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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
    Ok drivers soldered in and clipped to heatsink, powered up, nothing went pop! Black probe on T4,
    Meter set to 2000m,
    R30 = 000v
    R32 = 000v
    R62 = 021v
    R71 = 021v
     
    Last edited: Nov 28, 2014
  2. bikemech

    bikemech

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    Oct 23, 2014
    So what's my next step, pretty sure everything was ok on both output channels, ! The amp has a bridged switch on the back don't know if this would help or make it harder, but when it's in bridged position rather than stereo I get no reading from R30
    R32
    R62
    R71,
    is it possible that the fault it's now on the main side of the board?
    Is it basically a shorted component that makes the fault LED light or is it controlled by an IC of some sort?
     
  3. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK, sorry for the delay.

    I'm confused about the voltage readings from post #101. You set your multimeter to the "2000m" range in the VDC section? That means it measures voltages up to 2000 mV, or 2V. Does that sound right?

    So those figures, like "021" for the voltages across R62 and R71, those should actually have been displayed as "0.021". Is that right?

    In that case the voltages are actually 0.021V which is 21 mV. That is a lot less than I expected across those resistors, and the 0V measurements you're getting across R30 and R32 are also wrong. There should be around 0.6V across each one, because that's the base-emitter bias voltage for the output transistors.

    Can you confirm everything I've said here, then I'll suggest some more measurements to take, so we can diagnose the problem.

    Re the bridge switch. This should have no effect on the output stages at all. The bridge/stereo switching is done at an earlier stage. Don't worry about it at this stage but we'll need to check it before we're finished.
     
    bikemech likes this.
  4. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    Also can you confirm that all components are installed as normal except:
    • All four output transistors not installed
    • All four fuses replaced with resistor fuses made with fusible resistors and LEDs
     
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  5. bikemech

    bikemech

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    Oct 23, 2014
    ok kris here it is again

    meter set to 200m VDC this time,
    and the black probe clipped to T4, which is negatige to left chanel speaker connector,

    left chanel,
    R30 reads 000 either side
    R32 reads 000 either side

    right chanel
    R62 reads 22.5 either side
    R71 reads 22.5 either side,

    i dont think its important to mesure either side but did anyway,
     
  6. bikemech

    bikemech

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    Oct 23, 2014
    yes all componants apart from the output transistors are installed and soldered in, and resistor fuses installed
     
  7. bikemech

    bikemech

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    Oct 23, 2014
    it would seem i am back to the very same fault i origanally started with, IE protect light on and fault left LED lit ;(

    also, i have a Q21 and a Q41 mounted on each chanel!!
    i can not find these anywhere on the schematic
     
    Last edited: Nov 30, 2014
  8. bikemech

    bikemech

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    Oct 23, 2014
    Any ideas yet kris ? The other 2 transistors are not listed on the schematic, or I can't find them anyhow.
     
  9. bikemech

    bikemech

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    Oct 23, 2014
    Anybody got any ideas wher to go next with this, really don't want to throw it away!
     
  10. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Nov 28, 2011
    I haven't forgotten you! I'll get back to you within 12 hours.
     
  11. bikemech

    bikemech

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    Oct 23, 2014
    thank you kris, i thought you may have given up in it
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Hi again. Sorry for the delay. I try not to let my real life intrude into my online life but it happens occasionally :)

    I can't find Q21 and Q41 either. Where are they on the board? Can you take a photo?

    OK, I'm still not clear on all of these voltages. Here's a schematic marked up with various places where I want to know the voltages. You can save this file and edit it to add the voltages in the boxes.

    For red boxes, I need to know the voltage at the connected point, measured relative to T4, the 0V reference point in the amplifier. For red/black boxes, I need to know the voltage measured between the two connected points. In both cases, the colours match the probe colours.

    When you're measuring a voltage with a manual ranging multimeter like yours, use the lowest voltage range that gives a meaningful number, and include the decimal point.

    Start measuring on any voltage range. If the display shows overload ("1." or whatever), try the higher voltage ranges one at a time, until you get a valid number. If the display shows a valid number, try the lower voltage ranges until it shows overload, then switch back one range. This will give the most digits on the display and the most accurate reading.

    Two other important things about making measurements.

