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Student learning to troubleshoot has a couple of questions

Discussion in 'Pioneer Audio' started by WoodyJ, Jan 27, 2018.

  1. restorer-john

    restorer-john Addicted Member

    Messages:
    5,851
    Location:
    Australia
    Well, if the relay is engaging, it is most likely your power amplifiers are fine. Confirm after the relay clicks, measure DC offset at the speaker terminals (with the speaker switch actuated of course)- volume down. It should be 0-30mV or so.

    I want you to first check all the basics, tape monitor 2 switch is off and the speakers A or B are selected according to the switch and the rear terminals you are using. Select aux and feed a signal in, turn up the volume a bit. Just to get the obvious out of the way.

    If it is still dead, check the +/-32v rails at the emitters of Q29 and Q28 as they supply the low level EQ amp and the tone/flat preamps.
     

     

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  2. restorer-john

    restorer-john Addicted Member

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    Down the track, absolutely, but he's got a speaker relay actuating in the amp, so we clearly don't have any significant current draw that needs a DBT right now.

    I'm suspecting operator setup or low level preamp regulated supply failure.
     
  3. WoodyJ

    WoodyJ New Member

    Messages:
    21
    Thanks! I'll check all this as soon as I get out of class, I've learned something new here too, "DC offset", very interesting reading on that. I originally tested the amp using a CD player and headphones, I didn't use the speakers. I did check all the tape monitors. I tried the CD player in AUX and TUNER. When I turned the speaker selector the Flouroscan meter jumped a couple of bars the first time, then went completely dead.
     
  4. WoodyJ

    WoodyJ New Member

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    I'm thinking of building an Access VBA application to keep track of what I'm learning, have you found any software tools that aid you?
     
  5. WoodyJ

    WoodyJ New Member

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    DC offset on A right channel reads -348 mV. A Left channel reads -158 with amp set to Aux and Speakers set to A and volume set to 0. After it warmed up it was -320 R and -89 Left
     
    Last edited: Jan 29, 2018
  6. slimecity

    slimecity Super Member

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    2,691
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    Depending on what the manual tells you the ideal offset is, you need to bring both those channels down.

    Locate the DC offset adjustment pots on the correct PCB and using the procedure in the manual, bring them down to around 25-30mV (that number is my best guess).

    You want to ensure that you are NOT adjusting any bias pots when you do this!
     

     

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  7. loweran

    loweran AK Subscriber Subscriber

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    Nah, I just use evernote to jot down things. Usually, once you have seen and corrected something, it sticks anyway. I learn something new with every unit I work on. I keep notes on the specific unit that I work on including condition upon arrival, specific tests and results conducted, any corrections done to get it back to a working condition, and then a full list of parts replaced for the restore.

    I also tend to create a thread that can tend to ramble on about the process. Those come in handy too because I can go back and review the thread to see the nuances.
     
  8. rcs16

    rcs16 Super Member

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    Location:
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    You are assuming a standard OCL(o/p capacitorless) design, offset is always set for as close to "0" DCV as you can get it, if it is < 50mV it is usually fine.
     
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  9. kvining

    kvining Active Member

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    Location:
    Planet Houston
    Maybe he's reading the meter wrong and it's actually -34.8 and -8.9?
     
  10. WoodyJ

    WoodyJ New Member

    Messages:
    21
    Just a quick (probably dumb) question: Where the service manual says to connect an 8 ohm resistor to the speaker terminals, can I use a bare speaker rated at 8 ohms as the resistor or do I need like, a real resistor ?
     
  11. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    No DC Balance adjustments on this set.
    There is a Idle adjustment.
    Talking of the Idle adjustment:
    No 8 ohm resistor is needed.
    Volume to minimum.
    Follow the procedure for setting the idle.
    Best to check and report before you cut any wires or turn any trimmers.

    Can you explain how you determined the amp wasn't operating any channels?
    Deoxit switch pot cleaning?
     
    Last edited: Jan 31, 2018
    dlucy likes this.

