Connect with us

Stage Line 500W amp repair project

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

Scroll to continue with content
  1. bikemech

    bikemech

    397
    40
    Oct 23, 2014
    Still need to be removed, cleand and pasted before final fitment 20210113_182140.jpg
     
  2. WHONOES

    WHONOES

    1,034
    282
    May 20, 2017
    IT's good that you have the right hand side working now. Have you tested with any audio or a signal generator?
    It would be worth removing the fuse from the + supply and inserting your DMM across the fuse receptacle and switched to a current range, probably 200mA, and then switch on and see what current is flowing in the O/P stage. I would expect it to be in the range of 10 to 50mA. Do this with no load on the output and with ideally the the power Amp input shorted. If not turn the volume control down to minimum and the measure. Make a note of the value.
     
  3. bikemech

    bikemech

    397
    40
    Oct 23, 2014
    Sorry bit am not sure what youu mean, basically i fitted the outputs and connected a small speaker to the output just to see if there was amplification, left side does produce sound but no more than the input when on full gain, right side is much louder ,
    Pete
     
  4. WHONOES

    WHONOES

    1,034
    282
    May 20, 2017
    OK. That more or less answers my question. I was interested in how much quiescent current is flowing in the output of the working amp'. I suggest you now use the working amp as a template for the non working one. Take strategic voltage readings form the working one to diagnose and fix the non functioning one.
     
  5. bikemech

    bikemech

    397
    40
    Oct 23, 2014
    Yeah that was my thought's to
     
  6. bikemech

    bikemech

    397
    40
    Oct 23, 2014
    I think this amp is cursed , although the right chanel seemed to be working ok yesterday it, now sounds terrible, like its over deviating, even with the imput signal low, so annoying!!
     
  7. bikemech

    bikemech

    397
    40
    Oct 23, 2014
    Would it be possible to connect an input signal straight to the right side amplifier and cut out the normal input stage ?, my reason is to try work out if the distortion is pre amp circuitry or the actual amp ?
     
  8. WHONOES

    WHONOES

    1,034
    282
    May 20, 2017
    I think you need to go back a step and measure the quiescent current as I suggested earlier. Also, measure the d.c voltage on the output with no signal present and the input shorted. This should be low, certainly less than 100mV.
    You really must get one bit at a time working correctly otherwise you just end up chasing your tail all the time.
     
    bikemech likes this.
  9. WHONOES

    WHONOES

    1,034
    282
    May 20, 2017
    Do you have access to an oscilloscope and a signal generator?
     
  10. bikemech

    bikemech

    397
    40
    Oct 23, 2014
    Thanks for the advice,
    When you say input shorted, do you litteraly mean short + to -- where the input signal comes in,
    Also when you say output, where exactly do you mean?
    I can take as many readings and change as many parts as required but you need to be clear with your instructions,
    As i said before, i am still learning, i know 12VDC single phase and three phase AC electric realy well, i do it for a job now,
    Its the 0V part i have difficulty in understanding but i am getting better,
    So, what would you like me to test ?
    Thank you for your time and help, it realy is appreciated,
    Pete
     
  11. bikemech

    bikemech

    397
    40
    Oct 23, 2014
    No i dont sorry, although i was thinking of buying an oscilloscope
     
  12. bertus

    bertus Moderator

    1,336
    525
    Nov 8, 2019
    bikemech likes this.
  13. bikemech

    bikemech

    397
    40
    Oct 23, 2014
  14. bertus

    bertus Moderator

    1,336
    525
    Nov 8, 2019
  15. WHONOES

    WHONOES

    1,034
    282
    May 20, 2017
    Those two items will be of more use than you could possibly imagine. They will make your fault finding so much easier. You will have to take a little time to get accustomed to them, especially the scope where you will have interpret what you are seeing and how to set it up. It won't take too long though and will be worth all the effort you put into it.
     
    bikemech and bertus like this.
  16. WHONOES

    WHONOES

    1,034
    282
    May 20, 2017
    Ok. I will try to be more explicit with my instructions.
     
    bikemech likes this.
  17. WHONOES

    WHONOES

    1,034
    282
    May 20, 2017
    To check the output voltage, short the junction of R69 and C42 for the R.H. channel or R12 and C34 for the left hand channel to the nearest 0V connection on the PCB.

    You will sometimes see 0V referred to as Ground. These are not to confused with Mains Earth.

    Next make sure there is no load, test resistor or speaker, on the output of the channel you are testing.

    Connect + lead of your DMM to the O/P of the Channel under test and the - lead to 0V at the power supply.

    Set your DMM to 2V d.c. or whatever range you have in that region.

    Turn on your amplifier and check the voltage reading on your DMM.

    It should be less than ± 100mV or preferably less than ± 50mV. If you get substantially more then you have a problem.

    You will next need to check the quiescent current but we will deal with that on another post.
     
    bikemech and Martaine2005 like this.
  18. bikemech

    bikemech

    397
    40
    Oct 23, 2014
  19. bertus

    bertus Moderator

    1,336
    525
    Nov 8, 2019
    Hello,

    They are comparable, but the second one has some more auto calculations in it.

    Bertus
     
    bikemech likes this.
  20. bikemech

    bikemech

    397
    40
    Oct 23, 2014
    Are these points still valid ?
    I will try to xo this tomorrow and post my results
     
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day

-