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5F1 - Speaker Load Damage

Discussion in 'Shock Brother's DIY Amps' started by benjamburns, Jan 22, 2021.

  1. benjamburns

    benjamburns TDPRI Member

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    Hey all,

    First post! I've gleaned a lot of valuable info from many folks here, so thank you all for your generosity.

    I finished a 5F1 build last month (Boothill, everything was great, thanks Dave!). 15W Classictone OT to a 4ohm 10" Alnico from Weber. Sounded really nice.

    That is until my temporary cabinet-less setup pulled the speaker tabs off the speaker without me noticing - I fired up the amp and it ran with no speaker load for probably half an hour before I sat down. Nothing noticeably fried, though a toasty smell permeated the space and I've got nothing but hum with no effect from the volume pot.

    Replaced the 6V6, no change.

    Before I accept the reality that my OT is cooked with the keps nuts hiding under the turret board, are there any other components I should test that would have fried first? Capacitors in the power section? Otherwise I'll just run robrob's startup procedure again to try to pinpoint it.

    Please enjoy my avatar poolside chair shrine to Uncle Doug's grill cloth hack.

    Thanks!
     
    Last edited: Jan 22, 2021
  2. kbold

    kbold Friend of Leo's

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    Welcome to TDPRI

    Toasty smell sounds like OT got overly warm. First check rectifier capacitors (the big high voltage ones) and the rectifier diodes which may have failed. (These are high V diodes attached to the High V caps.)
    Even if they haven't failed, I would replace these caps and diodes. These will typically go before the transformer.

    Check OT impedences. Not sure what they should measure - fairly low. Someone else may dime in on that.

    Careful when poking around inside. These caps can hold high (lethal) voltages.
     
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  3. milocj

    milocj Friend of Leo's

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    This tip is too late to help right now but will be something you can do if you have to replace the OT. If I'm using keps nuts for the OT I always use some epoxy around the outside of the nuts when doing the initial fit up if the transformer just in case I want/need to change it at a later date after the board is in place.
     
  4. 2L man

    2L man Tele-Meister

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    You can test OPT. First use multimeter and verify primary and secondary are not shorted together. Then that primary has something from 50 ohms up to perhaps 200. Then connect 120VAC to primary input and measure output(s) and calculate winding ratio. You don't need to remove power tube(s) but if it is easy take them out. For example mains voltage input is 122VAC and 8 ohm output is 3,85VAC. Winding ratio is 122 / 3,95 = 31,7

    8 ohm loudspeaker Impedance comes 31,7 sqr x 8 ohms = 8033 which is close enough 8k that I would say OPT does not have shorted turns. Naturally be careful when wofking with mains voltage and it would be good to install a fuse between mains and OPT for this test if there are shorted turns it would burn before building fuse.

    120VAC to primary seems to be enough for accurate winding ratio calculation although OPT manufacturers often use much higher voltages.
     
  5. benjamburns

    benjamburns TDPRI Member

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    Great idea. My first thought was I wish I had tack welded them to the chassis!
     
  6. schmee

    schmee Doctor of Teleocity

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    If the chassis is steel they will solder to it. I've done that on some hard to reach chassis strap nuts. A dab of solder paste on the chassis and the nut, pre load either with some solder. Heat 'er up!
     
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  7. benjamburns

    benjamburns TDPRI Member

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    Assuming the rectifier caps and diodes have failed, should I also look into replacing the 5Y3?

    Also re: testing caps, this being my first run at that, I’m setting my multimeter to read capacitance and will expect to see values significantly lower than the capacity stated on the cap. Is that right? Thanks!
     
  8. NTC

    NTC Tele-Holic

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    Regarding keeping the nuts in place: how about putting locktite on the bolt before assembly?
     
  9. kbold

    kbold Friend of Leo's

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    First ensure caps are discharged. If OK, they should read at/near stated capacitance. If not replace. Typically a damaged cap (electrically) will be swollen or leaky (or exploded).
    Oh ... my bad. Didn't know it was 5y3 cct (5Y3 is the rectifier). No experience with this ..... I think a rectifier valve would be more durable (electrically) than a SS diode.
     
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  10. dan40

    dan40 Friend of Leo's

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    Loctite on the bolt itself will only make future removal that much harder unless you use a bit of heat to break the bond.
     
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