     

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  12. WoodyJ

    WoodyJ New Member

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    I got it at a garage sale, told it would power up but no sound. I bought it to learn how to read schematics. I tested it with head phones. No sound on either channel, but when I turned the Speaker switch the Flouroscan meter jumped a little once just a bar or two and I got a little crackle. Then both channels went silent. I soaked the potentiometers with Deoxit D5, followed instructions in the Deoxit thread, no change.

    I've had a week of classes all on schematics, I am actually beginning to understand the Pioneer schematic, and I think I'm ready to tackle the idle adjustment. I've ordered a transistor checker recommended by another poster, it should be here any day.

    I've thoroughly checked the boards with a magnifying glass and cannot find any burnt or obvious damage.
     
  13. WoodyJ

    WoodyJ New Member

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    21
    Another dumb question, what is an "AF Amplifier", can't find anything on it, is it some sort of Pioneer thing?
     
  14. mars_volta

    mars_volta Super Member

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    I always start by cleaning all the controls. A dirty pot can cause some funky business!

    Next check the power supply voltages.
     
  15. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Did you try all the inputs?
    Run a CD player through the AUX, TAPE, Tuner RCA jack's?
    It seems safe to connect a cheap set of speakers.
    You never really explained the receiving check you did. Just wondering.
    Mars_volta has a good suggestion.
    Basic Troubleshooting is check the power supply voltages. It's the heart of the set.
     
  16. rcs16

    rcs16 Super Member

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    Audio frequency
     

     

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  17. WoodyJ

    WoodyJ New Member

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    How does that differ from a power amplifier?
     
  18. WoodyJ

    WoodyJ New Member

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    Still studying how to read schematic drawings, there is a lot to absorb. I have tested the power supply voltages. There is 114 going in from the fuse and 32 V on the red and orange wire going out to the board, using white as neutral when measuring both. Schematic shows a 48V line as well but I cannot match that to what I am seeing on the board. Right now I'm trying to apply what I have learned in class. I'm trying to identify the stage coupling points.
     
  19. WoodyJ

    WoodyJ New Member

    Messages:
    21
    As instructed, I have built this ancient device, and used a 100 watt bulb. When I plugged in my now much studied SA-710-2, the unit powered on and the light did not light at all. If I pull the chain switch on the light, the amp powers off so I know my circuit is good.Should I have used a lower wattage or does this tell me the board has no shorts?

    Class was very good this week. I am now an ace transistor tester using Diode mode on the DMM on transistors that are not in circuit, which brings up the question which I am hoping will be answered next week, how do you test them in circuit? I now understand data sheets, Bipolar PNP and NPN, and two pole power transistors. Also big week for inductance and capacitance. I believe the next thing I will buy for my expanding work bench is a capacitor ESR tester, an expensive tool but it looks like it will save so much time spent desoldering capacitors it will be worth it.

    I have been immersed in schematics. We have now begun the chapter "Troubleshooting with Schematics" so hopefully by next weekend I'll be able to go through my little amp board a little more intelligently.
     

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  20. zebulon1

    zebulon1 Getting behind on work. I need help? Subscriber

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    Wish I was in class with you. Sounds like fun. Next week will be a little tougher.
    The DBT will light bright if there is a short in the unit plugged in.
    If the bulb is dim or you can't see it glowing the set is short free. The DBT is a current limiter where as a Variac keeps poring the coals to the unit even at the low voltages you are dialing in.
    I think the amp may work, You haven't really answered my questions. But that's OK. Your using the set as a learning tool.
    Oh, you have to remove the components to test them. If you understand the circuit you might save some time by checking them mounted but best practice is to desolder and check. The caps are cheap and replaced if suspect. Checking the large expensive ones is worth it. Must be isolated from the circuit. Resistors can be checked in situ if you know the values. If the value is off by a few percent lift a leg, isolate it and recheck. If there is a mirrored circuit in the unit being tested I'll use it as a guide, only if its known operational.
     

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