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Thread: Dreaded SVT3 Output Mistake....

  1. #1
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    Dreaded SVT3 Output Mistake....

    So I did a boo-boo...

    Successfully corrected bias on SVT3-pro, was measuring MV ratings across .047 resistors with alligator clipped probes. One of the alligator clips "snapped" off the resistor's lead and I saw a spark on a transistor where it shorted to the heat sink. R27 smoked before I could do anything.

    Measuring diode checks across each of the 8 OP trannys measure 0.000 except for one or two that measure .030... I'm guessing now I have made a very, very expensive mistake. Should I shotgun replace every transistor in the power section at this point? Any tips?

    -Thanks,

    A sad, sad man.

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    I also found this online: Ampeg 2041114 00 Power Amp PCB for SVT 3PRO | Full Compass


    I'm wondering if it's the best solution.

    Here's the schem since I forgodpro-svt-3-pwr.pdf

  3. #3
    Supporting Member The Dude's Avatar
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    Just repair it like you would anything else. Troubleshoot it. Shotgunning is never a good idea regardless of the cause of the failure. I'm not sure why you'd replace the entire module before you even find out what's wrong with the one you have. It could be as simple as one shorted transistor and a burnt resistor.
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    Yeah, well, you know, that’s just, like, my opinion, man.

    I guess the reason I was asking is that each of the 8 OP transistors measured 00.00 from all legs to all legs with the exception of two, that measured .030. I've heard this thing is a beast and thought It might be easier to replace all the transistors since it seems almost all are fried. I will take your advice though and measure them each independently and replace what needs replacing. Thanks for your time dude, always appreciate the input!

    -Matt

  5. #5
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    Did you desolder the transistors from the board? If not, you have to know that one shorted transistor will cause three other to appear as they are shorted. Fixing the amp will be much cheaper than $230 for the new module (+delivery fee).

    Mark

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    This is new information to me, which is exactly why I love you guys and this site. Thanks for the excellent starting point! SS amps are still a mystery to me.


    -Matt

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    Senior Member Enzo's Avatar
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    Do you realize that the row of transistors is in parallel? So if one is shorted source to drain, they will all appear so. Start pulling them off and testing them, you might find they are not all shot.
    Education is what you're left with after you have forgotten what you have learned.

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    Of course, even if you find that only one, or two transistors are shorted, it is advised that you replace all the transistors. The reason for this is that they should be matched (in order to have the same quiescent current). I usually buy 10 transistors of one type and select 4 for an amp. The other 6 are left for other amps. This still be much cheaper (maybe 10 or 15$) than the new module.

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    Yes, when I flipped that board I understood exactly what Markus meant . Never tested MOSFETS beyond using a diode checker, does this page seem pretty on the level? Any tips or tricks from the Pros? I saw another good thread here that involves hooking up voltage sources to each MOSFET to test.

  10. #10
    Old Timer J M Fahey's Avatar
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    Absolute worst case, a full set of transistors plus resistors plus micas/goop should cost less than $30 at Mouser. Not sure about shipping but in any case shoud be less than for a full module.

    Personally I would replace just the dead transistors and not worry that much about old (but good) ones or matching because the large 0.47 ohm emitter resistors voltage drop under load will *swamp* minute Vgs differences.

    We are talking 2.5V@5Apk compared to differences in the order of, at most, tens of millivolts.
    Juan Manuel Fahey

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by J M Fahey View Post
    Absolute worst case, a full set of transistors plus resistors plus micas/goop should cost less than $30 at Mouser.
    Last time I looked Mouser didn't have these output transistors. In fact no USA distributor did*. Maybe I'm looking in the wrong places? An outfit in China / Hong Kong offered a set of the same transistors aimed at kit builders but I'm reluctant to buy what could possibly be counterfeit parts.

    * except possibly at exorbitant rates from outfits that specialize in out-of-production parts or have absurd minimum order rules.

    I'd be delighted to find 'em at a reasonable price in USA, I have three SVT-3's here, dead in the water.

  12. #12
    Senior Member Enzo's Avatar
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    SVT3Pro uses IRFP240, yes?

    http://www.mouser.com/ProductDetail/...uHmAlkPuz34%3d

    Mouser shows 4000 of them in stock at $2 each. $1.74 each if you buy 10.

    And IRFP9240
    http://www.mouser.com/ProductDetail/...8rpRYOMim0Q%3d

    Mouser only has 800 of those at $2.50
    Education is what you're left with after you have forgotten what you have learned.

  13. #13
    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Enzo View Post
    SVT3Pro uses IRFP240, yes?
    Thanks so much for the lead Enzo. Last time I went to Mouser to find these, well maybe they had 'em but dam' if I could navigate their search. Now they're in my shopping cart & soon to be 3 happy(er) bass players. I'm getting a dozen each.

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    OK I have an order pending at Mouser. When you say ALL transistors, you mean ALLLL transistors right including the smaller non-jfets on the board? And then power up with a limiter for the first time just in case right?

    EDIT:
    4 of each of the JFETS:
    IRFP240
    IRFP9240

    Little trannys:
    TIP-50
    2N5415
    MPSA06 X3
    MPSA56
    2N3440
    Last edited by Mr_bibbles; 06-13-2017 at 05:23 PM.

