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    Blowing my Fuses!

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    GP49

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    Re: Blowing my Fuses!

    Post by GP49 on Sat Aug 15, 2015 2:12 am

    The reason why an electrolytic capacitor failure can be tricky to spot: one of the sections of the quad cap can suddenly arc over at a "weak spot." That spot undergoes greater stress as voltage rises, and heat increases, until suddenly it arcs over and the resulting amperage spike blows the fuse. The extreme current also vaporizes the capacitor dielectric at that spot so that by the time you realize what happened, replace the fuse and power it up again, that weak spot...and the tendency to arc over...is gone...vaporized. But there could be more, and if it is an old capacitor or one of unknown provenance, there likely will be more and it could very well happen again.

    deepee99

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    Re: Blowing my Fuses!

    Post by deepee99 on Sat Aug 15, 2015 10:10 am

    Holger (a/k/a MontanaWay, a/k/a Tube Nirvana) has done away with the quad-cap entirely in favour of discrete capacitors in his VTA-based amps. Not sure if the stock chassis can accommodate such a conversion, might check w/him.

    GP49

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    Re: Blowing my Fuses!

    Post by GP49 on Sat Aug 15, 2015 11:46 am

    Good idea.  I put updated discrete capacitors and equalizing resistors into the underside of Mark II chassis when I updated the drivers. It probably could not have been done with Mark III because of the space taken up by the choke, and I doubt that a Stereo 70 chassis would have enough room.

    The arcing-over could happen with discrete capacitors, too; but they dissipate heat better than the four sections encased in a quad cap.

    It is not clear from the original post whether the amp is a stock Stereo 70, or whether the filter capacitor is a quad cap, whether it is original, New Old Stock, or new-build; or if discrete capacitors have been substituted, most likely in an above-chassis sub-board.

    Tube Nube

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    Re: Blowing my Fuses!

    Post by Tube Nube on Sat Aug 15, 2015 7:04 pm

    Hi Guys,

    Thanks for your thoughts.

    It's a new VTA-ST 70 which I 've had about 6 years, I think. It has the quad cap that Bob supplies standard with his kit, and also the 4 upgraded PIO Russian caps he offers as an option.

    I read about using a digital ohm meter to check caps, and on some I get a reading, though it doesn't happen the way it's described on the net. On other caps I can't get a reading, no matter what range I select on the digital multi meter.

    For most of the caps on the driver board, I can't get at them -- I'll have to liberate the board from it's mounting screws, maybe even take it out.

    Nothing looks damaged, no connections are loose.

    It actually makes me wonder if, time and effort wise, I should just gut the thing, order a whole new board, and start over.

    Before I reach such a radical solution, I'll probably want to be sure that I'm still burning up fuses. Maybe I should re-solder all the joints on the driver board.

    Before I go for the new driver board in frustration, I'll probably elect to pack it up and send it to one of the local wizards on the forum to take a stab at it. We have some great electronics talent available as a resource.

    sKiZo

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    Re: Blowing my Fuses!

    Post by sKiZo on Sat Aug 15, 2015 8:37 pm

    Check the easy stuff first. Speaker wires are often overlooked, and an errant strand shorting to the other side can blow a fuse.

    Tube Nube

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    Re: Blowing my Fuses!

    Post by Tube Nube on Sat Aug 15, 2015 8:54 pm

    Good tip, sKiZo-man. Thanks for that!


    GP49

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    Re: Blowing my Fuses!

    Post by GP49 on Sat Aug 15, 2015 11:45 pm

    A standard VOM or DVM is not really the correct measuring device to check capacitors, though it will probably pick out those that are dead-shorted but those would probably blow the fuse instantly.  Problem is, an intermittent short may not develop until working voltage is applied, and when you are checking a capacitor it needs to be discharged, at ZERO volts.

