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SF Champ blown Output Transformer causes?

Discussion in 'Amp Tech Center' started by goldenhound, Sep 27, 2016.

  1. goldenhound

    goldenhound TDPRI Member

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    BTW, there was actually voltage to discharge on the cap can this time. Like 14, 15v. Not a lot, but that was leftover from when I pulled the red wire from the turret board.
     
  2. Ten Over

    Ten Over Tele-Holic

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    I regret to inform you that your new OT is kaput. You can take it off and test for resistance from the leads to the case as one last test, but I am not very optimistic.
     
  3. goldenhound

    goldenhound TDPRI Member

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    Can you explain how you know that? I don't doubt it, but I'd like to know how you arrived at that conclusion.

    Sob - more money down the drain. :eek:( I wonder which problem caused it? I've gotten the hum since the very beginning.
     
  4. goldenhound

    goldenhound TDPRI Member

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    Must be something that we haven't seen yet. Can you say what readings a healthy OT should have reported? The new OT must've blown immediately after it was turned on.

    There are tutorials on how to salvage a OT. I was unsuccessful when I attempted a repair on the original. The wires are very fine and the burned wire was too far down in the paper wrapping to work with. Very delicate work, and it looks like it requires a fine touch.

    The initial thought was that the 6V6 was bad, and so I started off with a new 6V6 and Rectifier tube. I'm going to check with the vendor to see if they will test the two 6V6's and the rectifier tube.
     
  5. RLee77

    RLee77 Friend of Leo's

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    Yeah I would have to agree with this. The OT winding near the blue side, 151 ohms in (about 23% of the turns), is shorted to the case. If you add 151 + 495 you get the total OT resistance.
    The corroborating clue is that when you removed the red OT wire, your b+ went up to normal values. The b+ is going through 495 ohms to ground, which is why the OT is getting hot.
     
  6. peteb

    peteb Friend of Leo's

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    RLEee, nice observation, 151+495=646.

    Why, how, would that happen?


    I would expect the primary on an OT for a champ to measure about 260 ohms.

    The resistance to the chassis should be in Mega ohms, at least that's what I measure on old champs, no connection would be best.

    If you get a new OT, ohm it out before you put it in, then check it again in the amp before you power it up. Then I would check it after it powered up.
     
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  7. dan40

    dan40 Tele-Afflicted

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    I would also agree that the OT appears bad unless the measurements were done incorrectly. It does appear as though you measured it correctly but it's strange that the light bulb limiter didn't prevent the damage, and that a 1amp fast blow fuse held without the limiter being used. I have a small 5 watt, SE OT that I just checked and it measures out at 266 ohms across the winding, just as peteb had mentioned.
     
  8. Ten Over

    Ten Over Tele-Holic

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    He didn't have a light bulb limiter when he originally installed the new OT. The old 6V6 probably shorted and damaged the new OT right there.
     
  9. Ten Over

    Ten Over Tele-Holic

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    The rectifier will only put out a certain maximum current and it isn't enough for all of the VA's on the secondary to add up to a value that will reflect a primary VA large enough to blow the fuse.
     
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  10. RLee77

    RLee77 Friend of Leo's

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    I would certainly not install a new OT until you find what the issue is.
    Could be excess current/heat caused by a shorted output tube, or a big OT flyback voltage spike caused by something like a bad connection. Are you sure the speaker load is good, no loose wires?
    Some of those solder joints in your pics look rather hinky. I would also check under the board -- if you flow excess solder on a connection, it could slop down and contact the chassis.
     
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  11. goldenhound

    goldenhound TDPRI Member

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    Seems very unusual that the original 40+ year old OT made such a show when it went out, and this one did so, so quietly.

    True, I didn't have the limiter on my first test. But the first test lasted 3 seconds, 4 seconds tops. That's when the rectifier was arcing. I did not try it again until I received and installed all the parts and ran it thru the light bulb limiter. Would the rectifier cause that kind of damage? Do bad tubes always take out entire amp?

    Other than order parts, I don't know where to go next?
     
  12. dan40

    dan40 Tele-Afflicted

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    It usually takes a rectifier tube about 10-15 seconds to warm up and start conducting current. Did the arcing in the rectifier occur the instant you powered on?
     
  13. goldenhound

    goldenhound TDPRI Member

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    Within a couple of seconds - meaning two seconds, I heard the tube crackling.
     
