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    Started down the road.

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    CNCfan

    Posts : 21
    Join date : 2018-08-13
    Age : 70
    Location : Houston Texas

    Re: Started down the road.

    Post by CNCfan on Fri Sep 07, 2018 5:07 pm

    J beede

    Can’t hear any hum from the power transformer.

    Checking a bit further as I did not check the right speaker. (thanks for your assumption) The right speaker is quiet. I have the FM3 plugged into the Preamp, with two prong plug. Preamp is plugged into multi power box with two prong plug. ST70 plugged into power box with 3 prong plug.



    PeterCapo,

    Thanks for suggestion and link will buy one.


    Peter W.

    Shorting input made no difference that I could detect.

    On checking with a VOM from main ground the builder used , I tested to multiple other ground points around the chassis and boards. I was getting .010-.012 volts.

    When I used a two prong plug the hum was a bit louder. Did not notice any difference when
    I reversed the plug in socket.

    All wiring that went to ground used same ground point. (lug of hold down for power transformer.)

    “ So, the dedicated receptacle *here* has same-gauge home-run straight to the panel, but the undedicated receptacle *there* is at the and of a 7-device daisy-chain. So, any device plugged in *there* will be 'above ground' relative to *here*. “

    I have always been a bit “hazy” when I watch a youtube videos with people repairing amps and old radios when they mentioned this. That one sentence has made it very clear to me now. Thank you for putting it in a way I could understand.

    Understand about a cover board for the driver board. We have three small dogs (20 lbs + -). The
    youngest like to jump from any surface she can land on then on to the next. I could see her landing on top of the ST70. I will use the cage I have for the parts chassis if I do not put it in
    an audio cabinet with glass front. Also have a grand daughter to worry about when she is around.



    Rjpjnk.

    On testing my unit the hum is less with a three prong plug. Will be using it unless in the future I find it is causing problems.






    Also did following:
    I also swapped the output tubes, made no difference. Swapped the ECC82EHs with no difference. Will have to get another 12au7 to test with.


    Went over the wiring a few times and could find nothing that looked like it may be casing the hum in a psychical nature.

    rjpjnk

    Posts : 114
    Join date : 2018-07-18

    Re: Started down the road.

    Post by rjpjnk on Fri Sep 07, 2018 8:17 pm

    So the hum is only in the left speaker. That is good news because it means when you fix it it will be as quiet as the right speaker is now.

    So we know the hum is not due to B+ power supply filtering since only in one channel. It may be 60Hz hum from the filament wires. Check to see if any filament wires on the Left channel are close to other wires and/or not twisted tightly. Maybe that is what you already checked.
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    peterh

    Posts : 1021
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Started down the road.

    Post by peterh on Sat Sep 08, 2018 1:50 am

    If the hum is only in one speaker, try to swap the two outer 12au7 tubes. If hum follows tube then replace that tube. If hum stays you might need to adjust your cabling underneath and/or groundings.
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    CNCfan

    Posts : 21
    Join date : 2018-08-13
    Age : 70
    Location : Houston Texas

    Re: Started down the road.

    Post by CNCfan on Tue Sep 11, 2018 1:14 am

    Tried the tube swaps and no change. (also all the other suggestion made in this thread)

    Also triple checked all the wiring and moved some around so see if anything helped. No luck.

    Not sure what to do now except to start swapping out parts.

    My parts chassis is in the planning stage. I think when I buy parts for it I will put them in
    this amp and move the current parts to the parts chassis build and see what develops. This will take a bit of time as I want to replace the power
    supplies in the PAS and FM3 and to recap and/or maybe board upgrades first. Also have to find some speakers for current amp.

    Want to thank everyone for your help and suggestion. Really appreciate it.

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    Re: Started down the road.

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