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    The output transistors (IRFP....) are MOSFET transistors and not JFETs. When I said "replace all transistors", we were talking about output transistors and I meant exactly output transistors (power MOSFETs). But... any of the other transistors could have failed as well. It's your responsibility which of the other transistors require to be replaced.

    Mark

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by MarkusBass View Post
    The output transistors (IRFP....) are MOSFET transistors and not JFETs. When I said "replace all transistors", we were talking about output transistors and I meant exactly output transistors (power MOSFETs). But... any of the other transistors could have failed as well. It's your responsibility which of the other transistors require to be replaced. Mark
    Add to that, you must replace any resistors, diodes, thermistors etc that may have perished. The trick in solid state amp repair is, "everything is important." A defect in any part may result in your amp not working, in fact may cause the amp to self destruct again.

    No wonder I try to avoid repairing transistor amps.

    Looks like you have a complete inventory of transistors ready to order Mr_b, check the rest of the parts on that board in case there's anything else. And I'll be doing the same here.

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    Tight! Thanks doods.

    bd226792af32eb29da3998b9f6558a50.jpg

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    OK So i've checked all the diodes and the transistors in the power board and removed shorted offenders. However, with just the power board plugged in, the current limiter is still brightly lit. There are only three transistors left on the board that all pass the "double diode check" so my question is... what could cause this? The shorted diodes been removed from circuit until the new ones arrive.


    1. Would I have to replace all of the components in this amp to see if the current would stop being drawn? Wouldn't removing them from the circuit effectively stop the shorts?

    2. Since damage was caused by a transistor shorting to heatsink, we can rule out the big caps and the transformer, yes?

    3. We can also rule out rectifier, since I powered it unlatched from the board and the bulb stayed dim... yes?

    4. Most of the .47 OT resistors are shorted so I'm going to replace them all, but that wouldn't cause infinite current draw right?

    Thanks everyone, I am over my head with this one. SS stuff is not very simple.

  19. #19
    Senior Member Enzo's Avatar
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    I seriously doubt the 0.47 ohm resistors are shorted. They are only half an ohm to start with, your meter is likely having trouble measuring that low, your probes might have that much resistance. Those resistors go bad by going open, not shorted. I am unsure how they could physically short anyway.

    I hope you realize the output transistors are MOSFETs and not bipolar transistors. ANy that measure shorted source to drain need to come out, and yes you can remove them all for testing the board.

    Shorting power rail to ground COULD have damaged the Q3 or Q4 circuits.
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  20. #20
    Supporting Member The Dude's Avatar
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    I'm not sure which diodes you have removed from the circuit, but it's generally not a good idea to run a solid state amp with components missing. The diodes may set up bias for something, provide a supply voltage that (if missing) may cause a rail imbalance, etc., etc. I would not fire it up until all parts are replaced and checked good. There are, of course, a few exceptions.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Quote Originally Posted by Enzo View Post
    I seriously doubt the 0.47 ohm resistors are shorted. They are only half an ohm to start with, your meter is likely having trouble measuring that low, your probes might have that much resistance. Those resistors go bad by going open, not shorted. I am unsure how they could physically short anyway.

    I hope you realize the output transistors are MOSFETs and not bipolar transistors. ANy that measure shorted source to drain need to come out, and yes you can remove them all for testing the board.

    Shorting power rail to ground COULD have damaged the Q3 or Q4 circuits.
    Thanks Enzo, excellent information as always.

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    Quote Originally Posted by The Dude View Post
    I'm not sure which diodes you have removed from the circuit, but it's generally not a good idea to run a solid state amp with components missing. The diodes may set up bias for something, provide a supply voltage that (if missing) may cause a rail imbalance, etc., etc. I would not fire it up until all parts are replaced and checked good. There are, of course, a few exceptions.
    I've only run it on the current limiter, but that makes a lot of sense. I'll get the new ones in and make sure all diodes are %100 before moving on to transistors. Thanks Dude!

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    OK so I've replaced all questionable components except for lm35 (in the mail...) , turned the bias pot all the way to the left and powered it up- Lightbulb stays dim! Fan goes full blast but I'm guessing temp sensor lm35 will fix this.

    So, moving on, what is your "brand new OT transistors" checklist before adding a load and then finally removing it from current limiter?

    Rail voltages should be + /- 65vdc, OR + / - 100vdc but I'm reading + / - 81 on a limiter FOR BOTH RAILS (measured at r22/r20 r19/r14)

    DC offset is... 37 volts! Have not hooked this up to a load yet or removed from current limiter, bias resistor is still all the way clockwise. ALL transistors in this amp are new.

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    Anyone? Is it possible d2 or d3 has become compromised merging the two rails? My theory is pitiful and but I know Zeners can be used to control voltages for different sections, yes?

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    Senior Member Enzo's Avatar
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    If they test OK as diodes, they generally are OK. But if in doubt, they cost pennies apiece, so swap them out to be sure.
    Education is what you're left with after you have forgotten what you have learned.

