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Stage Line 500W amp repair project

Discussion in 'General Electronics Discussion' started by bikemech, Oct 23, 2014.

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  1. bikemech

    bikemech

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    Oct 23, 2014
    Will the new caps have to go a certain way round? Ie have cathode and anode . +. -
     
  2. davenn

    davenn Moderator

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    that is why so much stuff is rarely fixed by service companies these days

    and that is the only good reason :)
     
  3. davenn

    davenn Moderator

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    they are non-polarised caps, orientation is unimportant :)
     
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  4. bikemech

    bikemech

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    Oct 23, 2014
    Ok so just learnt there non polarised unless stated ??
     
  5. davenn

    davenn Moderator

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    LOL yup

    there are 2 main types of polarised capacitors, electrolytics and tantalum
    and just to be confusing, there is a range of non-polarised electrolytics but they should always have
    for example say 220uF 50V NP the NP indicating non -polar

    all other types of caps ... monolithic, mylar, polyester, polystyrene, ceramic are all non polarised
    ( probably missed a couple) .... BUT they all have a max voltage rating

    SMD tants .....
    see the bar across one end ? that's the positive end

    tant1.jpg

    legged tants ...
    see the + symbol ? that's the positive leg

    tant.jpg


    Electrolytic caps

    virtually always have a NEGATIVE stripe down one side

    electro.jpg

    and on new ones the positive leg is always longer than the negative leg
    That usually applies to legged tants as above too

    cheers
    Dave
     
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  6. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Yeah, I'm sorry about that.

    These days, you pay a price if you want to be responsible and sensible, and repair your old electronic equipment. Most people simply throw it away and buy a new, cheap, poorly made replacement (unless they're wealthy enough to buy a properly made replacement). It may not last long, and it's false economy, but at least it has nice shiny metallic paint, right?

    Back in my day (he says, pulling on his long white beard), there were lots of shops selling replacement parts. We even had a specialist shop called Wisemans (Wiseman Electric, actually) that had almost any IC you wanted to buy, individually foil-wrapped in a little plastic drawer in a big grid on the wall behind the counter. Old Mr. Wiseman would charge like a wounded bull, but he needed to, really, to keep the place going.

    Wisemans was nearly unique in Wellington because the staff were electronics folks themseves, and could answer technical questions and give good advice. So the staff at other shops would send customers to Wisemans if they had difficult questions, and that wasted quite a bit of time.

    In the mid to late 1990s the writing was on the wall as far as local specialist electronic components suppliers were concerned. Farnell and RS Components had a wider selection, a nice colour catalogue, and often, better prices. If you had Internet access, and could wait a day or two, they were a better choice than Wisemans. Courier companies were the big beneficiary.

    Wisemans tried diversifying into security and spy equipment and other consumer products, then finally shut up shop, over ten years ago I think. Valve (toob) collectors all over New Zealand rejoiced! Cheap NOS ("new old stock") valves!

    Nowadays a place like Wisemans would be totally impractical, given the sheer number and variety of electronic components available. It makes sense to have companies like Digi-Key and Mouser with huge warehouses on cheap land, supplying huge numbers of parts all around the world.


    In this case, a thorough and careful inspection of the board earlier on could have identified those resistors, and you could have ordered their replacements along with other batches, to save on shipping costs (which are the biggest factor for small repairs now). That's a lesson learned I guess.

    And you certainly are learning. Not much about electronic theory yet, but a lot about practical things like measuring components and soldering and desoldering. This is really important hands-on experience and you will get a lot of satisfaction when the repairs are complete. Whether it's worth the price is a decision only you can make, but I think it's too soon to say at the moment.

    I'm going to try to sneak some theory into my explanations, since you're taking to this stuff pretty well and you obviously learn quickly. I hope you'll find it interesting. I can pretty much guarantee that you will :)
     
    Last edited: Nov 22, 2014
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  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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  8. bikemech

    bikemech

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    Oct 23, 2014
    Ok hopefully this is the last of the new parts
    [​IMG] Going to start fitting the later tonight
     
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  9. bikemech

    bikemech

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    right all the new componants are fitted!!!

    new driver transisters in place but not soldered in, put 2SA1859 PNP IN Q13 AND Q34
    and 2SC4883 NPN in Q12 and Q26,
    think this is right,
     
  10. bikemech

    bikemech

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    Oct 23, 2014
    must remember to get some thermal paste to!!
    think i might have a trip to maplins tomorrow.
     
  11. bikemech

    bikemech

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    Ok got some thermal paste today, fitted the board back in the case, fitted the resistor fuses, and turned it on , no blown fueses :)
    But still have fault left led lit, going to test voltages later tonight, will post my results.
     
    Last edited: Nov 27, 2014
  12. bikemech

    bikemech

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    Oct 23, 2014
    kris are you sure its T4 and T9?? i am sure the negative outputs to the speaker sockets go to them!
     
  13. bikemech

    bikemech

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    will test anyway,

    meter set to 200VDC, black probe on T4
    D9 cathode=66.9v
    D10 anode=66.9v
    D21 cathode=66.7v
    D32 anode=66.9v

    with the meter set to 2000m
    R39=00
    R68=008

    no blown fuses anywhere but fault left led is lit !
     
  14. bikemech

    bikemech

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    Oct 23, 2014
    will it make any differance that i have refitted the heat sinks? think you said not to!
    no drivers or outputs soldered in tho
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK good! Sorry for the slow reply.

    So the drivers aren't fitted? OK. Did you clean the heatsinks carefully? If not, do that. But leave the output transistors out.

    Then fit the drivers and clip them to the heatsink, then solder them. Then power up again. Assuming no fusible resistors go pop, check the voltages across R30, R32, R62, and R71. Be careful not to let the multimeter leads slip. You may want to measure the voltages on the underside of the board, to reduce the likelihood of probes slipping.

    Whenever you work on this board, remember that there are some fairly high voltages there - not mains voltage, but still enough to give you a shock. And currents can be high too, although they will be limited by those resistor fuses. So be careful not to touch anything when you're working on it!
     
  16. bikemech

    bikemech

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    Oct 23, 2014
    It's not possible to get to the underside of the board while its in the case, also I don't think the resistor fuses are working properly, I removed one of the fuse able resistors and turned it on, the led flickered on then went off! Possibly because I used 2 LES's farther than 1 bycolour one?
     
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Two LEDs reverse-connected like that is fine. It's possible that the LED will flash ON for a moment at power-up while the circuit settles down with the right DC conditions.
     
  18. bikemech

    bikemech

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    Oct 23, 2014
    if i remove the resister then turn it on they do the same??
     
  19. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK, then don't remove the resistor. Why did you remove the resistor? I never suggested that. Why are you surprised if something odd happens when you remove the resistor?
     
  20. bikemech

    bikemech

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    Oct 23, 2014
    Because I tested them with a 12v power supply just after I made them, with the resistors removed, I assumed with them being
    Fuseable they would brake the circuit when they blow, as I seen them flash I didn't know if they were working or not and assumed by removing the resister this would tell me,
     
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