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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    Dynaco FM-1 Hum on high bass

    bubbasweet
    bubbasweet

    Posts : 87
    Join date : 2016-03-14

    Dynaco FM-1 Hum on high bass Empty Dynaco FM-1 Hum on high bass

    Post by bubbasweet Fri Aug 20, 2021 12:51 pm

    Just did a new Can Cap on my FM-1. Unit Sounds great, tunes well. Transformer has small vibration. When I peel the bass back all the way with loudness on on the amplifiers it has no hum. It is bassy enough this way even with the bass off. When I turn the bass to middle. I can hear a faint hum in the speaker not tuned in to a station. Is there a filament hum or hum adjustment on this unit? I also notice on the Can Capacitor the lowest voltage 2 lugs the voltage is 20V less than document says. 2 highest voltage lugs are bang on within a volt or 2. Wonder if I have a problem. Any tips on hum in this unit?
    Bob Latino
    Bob Latino
    Admin

    Posts : 3074
    Join date : 2008-11-26
    Location : Massachusetts

    Dynaco FM-1 Hum on high bass Empty Re: Dynaco FM-1 Hum on high bass

    Post by Bob Latino Fri Aug 20, 2021 2:29 pm

    bubbasweet wrote:Just did a new Can Cap on my FM-1. Unit Sounds great, tunes well. Transformer has small vibration. When I peel the bass back all the way with loudness on on the amplifiers it has no hum. It is bassy enough this way even with the bass off. When I turn the bass to middle. I can hear a faint hum in the speaker not tuned in to a station. Is there a filament hum or hum adjustment on this unit? I also notice on the Can Capacitor the lowest voltage 2 lugs the voltage is 20V less than document says. 2 highest voltage lugs are bang on within a volt or 2. Wonder if I have a problem. Any tips on hum in this unit?

    The Dynaco FM-1 came out in 1961 so these units are 55 to 60 years old. They discontinued the FM-1 in the mid '60's when the FM-3 (stereo version of the mono FM-1) came out. You should probably rebuild the unit and go through the alignment process again as outlined in the assembly manual. Many of those carbon composition resistors on the PC-7 and PC-8 boards have probably drifted from their original value. This drift would hinder a good alignment using the resistors that are on there now. See FM-1 assembly manual at the link below ..

    Dynaco FM-1 assembly manual

    Bob
    bubbasweet
    bubbasweet

    Posts : 87
    Join date : 2016-03-14

    Dynaco FM-1 Hum on high bass Empty Re: Dynaco FM-1 Hum on high bass

    Post by bubbasweet Tue Aug 24, 2021 1:04 am

    Wow ! you're not kidding Bob ! Both 2 W 10 K resistors R11 and R13 one was at 750 ohms and the other 3.4K...that was the issue. Can cap was even getting warm because of the excessive current draw.
    bubbasweet
    bubbasweet

    Posts : 87
    Join date : 2016-03-14

    Dynaco FM-1 Hum on high bass Empty Re: Dynaco FM-1 Hum on high bass

    Post by bubbasweet Tue Aug 31, 2021 11:53 am

    I am copying this over from Audio Karma page as they had the answer to the hum on my FM1...

    I was slaving over it. In the end it was a good thing, it made me look over the entire set anyway as I found 2 bad reisistors (the 10Ks off the Can were super low likely because of heat over the years) and 2 tubes with shorts in them that were somehow still working. But you nailed it. Mine does not have the multiplexer board but: I isolated the RCA jacks in the back from the chassis (you have to do this just adding the ground will not remove hum) and grounded the RCAs Ground lug to the center pin of the 12ax7 and BOOM. Hum gone THX. I guess it is because they used the chassis side as the low side of the filament as someone said. THX so happy I plan on using this for years to come.

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