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How to test a Bi-Directional Triode Thyristor

  1. Apr 1, 2016 #1
    Hi Guys,
    I am working on a Carver model M-1.0t stereo amp. There is no AC voltage to the input transformer primary.
    The AC input is controlled by a Bi-Directional Triode Thyristor. This is the data sheet.

    This is the schematic.

    This is the full schematic.
    http://bmamps.com/Schematics/Carver/Carver%20M-1.0t%20Service%20Manual.pdf [Broken]
    EDIT: a full schematic of improved clarity is available here.

    Is there a way to test this device?


    Last edited by a moderator: May 7, 2017
  2. jcsd
  3. Apr 2, 2016 #2


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  4. Apr 2, 2016 #3
    why would a mains power transformer in an amplifier have a triac before it?
    looks like some sort of a safety feature.
  5. Apr 2, 2016 #4


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    I haven't examined that tiny diagram, but a triac in the transformer primary could be the electronic on/off switch, e.g., as used by an IR remote control or a low-power touch on/off switch.
  6. Apr 2, 2016 #5


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    It is the power control and voltage regulator. It (via the TLP631 opto isolator) regulates the power supply. Not sure where I'd start measuring.
    If the fuse is not blown, I'd suspect something wrong on the other side of the transformer.

    The block diagram will be useful in tracking it down.
  7. Apr 2, 2016 #6

    jim hardy

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    In circuit or out ?


    Voltage across R409 tells you gate voltage.
    No gate voltage no conduction. About two diode drops will usually turn one on but i've seen them as high as 4 volts.
    So you're looking for AC of almost a volt across R409 .
    Can you read voltage across the thyristor itself ?

    Out of circuit test
    put it in series with a 100 watt lightbulb
    When gate voltage exceeds about a volt, voltage across thyristor should collapse to near zero and the lamp illuminate.

    here's a tutorial


    in circuit test
    I'm not sure i'd force that triac full on, something might be turning it off because it detects a fault. I havent studied that schematic in any detail.

    Seems you could unsolder the primary wires to transformer , lift R408 and rig a lamp test as in that sketch.

    If you do that, measure voltage across the lamp with DMM set for DC. You should read less than a volt.
    Is this thing blowing fuses ? Triacs are notorious for asymmetric firing which gives small DC offset in their output which will overheat an inductive load and fry it.
    That effect gets worse with temperature so warm the triac with your heat gun and watch for DC.
  8. Apr 2, 2016 #7


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    The triac is being used to regulate the power supply, so forcing it on will likely destroy stuff. There is an opto isolator above the bridge that provides feedback
  9. Apr 2, 2016 #8
    a transformer designed for this application , if running properly can be directly connected to mains , if everything is correct the amp will just turn on.I would do as Jim said and put a small light bulb in series and turn the transformer on and see whether the amp itself works , it could be that it's used as a turn on of and or safety switch of some sort.theoretically the amp should work without it , see if the transformer is okay , I would probably first connected the transformer to AC and remowe the secondary wires going to the amp and measure the output voltage to check if it's okay.
    then connect the amp and check that and then sort the thyristor issue as you may have more than that in the first place.
  10. Apr 2, 2016 #9
    I've had a look at the circuit, briefly, there is is useful block diagram also, this part is "AC control"
    it is a thyristor driven transformer, I don't know how familiar you are with thyristors, basically they are an arrangement of transistors exactly like Q402 Q403, and when turned on, they latch on until power is removed.
    Actually TR401 is a triac, which is just a pair of thyristors in antiparallel. The idea is that you switch it on some way through each mains half-cycle, and at the zero crossing, it switches itself off. This will control the amplifier rail voltage, a variable rail is the key design concept here, class "H" or somesuch.
    Its odd that they've chosen to drive the gate of the triac with a pair of discrete thyristors, Q401 Q404 is the other one - a designers joke perhaps..?
    anyway, triacs do tend to fail, so you can replace with a modern part. I don't see much risk of blowing stuff up, unless the triac blew because of a fault in the downstream power amps. I would replace the triac with a 60W bulb, see if it lights brightly (then you've a downstream fault) - or wire the 60W bulb in the mains input and replace the triac. In the worst case it might be permanently on, but this would just put the amplifier DC rails at their maximum, which should be OK, like won't blow anything up, though if used like this it might overdissipate.
    the other things that seem to go is opto-isolaters, a straight replacement is an idea, or you can look a the diode current on the isolated side ?(voltage across R301) and see if this causes the triac to switch on/off at some part in the mains cycle.
    keep a bulb in the mains lead, even if you need to up it to 100W, its a very good non-linear load, with the amp doing nothing, the current shodl not light the bulb and the circuit gets fulll volts, if you have a fault, most of the energy willl be in the bulb, giving you time to diagnose (switch it on only for a few seconds at a time)
  11. Apr 2, 2016 #10
    Good morning guys.