    Excess current is what is causing your fuses to blow.  Several faults can cause this in an amplifier.  These are some of the more commonly seen:

    Bad rectifier, internally shorting while at working voltage. Applies to tube rectifiers only. A solid-state rectifier, once shorted, is destroyed and permanently shorted; it will blow the fuse instantly.

    Bad power supply filter capacitor, shorting while at working voltage.

    Excess current through one or more tubes.  Possible causes:

      A bad tube...though if a tube runs away or red-plates, it is not likely to work properly again.

      Loss of the NEGATIVE bias voltage on the grid of one or more output tubes.  Possible cause:
          -Bad socket-to-pin contact at the grid pin of the output tube.
          -Failure in the bias supply.  Loss of the negative bias voltage will cause current through the
             output tubes to increase to dangerous, possibly destructive levels. 
          -Short circuit in a coupling capacitor.  The coupling capacitor isolates the negative bias
             voltage from the positive plate voltage of the preceding stage, while passing audio.
             If it shorts, the positive plate voltage (250-290 volts or so, typically) overcomes the
             negative bias voltage (-40 to -50 volts or so) resulting in a positive voltage on the
             grid of the output tube, causing grossly excessive current through the tube.

    Shorting transformer.  If a winding of either the power transformer or an output transformer
      shorts to its frame or laminations, a dead short to the chassis results and will blow a fuse.  
      However this is not likely to "self-repair" or be intermittent.  Once this happens, it is
      generally permanent and terminal.

    I would set aside some time when you can run the amplifier, and watch it closely.  You may see or hear something when it fails, that will help trace the problem.  Maybe you will hear an arcing sound, or maybe a sharp increase in hum accompanied by red-plating of the tubes.  Sitting by the amplifier is inconvenient but your problem is apparently only happening when the amplifier is fully warmed up; my guess is that measurements or tests on a cold amplifier will turn up nothing.  It happens!

    Kentley

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    Re: Blowing my Fuses!

    Post by Kentley on Sat Aug 15, 2015 11:53 pm

    GP49 wrote:A standard VOM or DVM is not really the correct measuring device to check capacitors, though it will probably pick out those that are dead-shorted but those would probably blow the fuse instantly.  Problem is, an intermittent short may not develop until working voltage is applied, and when you are checking a capacitor it needs to be discharged, at ZERO volts.

    Excess current is what is causing your fuses to blow.  Several faults can cause this in an amplifier.  These are some of the more commonly seen:

    Bad rectifier, internally shorting while at working voltage.

    Bad power supply filter capacitor, shorting while at working voltage.

    Excess current through one or more tubes.  Possible causes:

      A bad tube...though if a tube runs away or red-plates, it is not likely to work properly again.

      Loss of the NEGATIVE bias voltage on the grid of one or more output tubes.  Possible cause:
          -Bad socket-to-pin contact at the grid pin of the output tube.
          -Failure in the bias supply.  Loss of the negative bias voltage will cause current through the
                 output tubes to increase to dangerous, possibly destructive levels. 
          -Short circuit in a coupling capacitor.  The coupling capacitor isolates the negative bias
             voltage from the positive plate voltage of the preceding stage, while passing audio.
             If it shorts, the positive plate voltage (250-290 volts or so, typically) overcomes the
             negative bias voltage (-40 to -50 volts or so) resulting in a positive voltage on the
             grid of the output tube, causing grossly excessive current through the tube.

    Shorting transformer.  If a winding of either the power transformer or an output transformer
      shorts to its frame or laminations, a dead short to the chassis results and will blow a fuse.  
      However this is not likely to "self-repair" or be intermittent.  Once this happens, it is
      generally permanent and terminal.

    I would set aside some time when you can run the amplifier, and watch it closely.  You may see or hear something when it fails, that will help trace the problem.  Maybe you will hear an arcing sound, or maybe a sharp increase in hum accompanied by red-plating of the tubes.  Sitting by the amplifier is inconvenient but your problem is apparently only happening when the amplifier is fully warmed up; my guess is that measurements or tests on a cold amplifier will turn up nothing.  It happens!

