Bias Circuit Parameters...INCREASE/DECREASE

Discussion in 'Pioneer Audio' started by ic-racer, Feb 9, 2019.

  1. ic-racer

    ic-racer Super Member

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    Just so I have this correct I'd like a second look.

    I'm working on a Pioneer SA-800 with the bias trimmer pot all the way over and the output transistor resting current cannot get up to the recommended 100mA.

    The SA-800 bias circuit is similar to this generic diagram, with the NPN TR3.

    Is this correct?

    When Resistor R3 is INCREASED, I think the current flowing through it will be LESS, which will cause the voltage drop across it to be LESS, which will make the base of the output transistor TR1 to see a HIGHER voltage which will INCREASE its idle current.

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  2. merlynski

    merlynski Curmudgeon Electronicist Subscriber

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    Transistors are current controlled devices. Increasing R3 decreases the current through it, thus decreasing the current through D1 and D2, and decreasing the current through the b-e junction of TR1, which deceases the collector current of TR1 (and TR2), i.e. decreasing the bias. The voltage of PN junctions depends on the current through them (and temperature).
     
  3. ic-racer

    ic-racer Super Member

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    My reasoning in the first post is based on this schematic. INCREASING VR3 (to 100r) causes INCREASES voltage across the two 0.5 ohm 2W resistors across which the bias current/voltage is measured.
    Decreasing VR1 (to 0 ohm) causes a DECREASE or shuts off completely any current through the two 0.5 ohm resistors.

    Screen Shot 2019-02-09 at 5.19.54 PM.png
     
  4. merlynski

    merlynski Curmudgeon Electronicist Subscriber

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    In this (second) schematic you are correct, because the VR is in between the driver transistors bases (inside the bias loop), so an increase of resistance does cause an increase in voltage across it. The current that would have gone through it now goes through the driver transistor bases instead, is amplified and increases the bias current. If you decrease the VR enough all the available current goes through it, the voltage across it drops, decreasing the bias and dropping the output transistors into cutoff, no bias current through the two 0.5Ω resistors. The circuit in the first post is different, R3 is not between the output transistor bases.
     
  5. ic-racer

    ic-racer Super Member

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    Thank you!!
     
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  6. chazix

    chazix AK Subscriber Subscriber

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    I believe that is an error in the service manual - no way this amp was intended to run 100mA of idle current. If you look at the schematic, it shows voltages of + and - 0.02V at the output transistor emitters, which means 40mV across about an ohm's worth of emitter resistors, therefore 40mA idle current. That's more reasonable (though still kind of high for an amp of this era).

    Cheers,

    chazix
     

     

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  7. ic-racer

    ic-racer Super Member

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    You are correct. In fact, in the last 2 weeks I have discovered multiple errors in the schematic, layout and service manual. Yesterday I almost gave up, but found a bad trimmer pot which lead to finding a second bad trimmer pot. So, in spite of the errors, I may have fixed the unit (waiting on the new trimmers). If I get it running, maybe I'll start a thread showing all the errors as a reference.

    For example the schematic shows the trimmer pot and STV-3 interchanged in position from how it is on the PC board. Also, the PCB layout in the manual shows the STV-3 with opposite polarity and R33 as 47r (68r is in the unit).

    Screen Shot 2019-02-13 at 5.57.53 PM.png
     
  8. ic-racer

    ic-racer Super Member

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    So, 'good is bad.' In that, what I thought the 'good' channel is the 'bad' channel. Because the bias trimmer on one channel became open circuit on the wafer, I was able to set that channel to 100mA idle. But the other channel could not go that high because its trimmer still read 100r across the wafer. So it is the channel that could go to 100mA that is the problem.

    I also checked he DC offset trimmers and they are probably bad also. So, I'll see if I can get 40mA idle on each channel when the new trimmers come in.
     
  9. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    With regard to R-33, PIONEER, along with almost everyone else, made "undocumented" changes on the line, either due to engineering changes OR part substitutions when short of certain parts, that made Service bulletins for shops(sometimes not even then), but the Service manuals were NOT updated (except for the SX-626, which had a complete re-design 1/2way thru it's production run, hence 2 different manuals for different units). The general consensus is to match the board # with the schematic, check parts against the parts list and make changes (if any) to the parts list so your particular manual matches the unit. If there is a discrepancy between the actual part and the parts list, go with the value / voltage on the actual part(provided it appears to be an original part.)
     
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  10. ic-racer

    ic-racer Super Member

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    Yes, and it looks like R33 could be used to calibrate the trimmer range to the actual voltage drop of STV-3, assuming there are different types of STV-3.
     
  11. ic-racer

    ic-racer Super Member

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    So far good news. I'm listening to the SA-800 right now and it seem to be sounding better than ever. I replaced the 4 trimmer pots. Two for the DC offset and two for the bias. I got the bias set right at 40mA for each channel. Dc offset as close to zero.
     

     

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