    I just woke up so I will read all the feedback.
    The fuse was not blown. I replaced the part in question which did not solve the issue. In fact I would like to test both the old part out of the circuit and the new part in the circuit so I will go read what Jim said first.
    I had started to measure the voltage at R409 last night but it is hard to get to as the resistor is under the heat sink of the triac.
    The data sheet for the device indicates a max gate voltage of 10 volts and I have no idea what the "normal" gate voltage should be. Got some reading to do as I do not understand this device and it's use.

    This amp failed due to a defective pre amp so I also assume some other fault past the transformer. I was able to get a new SM16GZ41 (the triac) but that was not the issue.

    Be back after some reading and testing.


  12. Apr 2, 2016 #11

    jim hardy

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    I'd use that test ckt in post 6, use your 2 channel 'scope to observe gate voltage directly
    both channels on 5 volts /div
    vertical display A minus B
    A to gate, B to MT1

    I'm used to seeing around a volt on thyristor gates
    in my control rod drive system the SCR's were nearly all between 1 and 2 volts
    three outliers , 0.5V, 4V and 5V ran fine for decades
    a very few volts is typical
    see this datasheet Google picked at random for me, it's a small 8amp triac in TO220
    http://www.nxp.com/documents/data_sheet/BT137_SERIES.pdf [Broken]

    try increasingly larger resistors in series with gate and you'll see turn-on delayed to later in the cycle.

    Last edited by a moderator: May 7, 2017
  13. Apr 2, 2016 #12
    Thanks Jim,

    Still reading...

    There is almost no voltage at R409....50mv
  14. Apr 2, 2016 #13
    I just got a call with a issue on a complex potting machine which I maintain on a farm near me. I have to run out to fix it.

    Be back later today.
  15. Apr 2, 2016 #14

    jim hardy

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    I will be away too.
    I'm fixing the "famous" leaky water outlet on a Ford Escort (also Focus & Escape) motor

    Am trying a design change from Ford's flat self energizing O-ring to old fashioned flat surface w gasket.

    If anybody else has same trouble send me a conversation or start a thread....
  16. Apr 2, 2016 #15


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    How can you say that. You are essentially saying that if you remove all voltage regulation you will not damage anything. Do you understand how this regulator works and what part the TRIAC plays in it all?

    BTW traics are not simply "ON/OFF devices as described, especially in AC circuits. They are used in light dimmers to control power. That is obviously what this circuit is doing. They turnoff on the reverse AC cycle.

    Why do people make radical statements about things they don't really understand?
  17. Apr 2, 2016 #16

    jim hardy

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    One would HOPE they designed it so , but the truth is Murphy was an optimist.

    That was the thought behind this
    but in hindsight i see i failed to express it unambiguously.

    Symmetry in firing i suppose, you want positive and negative half cycles balanced......

    Yes , closed loop voltage control is right there in the block diagram. Seems extravagant to me but i'm stuck in the 60's .

    200 real watts ? I'd like to hear it just to be able to say i did.
    A friend of mine back in 80's had a similar amp and a set of "Voice of the Theater" 's . Fortunately he lived on an acre in NE Homestead(not far from Planobilly) and could enjoy the system.
    Genuine Audiophile stuff usually does sound good...

    Keep us posted Plano...-

    old jim
  18. Apr 2, 2016 #17


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    Carver amps are special. When they say 200, they mean 200. Bridgeable professional quality stuff.
    There are probably so many subtle circuit tricks going on to deal with transient power surges, etc. Definitely worth fixing.
  19. Apr 2, 2016 #18
    Come on bigGuy, I'm not at fault here, you've only quoted half what I said :

    Actually TR401 is a triac, which is just a pair of thyristors in antiparallel. The idea is that you switch it on some way through each mains half-cycle, and at the zero crossing, it switches itself off. This will control the amplifier rail voltage,...

    Its my first post, so I don't see any reason to be casting aspersions, I know enough to spot the two discrete thyristors formed from a PNP and an NPN - and there's all the secrets of their operation. Thyristors (and thus triacs) are simply on/off devices, they have no linear region, the NPN forces the PNP hard on, and vice-versa, they rely on the zero crossing to un-latch.
    Yes, granted you can use them to effect linear control, like lamp dimmers and like this circuit, but you know that.

    I was describing the thyristor (half a triac) under DC conditions when I said :
    basically they are an arrangement of transistors exactly like Q402 Q403, and when turned on, they latch on until power is removed.
    I thought I was making it clear that in AC, "power is removed" at every zero crossing.
  20. Apr 2, 2016 #19


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    You said Voice of Theatre.
    We built our own Voice of Theatres (cabinets) back in 70's. I didn't like the horn mids, added a JBL tweeter to brighten it up. Bass was good though. Drove it with a 600 Watt Phase Linear. We built one of the first 24-in 8-out solid state mixing consoles ever made. Traded beer for Fairchild 709's. Soon afterwards Allison research made good solid state modules available for all. (http://recording.org/threads/history-of-allison-research.58907/)

    I would sit between the VOT's like they were headphones and listen to ear damaging rock (Ummagumma, etc)

    My bad for over reacting. Sorry.
  21. Apr 2, 2016 #20

    jim hardy

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    I dont know where Al got his VOTT's but they were genuine Altec.
    He said he learned from his Dad to appreciate squawky violins.....

