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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:
    306
    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

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    3,602
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    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.
     
    Retrovert and Bill Ferris like this.
  3. redpackman

    redpackman Active Member

    Messages:
    306
    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:
    3,602
    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:
    306
    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

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    488
    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.
     
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  7. redpackman

    redpackman Active Member

    Messages:
    306
    Location:
    St. Paul, MN
    Update: OK. Put in the new Triac. The old one had a dead short between its two terminal legs (in both directions). Using a DBT the bulb glowed slightly and then went dark. Good sign. Started to check what the DC current at the speaker terminals is. Left channel is around 10 mV and Right is around 60 mV. Plugged in an expendable pair of headphones and listened to some music plugged in through the AUX channel. Sounds pretty good.

    Then the DBT light comes on.....bright. Immediately shut off the amp. Is the NEW Triac blown now? Remove it. Yes. Without the Triac the amp works and the DBT is not lit up. Check the DC current. Low again in Left and moderate in right. Begin to inspect the circuit board. Looks like something has been spilled on the underside of it, not on the top side. Not sure how someone engineered that. Like dried out pop...sticky (this is not my amp, I picked it up on Craigslist). Turn it all off. Begin to clean the circuit board where its sticky, with q-tips and alcohol. Check out the DC at the speakers again. It's dropping on the right channel (and most of the goo was on the right side of the board...humm). Now both sides are around 10-12 mV. Try the sound check again using the headphones. Sounds good. Don't want to hook up speakers (other than expendable headphones) yet.

    But the Triac (which is still out of the circuit...thus the no-glow DBT) did blow (without blowing the fuse).

    Could the Triac circuit be bad, and thus destroying the Triac? Did cleaning up the goo on the board correct the problem? ALL 4 rectifier diodes were blown on this unit when I got it. Replaced them. They're still just fine.

    Ideas?
     
    Last edited: May 30, 2018
  8. NAD80

    NAD80 Super Member

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  9. redpackman

    redpackman Active Member

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    306
    Location:
    St. Paul, MN
    Checked the transistors (out of circuit) and they checked out OK. ????? Any other ideas as to what ELSE would cause a new triac that's the exact replacement specified in the schematic, to blow or short out so there's continuity BOTH ways between the two terminal legs within 5 minutes of installation when the DC voltage at WORST was under 70 mV (and now since the cleaning...see above) is around 10 mV in both channels. I've also checked some of the resistors in the protection circuit and they seem to be within specs, too. There aren't that many components in the protection circuit, but I know it's hooked into the rest of the unit's circuitry...that's the point, after all. Could a pop (or some other kind of spill have caused a short/conductivity even after it dried and thus provided a conductive film in the right channel to the extent that the triac would blow?

    Any insight is very welcome. I have a new component on the way (they're not easy to come by) but don't want to cook this one in 5 minutes too.

    Without the triac in (and thus no DC protection) and listening to the unit on headphones it sounds great. ?????
     
  10. avionic

    avionic " Black Knights " Subscriber

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    Remove one leg of R110 and R111 ( 0.5 Ω 2 watt ) and check there present resistance.
     
  11. avionic

    avionic " Black Knights " Subscriber

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    While you have the soldering stuff out. . Check R112 (1KΩ 1/4 watt) and R113/114 both ( 470Ω 1/4 watt )

    Look the board over real good for solder bridges or broken solder pads.
     

     

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  12. redpackman

    redpackman Active Member

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    Thank you, Avionic, will do!!
     
  13. Moving Ahead

    Moving Ahead Active Member

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    whats the original Triac part number?
    and what did you replace with?
     
  14. Hyperion

    Hyperion Roobarb & Custard Subscriber

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    I think it will have been a Thyristor not a Triac - Triacs are bi-directional devices used for AC power control, whereas Thyristors are uni-directional devices often used in 'crowbar' and other types of protection circuits for vintage amplifiers.

    There is wide variation in the specifications of Thyristors, it is important to get the correct type (just as it is with transistors), you can't randomly pick a Thyristor and expect it to work.
     
    Last edited: Jun 4, 2018
  15. arts

    arts Super Member

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    Location:
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    Part specified by manufacturer is a 200V 3A Triac. Seems rather underrated for the task...
     
  16. redpackman

    redpackman Active Member

    Messages:
    306
    Location:
    St. Paul, MN
    This is what was in it originally, is in the schematic and what I installed in the circuit: TRIAC 3 Amp MITSUBISHI BCR3AM.

    The eBay ad on which I picked up the part listed it as a "Triac" as you can see, but the part number was identical AND the manufacturer was listed as "Mitsubishi." The two (original) and the one I bought which blew quite soon after installation looked identical too, complete with Mitsubishi logo. I bought it from an American supplier who said it was NOS, but I cannot guarantee it was not a counterfeit. The price asked would lead one to believe it had a good pedigree.
     
    Last edited: Jun 4, 2018

     

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  17. redpackman

    redpackman Active Member

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    I think you are right. Though listed in the eBay ad as a Triac, I believe it is a Thyristor.
     
  18. Moving Ahead

    Moving Ahead Active Member

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  19. Moving Ahead

    Moving Ahead Active Member

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    Thyristor......Triac.... What I suggest is you get on of those 5$ transistor tester and put it in to test. It will tell you if its a thyristor or a triac or if its toasted. I don't know your DMM skill level but this thing makes it a breeze.
     
  20. redpackman

    redpackman Active Member

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