The Dynaco Tube Audio Forum

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


    Dynaco FM3 Hum "issues"

    Share

    eickmewg

    Posts : 41
    Join date : 2014-08-29

    Dynaco FM3 Hum "issues"

    Post by eickmewg on Wed Feb 04, 2015 11:05 am

    I got an FM3 for a very reasonable price about 6 months ago.  My recollection was that there was negligible hum from the tuner at that time, but who knows.  Since that time I've done a few things to the tuner.  I got a new 6AQ8/ECC85 tube as I had read these were stressed and often bad.  I put a new power cord on it.  I replaced the can cap with a CE can with the same values as the original but with a higher voltage rating.  I replaced the output jacks with nice gold-plated units.  I cleaned all the tube pins but in pulling out one of the 6AU6 tubes, the grounding strap for the tube shield pulled out.  Recently, the tuner developed a rather serious hum.  With the tuner's volume control turned down, when I used the preamp volume control the hum got bad when it was turned up.  However, with the tuner volume pot turned up to maximum, there was very little hum with the preamp volume turned down to a low level.

    I've read quite a lot about this tuner and the list of things contributing to hum is a long one.  I know that hum is often an issue of bad grounding so I attacked that first.  Using the schematic I reflowed all the ground connections on the bottom of the boards and those to the volume pot on the top.  I then noticed on the assembly diagram what appeaed to be a solder bridge from eyelet 37 on the PC-8 board and the chassis.  I decided to try to fashion a chassis grounding strap using desoldering braid from eyelet 37 to the nearest bolt on the chassis.  I also was able to resolder the loose tube shield grounding strap to the PC-8 board for V6.

    Everything looked pretty good, so I put the bottom cover back on and gave it a test drive.  All the hum was virtually gone!  cheers   So, I don't know what if anything I did directly helped the problem, but I am very happy with the result.  I really do like this little tuner for our local jazz stations.

    Dynalover

    Posts : 12
    Join date : 2010-09-08
    Age : 52
    Location : Waterloo, Ioway

    Re: Dynaco FM3 Hum "issues"

    Post by Dynalover on Sun Mar 01, 2015 7:58 pm

    A properly aligned FM3 is a real jaw dropper on a good station!

    eickmewg

    Posts : 41
    Join date : 2014-08-29

    Additional information on the hum

    Post by eickmewg on Sun Mar 01, 2015 9:37 pm

    I was reading some information on the FM3 written by FM guru Bill Thomas and he suggested directly grounding the outputs to the ground pin for the V-72 12AX7 tube. I isolated the outputs from the chassis ground with the plastic inserts and ran short grounding wires from my new output jack ground lugs to this grounding point. As Bill suggested this also really helped with the hum issue. One day I will get the appropriate alignment tool and give my tuner a realignment. It sounds really nice with good stereo separation, even with the original PEC modules. The only real problem seems to be with dial tracking. The dial corresponds pretty well to stations around 88 and 89 megacycles but it is off quite a bit at the other end of the dial. I only listen to three or four stations, the highest frequency being 91.1 megacycles, so the mistracking is minor. Someday I'd like to replace the PEC's and I'm sure a realignment will be necessary. There also seems to be slightly more gain for the left rather than the right channel; maybe the V-72 amplification tube? For now, I think it is fine, I can live with it, and I won't take the chance of messing things up.

    Sponsored content

    Re: Dynaco FM3 Hum "issues"

    Post by Sponsored content Today at 10:29 pm


      Current date/time is Sun Dec 04, 2016 10:29 pm