  14. goldenhound

    goldenhound TDPRI Member

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    The speaker wire is solid - no breaks. Even twisting it, turning it while a song was playing through it.

    Tubes are being shipped off tomorrow, as well as the output transformer, to be checked.

    I can't imagine that an OT could short out within the short sessions I ran the amp to take MM readings. Keep in mind, the hum running thru the speaker appeared and manifested itself each time after the initial short session. Even when it was on the current limiter, which was used after the 1st session.

    The turret board is two layer. It would take an awful lot of solder to reach another component...especially the chassis.

    My soldering tools consist of a 35 watt Radio Shack iron that I bought before I bought those amp chassis back in the early 80's, and a 300 watt gun for the cap can ground, and bigger jobs. Love to have a Weller but I've never been able to justify the cost. My solder technique isn't up to sweatshop assembly standards, but far better than the early Electro Harmonix fx boxes. I do use flux to clean the tip, and keep from letting things get too hot as I hate melting wire coverings, etc.
     
  15. Darkness

    Darkness Tele-Afflicted

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    Have you returned everything to stock? Replaced the ancient cathode bias cap on V1?

    When I was working/learning about my champs, I had a severe volume deficiency and hum that was the result of a bias cap that I'd overheated on installation. I'm not saying that is anywhere near your problem, but you still shouldn't leave that one on your board. I tried everything, even considered trannies being bad, on that particular champ until I finally fixed it by replacing the bias cap. I was so invested in the fact that it was a more severe problem that I didn't see the forest for the trees.
     
    Last edited: Oct 25, 2016
  16. Ten Over

    Ten Over Tele-Holic

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    They went down under different circumstances. The original one blew with a fully charged power supply on it and the second one blew with a power supply that went from zero to about half.

    It's the other way around. The short caused the rectifier to start arcing. The 5Y3 is directly heated so that it starts to conduct almost as soon as it is turned on. Indirectly heated rectifiers take awhile to start conducting.

    The ripple on a power supply will increase with the current drawn. In this case, there was a huge current going through the OT to ground causing a large ripple. The hum that you heard was the ripple caused by the shorted OT.

    Shorts in the circuit boards are not consistent with the symptoms described.
     
  17. Ten Over

    Ten Over Tele-Holic

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    I don't think that the bulb should come back to semi-bright. I think it should be way far and away the brightest at first and quickly go to low. It may fluctuate some at the low brightness, but it shouldn't go back to the original brightness.
     
  18. goldenhound

    goldenhound TDPRI Member

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    Ten Over says "They went down under different circumstances. The original one blew with a fully charged power supply on it and the second one blew with a power supply that went from zero to about half.

    A fully charged power supply? What caused a fully charged power supply to dump its charge into the OT?

    It's the other way around. The short...

    Ten Over, detail here please - which short?

    caused the rectifier to start arcing. The 5Y3 is directly heated so that it starts to conduct...

    The ripple on a power supply will increase with the current drawn. In this case, there was a huge current going through the OT to ground causing a large ripple. The hum that you heard was the ripple caused by the shorted OT.

    What do you believe caused the short in the OT?

    Thanks for your input, Ten Over. I guess what I'm looking for is a direct answer to what caused the OT to short? Due to my pride, I'd rather not share details as to why buying another OT (or anything else) is difficult for me right now.

    On a tangent - I checked the OT again, and now there is no continuity from either the red or blue wire to the chassis. None. I guess its possible that once it cooled inside the wire pulled away from the frame so its now not making contact? Obviously, this means nothing from a functional standpoint, but I thought it was curious. I'm sending the OT back to see what can be done.
     
  19. goldenhound

    goldenhound TDPRI Member

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    Can I ask what brand you replaced it with - and what value?
     
  20. Ten Over

    Ten Over Tele-Holic

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    I think that the first 6V6 shorted while you were playing and while the power supply was fully charged up. Under these conditions, the capacitors deliver a huge current through the OT as well as whatever the rectifier can muster. With the second OT and a shorted 6V6, the current through the OT is only whatever the rectifier can supply without the kick from the capacitors because you started with the capacitors discharged. In either case, there is enough current to melt the primary wire insulation and maybe even the wire itself. The rectifier arced because the shorted 6V6 was drawing current in excess of what the rectifier was normally able to supply.
     
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