Fender Hotrod deluxe 3 didn't know the words

Discussion in 'Amp Tech Center' started by Burning Fingers, Jun 3, 2020.

  1. Burning Fingers

    Burning Fingers TDPRI Member

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    A fender Hotrod deluxe 3 stopped by with its owner who was fed up with it humming loudly.

    Humming badly with even the PI removed....hope it's not the OT !.

    New power tubes did nothing so I checked the bias...what bias !!..zilch.
    Remove power tubes and recheck bias...bias MIA.

    Thought I may have to chase down a faulty component in the bias circuit but my gut told me to check the power supply filter caps.

    Yep the B+ filter cap was stuffed...a new cap and... silence...no hum , no hiss , good tone and yes we have bias again.

    Hopefully this may save someone else some time.

    Hotrod Ps .jpg
     
  2. Fluddman

    Fluddman Tele-Meister Silver Supporter

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    Thanks for posting. I think i've come across this before. Cheers
     
  3. That Cal Webway

    That Cal Webway Tele-Holic Silver Supporter

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    I'm not a tech head that I still love the read the details on stuff like that. Thank you
     
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  4. Wally

    Wally Telefied Ad Free Member

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    Kudos on getting the amp going, but the problem was not bias voltage supply but that bad first stage filter cap. The bias voltage is supplied by the brown secondary winding, which also supplies the voltage for the solid state elements in the circuit. I can see where you may have not been able to measure any current due to a lack of voltage on the power rail?????
     
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  5. Burning Fingers

    Burning Fingers TDPRI Member

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    That bad filter cap had gone open circuit...thus the bad hum...still had B+ ( with a big ripple ) but I would not have thought that would have affected the separate winding supplying the bias voltage..but it did...something learned is a bonus when repairing amps.
     
  6. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Time to change the other electrolytic caps as well.
     
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  7. Burning Fingers

    Burning Fingers TDPRI Member

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    Yes..all the HT power supply caps have been renewed after finding that open circuit one.
     
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  8. Wally

    Wally Telefied Ad Free Member

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    If there is an open cap in that power supply, then you have no voltage on the power rail. There would have been voltage to that first cap from the rectifier, but B+ comes off of the first filter cap and feeds the center tap of the OT....or one end of a single ended design OT. If you had any voltage past that first cap, then you did not have an open cap but may have had a shorted cap that was passing AC voltage...and that would be a source for hum.
     
  9. Burning Fingers

    Burning Fingers TDPRI Member

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    Umm.. Nyet
    Without that cap there is still B+ ( with a massive AC ripple )...with a shorted cap there would be little or no B+ but smoke and maybe a bang as the power supply gets shorted to earth.
    Your post had me wondering for a minute or two if I had messed up checking the faulty cap..but I pulled it from the bin and rechecked...definitely open not shorted.
    Just to prove I haven't gone mad I removed that new first cap and yes there is still B+.
    My only puzzlement was that having an open cap at that location wipes out the bias.
     
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  10. Musekatcher

    Musekatcher Friend of Leo's

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    The ability to trouble shoot and find faults is very valuable in about everything. I like reading these too.
     
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  11. Wally

    Wally Telefied Ad Free Member

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    Thanks for correcting me....there will be voltage in the rail. It will have AC ripple on it causing the hum. I was not thinking correctly.
     
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  12. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Whenever someone brings me something that's 15-20 years old I give them the full service speech. I'm cheap enough where they usually go for it without too much hassle, especially when it's likely the cause for the issues they're seeing anyway. No point in my troubleshooting stuff I know needs to get replaced :D
     
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  13. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    Help me to understand why an open filter cap would affect the bias voltage.

    Is the AC ripple enough to turn to DC through the 6L6 plate diodes? Would that drop the bias to zero or is there another reason?

    Would this happen on an amp that does not sport the plate diodes?
     
  14. schmee

    schmee Doctor of Teleocity Silver Supporter

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    Something about today's filters (last 20 years?) that Fender uses are not good. This seems to happen a lot in their more modern amps. Heck, you can often play a 50 year old Fender with original caps (not that you should!)
     
  15. Burning Fingers

    Burning Fingers TDPRI Member

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    Totally agree with you there... I have seen many amps that have the original caps from the 60's and they are fine and the owners don't want me to replace them...modern caps though..that's another story.. I try to get new caps that have many thousands of hours life span ( supposedly ).
     
  16. Burning Fingers

    Burning Fingers TDPRI Member

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    I really don't know off hand but it seems like a possible scenario.:confused:
    I'm too busy to delve into it so I will ask the guy who created the Lenard tube amps in Australia in the 60's when I see him again...there's nothing he doesn't know about tube amps.
     
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  17. Andy B

    Andy B Friend of Leo's Silver Supporter

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    The Illinois caps Fender uses in the HR series are prone to failure. I routinely replace them when servicing these amps. F&T caps are my go to replacements.
     
  18. Lowerleftcoast

    Lowerleftcoast Tele-Afflicted

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    @Andy B

    Look up *Capacitor Plague* on Wikipedia.

    From 1999 to 2007 higher-than-expected failure rate on electrolytic caps. Possible espionage of a stolen formula has been alleged.

    It has been said Fender used caps that were products of the *plague*.
     
    Last edited: Jun 5, 2020
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