    1. When you have the reading, use the indication and the voltage range to work out what the actual measurement is. The range marking on each position of the selector switch tells you the highest voltage the meter can measure on that range, and the unit (mV or V) tells you the unit that the number is displayed in. So for example if the meter is on the 20V range and it shows 8.43 then the actual voltage is 8.43V. If the meter is on the 200 mV range and it is showing 030 then the actual voltage is 30 mV.

    2. If there's a negative sign showing on the display, include that in the figure your post.

    epoint 270979 for voltage annotation.png
     
    Last edited: Dec 7, 2014
    bikemech likes this.
  13. bikemech

    bikemech

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    Oct 23, 2014
    here is a photo of Q21, this is on the LEFT chanel, there is also one on the RIGHT chanel
    [​IMG]
     
  14. bikemech

    bikemech

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    Oct 23, 2014
    this is the pinout for Q1 am i testing the base?[​IMG]

    the base has no reading but the collector has 67.0vdc

    will cary on testing the rest you asked me to, and gat back with my results.
     
  15. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Aha! They're not transistors; they're triacs. They're a special type of semiconductor that is used to "crowbar" a circuit if a fault is detected.

    q21 marked.png

    There's one for each channel, connected between the output and the 0V rail. If a fault is detected, the triac is triggered, and it puts a big fat short circuit on the output, like throwing a crowbar across the speaker wires. This protects the speakers from overvoltage, and causes at least one fuse to blow in the amp.

    One or both of these may have failed short. You should remove them to test them. There are several ways to test a triac, some more thorough than others. But you can check them for short circuit by measuring resistance (on any range) between the left lead and the middle lead (holding it so you're looking at the marked front face and the leads are pointing downwards), both ways round. It should measure open circuit both ways.

    Don't put them back yet.

    Yes, that's possible.
     
  16. bikemech

    bikemech

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    Oct 23, 2014
    [​IMG]
    i will remove the triac's tomorrow and test them:)
     
    Last edited: Dec 8, 2014
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    OK thanks for that.

    I'm only interested in the voltages at the marked points. So if I've indicated a wire that goes to the base of a transistor, just measure the base.

    Those voltages in the bottom half of the circuit should be negative!

    Can you also measure the voltage across R20, and the voltage between the base and emitter of Q6 please.
     
  18. bikemech

    bikemech

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    Oct 23, 2014
    i have already removed the triac on the left chanel,
    triac Q21 using the diode test range has no mesurement fron MT1-MT2 either way round so think its good,
    going to reinstall it and refit the board now and mesure across R20, and the voltage between the base and emitter of Q6
     
  19. bikemech

    bikemech

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    Oct 23, 2014
    R20 reads 00.0
    base and emittor of Q6 reads22.5--23.0
     
  20. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

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    Sounds like Q6 is shot.

    The base and emitter terminals of a transistor have an internal diode connected between them. It is called the "base-emitter junction". A diode conducts current in one direction: when the anode (the blunt side) is positive relative to the cathode (the point and bar side). The diode clamps the voltage to about 0.7V.

    Here is a diode with its terminals labelled.

    Anode-cathode.jpg
    Here's how a transistor is constructed in terms of diodes. If you use the diode test range on your multimeter and measure between every combination of leads on a transistor, both ways round, you will get readings that correspond to these diodes.

    bjt2.png

    Here is that section of the circuit, with Q6's base-emitter diode shown in red.

    Q6 B-E junction marked.png

    When a diode's anode is more positive than its cathode, it conducts, and clamps the voltage across it to no more than about 0.7V. If you measure more than a few volts across a diode, when its anode is positive relative to its cathode, it is damaged.

    So if you measure the base-emitter voltage of Q6, with the positive probe on the emitter (the anode of the diode), and the negative probe on the base, and you get a positive reading that's more than about 1V, Q6 is definitely damaged.

    So check the polarity of the voltage you measured across the base-emitter junction of Q6, and if it's out of range, Q6 is damaged.

    You can check the base-emitter voltage of any other transistors as well, but remember that Q6 is a PNP (the emitter arrow points towards the base); the polarity is reversed for NPNs (emitter arrow pointing away from base).

    I hope this makes sense. Please ask if anything is unclear.

    Edit: I should have pointed out: the arrow on the emitter points in the same direction as the arrow on the equivalent diode.
     
    Last edited: Dec 9, 2014
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