
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    Oh, I'll def try it. Too bad Amazon doesn't ship to Israel... Do you think it's a good Idea to go on and clean the solder joints in the phono/tape connections with isopropyl alcohol? As you can see, the 2nd photo features the PCB for the tape connections and it has some white crap on it. Hope to hear from you soon!
     
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