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    OK- First I want to thank all of you for your help so far. I can see the light at the end of the tunnel!


    The amp is stable without a load now. Rails with current limiter read 84 /-84 55/-55 (Healthy, about 20v +/- which seems like a result of the current limiter.)

    NOW the issue is, the INSTANT you plug a load into the back, the lamp limiter spikes bright and I hear a relay click. You can hear a DCish hum from the speaker. This is the first real complicated SS amp I've ever fixed, so to be honest I'm not even sure where I should be looking in the output section. My only thought is that I have the bias set at it's lowest point until I'm sure this thing is stable and working, could that cause the relay to blow with a load???

    Help me M.E.F. Kanobi, you're my only hope.

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    Supporting Member The Dude's Avatar
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    Quote Originally Posted by Mr_bibbles View Post
    ......DC offset is... 37 volts! Have not hooked this up to a load yet or removed from current limiter.....
    If you have 37VDC offset, there is still something wrong with the amp. Don't hook it up to a load until you find the problem.
    “Yeah, well, you know, that’s just, like, your opinion, man.”

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    Ok- So I've done a lot of poking around.

    1. Removed v2, as it's driving the output section yea? No change in offset.
    2. -/+ 84 Voltscoming in at q4 / q3, but as soon as it passes through them I lose the + half of voltages in the output section. -44 on all the flameproof resistors, at the speaker, etc. Replaced q3 and q4, no change.
    3. Can anyone help explain to me the functions of d4/d5 and d6/d7? From a guess it separates the - or + rail from the 'merged' rail??? I have no idea though. Replaced them all, no change.
    4. I did notice there is -44 present on the trimpot AND the legs of q2. This means the bias circuit is feeding the output section -44 right, so the output may be functioning correctly and the bias may have an issue?

  29. #29
    Senior Member Enzo's Avatar
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    Repeat: do not connect a speaker or load until there is NO DC on the output.

    V2 drives the output in terms of signal,but note it is DC isolated from the outputs by caps C6, C7.

    The diodes are back to back zeners in pairs. They keep the voltage between source and gate of the outputs from going over 13 volts. Did you replace them with 13v zeners? DO NOT replace them with plain diodes.

    It sounds like your +65 main rail is missing.
    Education is what you're left with after you have forgotten what you have learned.

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    Thanks for the help Enzo,

    Question 1: So C6 and C7 are blocking the DC, and if they're functioning the output should have it's own supply independent from the previous stage? If C6 had failed, could that possibly merge the + rail with the previous one, accounting for the offset?

    I did replace them with In4743 13v 1w Zeners. It definitely appears that the +65 is missing, but it is present on the other side (top of schematic) of q3. When Measured at R22, there is +55v present (lightbulb limiter connected).

    Question 2. Since all of the output transformers and flameproof resistors have been replaced, as has q3 and q5, what part of the + supply circuit should I be examining for failure? I noticed the voltage across r16 and r17 was asymmetrical- 3v and .5v. Conversely, the voltage drop across r14 /r19 matched up at 2.1v. This means the issue is within the output circuit, and not from the rails themself?

    Question 3. What are the functions of q5 and q6 in this circuit?? Is there a name for this type of circuit I can google? To be frank I have no Idea what's going on here and would like to understand how it works.

  31. #31
    Senior Member Enzo's Avatar
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    I mention C6,7 only because they block DC, and that means the tube didn't affect the DC problems of the outputs.

    Q5,6 are limiters. If the current through R32 rises too high, the voltage dropped across it rises enough to turn on Q5. That in turn pulls down the gate drive with the help of D28.

    You may have +65 on R22, but that doesn't mean it is getting to the output transistors or R10.
    Education is what you're left with after you have forgotten what you have learned.

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    Ok... So the only way that +65 could get to r10 is through q3 though, right? +65 is definitely NOT hitting r10, and since q3 had been replaced with a brand new 2n5415, the + voltage is being dissipated to ground somewhere WITHIN the output block???? What could possibly cause that??? All resistors seem to be in spec...

    Also, I've removed q5/q6 for testing purposes.

    Other things I've checked in the OP section:
    R10 3.1k
    R11 14k in circuit, 22k (schem value) out of circuit
    R16/R17 - 15k each
    R24/R26 2.7k Each
    d9 / d8 Both .515 / .935 in circuit
    D28/d29 Both .525/OL in circuit
    R25/R27 327 Ohms / 315 Ohms (r27 Originally went up in flames.)
    R13- 2.11k
    R15 - 4.7
    All Flameproofs measure between .4 and .5 ohms
    IC1 has +16 on pin 4 and -16 on pin 11

  33. #33
    Senior Member Enzo's Avatar
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    I mis-spoke, R10 does not connect to the +65, it gets its voltage through Q3, yes. But we need to verify we have +65 at the output transistors.

    The voltage on the base of Q3 turns it on bringing some of that +100 down to R10, so what is on the collector of Q3?

    You replaced some bad parts, but remember, not all problems are bad parts. If the copper trace between Q3 and R10 is cracked, the circuit won't work, even if the parts are good.
    Education is what you're left with after you have forgotten what you have learned.

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