    My cap {pun?} goes off to you for one of the more detailed troubleshooting analyses to yet grace this forum. It has been bookmarked by me, as it it should. We ARE worthy!!!

    Tube Nube

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    Re: Blowing my Fuses!

    Post by Tube Nube on Sun Aug 16, 2015 2:39 am

    GP, thanks for posting this excellent explanation of what might be happening in my amplifier. I'll follow your advice tomorrow and just play it to see what happens!

    GP49

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    Re: Blowing my Fuses!

    Post by GP49 on Sun Aug 16, 2015 3:11 am

    My cap {pun?} goes off to you for one of the more detailed troubleshooting analyses to yet grace this forum. It has been bookmarked by me, as it it should. We ARE worthy!!![/quote]


    Thanks for the kind words.

    But we don't have his amplifier fixed yet, and there are other possible causes, though less common. I'll really celebrate when his amplifier is working and no longer blowing fuses!

    deepee99

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    Re: Blowing my Fuses!

    Post by deepee99 on Sun Aug 16, 2015 9:59 am

    GP, I second Kentley's good words. That's a superbly helpful check-list. And I think Skizo's "Occam's Razor" approach is worthy of consideration, too.
    Tube Nube, good luck and keep us posted. If you can't find any obvious issues top-side and have to go under the hood, you might re-butter your bias pots as well. They are very tiny connexions and it's almost impossible to detect a bad solder joint which can give you a ton of grief.

    Tube Nube

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    Re: Blowing my Fuses!

    Post by Tube Nube on Sun Aug 16, 2015 12:31 pm

    Since I do have the bottom off, I think I'll re solder the circuit board connections.

    Dave, are you referring to those little pots on the underside of the board? Im sorry to have to ask how I do it, but I lent my Dynaco documentation to a friend South of town.

    deepee99

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    Re: Blowing my Fuses!

    Post by deepee99 on Sun Aug 16, 2015 12:39 pm

    Noob,
    There's a down-facing pot that you set and forget. I wouldn't mess with that.
    The two bias pots face upwards, those are the ones that can give you grief if not properly soldered.
    I have a soft copy of the M-125 build manual, if you'll PM me your email address I will forward promptly.
    Cheerio,
    d

    sKiZo

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    Re: Blowing my Fuses!

    Post by sKiZo on Sun Aug 16, 2015 2:00 pm

    Good idea to test the pots as well. Check for continuity all through the range. Only problem my ST120 gave me was blowing a bias resistor, and I tracked that back to an intermittent pot with a flaky wiper.

    PS ... as with any rotary control, you should work these slowly stop to stop a few times every now and then to keep the contact areas clean. Put it on the same schedule with your smoke alarms. Goes without saying, you'll probably want to do that with the amp powered down.  ;-}

    ... and make sure your smoke alarms are working before you power it back up. clown

    Tube Nube

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    Re: Blowing my Fuses!

    Post by Tube Nube on Fri Aug 21, 2015 3:56 pm

    So I ran the amp today with the power tubes out of it at the suggestion of a ham radio friend. I think he was implying this might help differentiate between problems with a transformer, versus filter capacitors, versus screen grid capacitors.

    The amp lit up and was on for 5 or 10 minutes when the fuse blew again. I was in the next room for a minute when it blew, and didn't hear a thing.

    I don't know if that tells us anything.

    GP49

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    Re: Blowing my Fuses!

    Post by GP49 on Fri Aug 21, 2015 5:30 pm

    To me it says, "Filter Capacitors," especially since they were seeing a higher-than-normal voltage with the output tubes out.

    Bob Latino
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    Re: Blowing my Fuses!

    Post by Bob Latino on Fri Aug 21, 2015 5:38 pm

    Tube Nube wrote:So I ran the amp today with the power tubes out of it at the suggestion of  a ham radio friend. I think he was implying this might help differentiate between problems with a transformer, versus filter capacitors, versus screen grid capacitors.

