QL600A alignment

Discussion in 'Pioneer Audio' started by madders, Feb 13, 2018.

  1. madders

    madders Active Member

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    Hi,

    Does anyone know the procedure to set the idle current and offset for a QL600A?

    The power amp board has VR1-4 which I assume are for the alignment, but I don't know which pins to measure, and unfortunately there's no mention of it in the service manual. There are TP1 and TP2 pins and then it looks like left and right channel out pins. There's not much else though.

    Thanks,
    Michael
     
  2. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    It appears to be a little cap coupled amp.
    Test points are for the idle. TP1 and pin 6, VR3. TP2 and pin 7, VR4. Basicly a voltage drop across the 0.5 ohm emitter resistors. What should it be? :dunno: The B+ is running at 85mA, that's 43mA's per channel. .025 volts?
    Set these first before the idles:
    VR1 and 2 are for setting half the voltage of the collectors.
    VR1 would set half of the B+ voltage at the collector of Q11.
    VR2 would set half of the B+ voltage at the collector of Q12.

    You could measure the idles and report your observations.
     
    Last edited: Feb 13, 2018
  3. madders

    madders Active Member

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    105
    Unfortunately I already replaced all 4 trimpots. I thought I measured all the pins before replacing, but I didn't see TP1 and TP2.

    Right now TP1 is 20mv and TP2 is 30mv, and adjusting VR3 and 4 changes the values, down to 0 all the way counter clockwise.
     
  4. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    The idle is an ambiguous setting for us hacks on the Cap Coupled amps. The higher it is, the more heat when playing audio. If too low it sounds like crap. The 20mV's is a good spot. See how it sounds when you have it up and running.
    What is the B+ volts?
     
  5. madders

    madders Active Member

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    105
    I don't know what B+ is? Is that pin 8?

    VR1 and VR2 are changing the voltage on the collector of Q11 and Q12.

    On the schematic, it shows the collector should be 19.5V. When I adjust VR1 so it's 19.5V, then I can no longer adjust VR3 to 25mV - highest it goes it 7mV.

    I did make a note of all the voltages before I changed anything, and originally the collectors on Q11 and Q12 were both 18.1. If I set them back to 18V, then I can set the idle as high at 14mV.
     
  6. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Pin 8, yes.
    The Scat shows B+ as 42 or 36 volts, so your set must be the 36v B+, if 18v was the collector reading.
    We better get a second option on the idle setting.
    MTF was the guy who explained the procedure to me. I'll post his quote to me on my sx-770 in the next post.
     
  7. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    The MTF post on the idle and collector voltages on my 770 amp settings:

     
  8. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    It (Idle) should be around .021 volts using my maths. You have to be on full power to get the correct voltages.
     
    Last edited: Feb 13, 2018
  9. madders

    madders Active Member

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    105
    Aha! This was the problem. I was on the DBT.

    On full power, pin 8 is about 41V.

    The schematics show 19.5V for the Q11 and Q12 collectors though. So should I set them to 19.5V, or half of pin 8 (around 20.5)? Either way, I can set the idle to 0.021V.
     
  10. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Splitting hairs.
    Your in! :):):)
     
  11. madders

    madders Active Member

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    I put it into a voltage calculator on google and it came out as 0.02125 - I should have bought a 25 turn trimpot!

    You misspelt maths by the way :)

    Great! Thanks so much for your help with this. I'd no idea I should have been testing the collectors on Q11 and Q12.
     
  12. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    I corrected my silly mistake. :whip:
     

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