Thrystor question in an amp that's perpetually blowing fuses

Discussion in 'DIY' started by redpackman, May 17, 2018.

  1. redpackman

    redpackman Active Member

    Messages:
    283
    Location:
    St. Paul, MN
    I'm working on a Pioneer SA-6700 amp that's blowing fuses. I was given a tip by another AK expert member (thank you very much) that the protection circuit in this is very different than in many amps. He said to pull the thrystor to get things stabilized without it in circuit.

    I took the thrystor out of the circuit. The fuses stopped blowing whether plugged directly into AC or using the DBT. With the thrystor in circuit, the bulb on the DBT was BRIGHT so I shut the unit off immediately -- no magic smoke or flames.

    Anyway, here's my question: The thrystor in question, according to the data sheet, has three leads. The first two are called terminals and the third is the gate. When I check the two terminal legs using an ohm meter I get continuity - ZERO ohms resistance - in both directions all the time. Is this normal in a thrystor or does this indicate (along with the brightly glowing bulb in the DBT when this component is installed) that the thrystor is bad? Or does it indicate nothing certain?

    I know this is probably a novice question to some of you, but that's what I am, and that's why I'm here.
     

     

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  2. arts

    arts Super Member

    Messages:
    3,241
    Location:
    Qc, Canada
    When testing (in this case) a triac,you have to be sure that you are making no contact whatsoever between the gate and any other lead,and that includes with your fingers.
    Some are so sensitive that just touching the gate alone is enough to trigger conduction.

    If you are reading bidirectional conductivity with absolutely no contact with the gate,the device is defective and must be replaced.

    That is an interesting protection circuit; crowbarring the power supply is effective,but kind of hard on the rectifiers and transformer.
     
    Bill Ferris likes this.
  3. redpackman

    redpackman Active Member

    Messages:
    283
    Location:
    St. Paul, MN
    Thank you for your reply. I definitely have bidirectional conductivity (0 ohms). When I'm testing it, I have no contact with the gate and the component is completely out of circuit. I've got another thrystor/triac on order and I'll see (using the DBT) if there's been a change (with my hand on the "off" switch).
     
  4. arts

    arts Super Member

    Messages:
    3,241
    Location:
    Qc, Canada
    I would recommend that,with the triac removed and the unit powered up,you check for DC on the speaker outputs for each channel.

    While triacs (like any other semiconductor) do fail occasionally,it is rather uncommon. I wonder whether this amp had an output transistor failure and someone just kept putting in larger fuses until the triac expired from overloading.
     
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  5. redpackman

    redpackman Active Member

    Messages:
    283
    Location:
    St. Paul, MN
    I have checked the DC at the speaker terminals with no input and the volume at zero. They're both between 9 and 12 mV, so that's pretty good, I believe.

    When I got the amp all four rectifier diodes had blown and were open, too (I've replaced them). Don't know for sure what that indicates or which component failed first or if there's another bunch of components that maybe killed those two. It is a good sign that the DC on the speakers is very low. I've checked the output transistors and they seem good, and the DBT glows only briefly and the goes dark when I turn on the unit. As I mentioned elsewhere, when I had the (bad) triac in circuit, the DBT bulb would have lit up the whole neighborhood.
     
    Last edited: May 18, 2018
  6. Powertech

    Powertech Active Member

    Messages:
    329
    Location:
    South Wales, U.K.
    As said above - Crowbars are nasty pieces of work, but very effective. The principle being that they are quicker than an output fuse if the speaker leads are shorted out and so they can protect the output transistors by shorting the supply line and blowing the supply fuse. If you don't like them, you can get amplifier protection units as a kit for a few pounds/dollars. These literally connect between the amps output and the speaker terminals and have a disconnect relay built in. They usually sense overcurrent and high DC offset to protect speakers and amplifier. Obviously disconnect the crowbar if using these.
     
    Bill Ferris likes this.

     

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