    The amp lit up and was on for 5 or 10 minutes when the fuse blew again. I was in the next room for a minute when it blew, and didn't hear a thing.

    I don't know if that tells us anything.

    It is not a good idea to do what you mention above .. With no output tubes (a load) in the amp the B+ will go even higher than normal and cause more stress on the rest of the amp.

    Do you have a capacitor tester ? If not - get one and test the 4 sections of the quad cap to chassis ground with the amp off.

    Bob

    Tube Nube

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    Re: Blowing my Fuses!

    Post by Tube Nube on Fri Aug 21, 2015 6:58 pm

    Hi Bob,

    Well that was a bit reckless of me! :-/

    I have a Viot digital multimeter with capacitance test--no instructions, but following an on line instruction manual for an identical meter, I am unable to obtain a capacitance reading on about a dozen caps lying around, which gives me pause to suspect what I have is the POS model of multimeter. Alternately, it is possibly user error. Good evidence, I'd say, that it's the latter.

    Thanks for this tip. I'll obtain a proper functioning cap tester at the local electronics supply, and give this a go.

    GP49

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    Re: Blowing my Fuses!

    Post by GP49 on Fri Aug 21, 2015 11:09 pm

    As mentioned before: an electrolytic capacitor may test fine under the low voltage of a capacitor tester, but fail under the hundreds of volts in circuit.

    Tube Nube

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    Re: Blowing my Fuses!

    Post by Tube Nube on Sat Aug 22, 2015 1:19 am

    Right. Dohhh! I lost sight of the persuasive point of monitoring it under "ecologically valid" conditions!

    Dale Stevens

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    Re: Blowing my Fuses!

    Post by Dale Stevens on Wed Aug 26, 2015 4:47 pm

    Noob, you still there? Dale

    Tube Nube

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    Re: Blowing my Fuses!

    Post by Tube Nube on Wed Aug 26, 2015 5:11 pm

    Hi Dale,

    Well I was just gonna post an update.

    It appears there is a short in one of my KT 88's! The rectifier is partially -- damaged. It was new before I blew the fuse the last time, so I think it's a consequence of my the fuse blowing abuses of late.

    Time to get a new KT 88 or two.

    While something else could still be wrong, I am hoping it's a simple, one-cause problem of the tube, and that all will be right with the world when it is replaced.

    Of course, if the tube in the same position blows then . . . well, it'll be back to the drawing board.


    Frank111

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    Re: Blowing my Fuses!

    Post by Frank111 on Wed Aug 26, 2015 5:39 pm

    What brand is the KT88 and about how old and how many hours apx did it have on it?

    Tube Nube

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    Re: Blowing my Fuses!

    Post by Tube Nube on Wed Aug 26, 2015 7:04 pm

    They're Gold Lion reproduction KT 88's. I've had them about 4 years, and while they've had fairly light use playing music -- a few hours a week -- they've had a bit of a hard life, as I've been known to leave the amplifier "lit" and idling for weeks at a time. Bob has kindly advised me above to stop doing that, and so that's what I'll be doing (or not doing) from here on.

    deepee99

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    Re: Blowing my Fuses!

    Post by deepee99 on Wed Aug 26, 2015 7:20 pm

    Tube Nube wrote:They're Gold Lion reproduction KT 88's. I've had them about 4 years, and while they've had fairly light use playing music -- a few hours a week -- they've had a bit of a hard life, as I've been known to leave the amplifier "lit" and idling for weeks at a time. Bob has kindly advised me above to stop doing that, and so that's what I'll be doing (or not doing) from here on.
    Yeah, you're burning electrons whether the tubes are idling or going full-tilt. Leaving them on unattended is potentially dangerous, too. As my antique tube guru Andy Bowman at Vintage Tube Services says of lit tubes, "Never turn your back on the sumbitches."

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