    Do you still have them ? You can astound the kids with track 12 of Titanic. My little 12" EV's rattled the patio doors loud enough they came running in from the yard to see what Dad was doing.
  22. Apr 2, 2016 #21


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    Nah, they are long gone. It was part of a failed effort at building a Recording Studio. I liked the sound of the 604E's better.
    I always like the heartbeat in the beginning of Pink Floyd's Dark Side of the Moon Album.
  23. Apr 3, 2016 #22
    I do understand how a triac regulates voltage by "cutting" the sinus or enabling only one half of each half period through etc.
    the thing is why i said what I said let's look at the schematic , the transformer has two primaries in parallel with the mains , each one has the triac in series , now suppose what you say is true and the transformer is designed such that it outputs its intended voltage levels only when the triac controls the input in the primary , now suppose that triac fails short as semiconductors tend to do very often when they fail, now you have full mains ac across that transformer primary , now it would be stupid to make that transformer that if this happens either the output voltage at secondary goes above the limits and toasts the whole amp (80's semiconductors were not as rugged as they are now) and or the transformer itself fries.

    also they might have just put the triac on the secondary side , it would then be under less stress than when on primary ad would make no difference in the waveform going into the bridge rectifiers , this is the first time I honestly see that a transformer is driven using a triac.
    forgive my ignorance please but I can't understand the benefit of the triac being there , whats the benefit of controlling a iron core mains transformer with a triac you still need the transformer as big and powerful to support the max load when it's there , so i again think the primary can be directly put to the mains and nothing bad should happen.but then again don't take my word for it.

    also I don't want to brag about it but its rather hard to read the schematic because here are black dots were the traces connect (supposedly) and there are also similar dots were clearly the traces don't connect simply overlap it's kind of frustrating.
  24. Apr 3, 2016 #23


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    Think of it as a switching regulator. It is quite elegant, very efficient (no high power linear regulators wasting power), and probably VERY responsive to deal with transient voltage demands that the filter caps couldn't handle.

    As for a triac on the secondary, which winding on the secondary do you want to regulate. I'm astounded you are taking pot shots at a circuit you flat out don't understand (and that was designed by one of the foremost audio geniuses in the audio amplifier industry). https://en.wikipedia.org/wiki/Bob_Carver

    BTW, when you put the mains on the transformer, what voltage do you think will appear on the secondary supplies if they are unloaded (not playing loud audio)? You are essentially saying that disabling the voltage regulation element will not cause too high a voltage in the secondary circuits. I'm glad you are confident enough to say that. Stressing a vintage amplifier like that is not something I would do.
  25. Apr 3, 2016 #24
    I do know about Bob Carver hence the company name Carver , also have taken some interest in information about his amps.
    haven't owned one though since im living in Europe and onkyo's luxmans and other things are more frequent here so have repaired those.

    Ok I take your argument about it being a low frequency switching regulator.also a turn on off switch instead of a real mechanical one through which all the primary power has to go.but it isn't an actual switching regulator much like a smps in many cases can vary its pwm or frequency to adjust it's load , yet i agree it can cut down to only one half of each half period and maybe even less and that would lower the secondary output voltage and current after the rectifier.
    yet still i'm not fully sure what benefit it has since a properly designed amplifier at idle doesn't waste much power anyway why would it need a lower voltage then ?
    also assume your playing classical music at mid levels at many cases there are passages very silent requiring very little power and then suddenly very loud parts requiring quite some power for a decent speaker system , does the triac regulates this in real time that fast according to that schematic or doesn't it ?

    and after all I still ask the same question , the amp is rated at given power max so that means the transformer is also rated to handle that power (as you said caver is not cheating on power data) so i suppose at full rated power the transformer is driven with simply the full mains AC and the triac is fully conducting each half period , i'm not perfectly sure but i suppose so as it would seem logical , so that is why i said that nothing bad should happen if one just connected the full ac across the primary.
    also I see three bridge rectifiers at the secondary side each for its own necessary secondary voltage but they all are connected logically to each of their respective secondary windings on the same transformer whose primary is triac controlled so lowering the power for one means lowering it for all of the secondary outputs if i'm correct and still i don't see what's the big point in that.
    a properly designed transformer of good metal quality is if i'm not mistaken very efficient at idle in other words wasting very very little next to no power so what benefit can there be made other than completely shutting it down ?

    now if my assumptions are so wrong would you please be so kind as to point out the opposite of what I said.
  26. Apr 3, 2016 #25


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    It IS a SMPS. Google triac waveforms.
    LOL --- I'm done discussing this with you.
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