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    FM-5 Tuner Weak & In/Out Receive

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    8xlaxx

    Posts : 18
    Join date : 2016-12-10

    FM-5 Tuner Weak & In/Out Receive

    Post by 8xlaxx on Wed Dec 21, 2016 9:45 am

    Since I'm waiting on an attenuator to install for the pre-amp (separate thread),...

    I recently acquired a Dynaco FM-5.  It powers on and I can hear stations with an antenna connected but even with the strongest of stations close by my home, the signal is still very weak.  The blue strength bulbs will light up and the Dyna-Tune function works.  Sometimes the signal will pop out then back in and it is loud with meter deflection, then all of a sudden it drops out again and is difficult to hear the strongest of stations again.  Currently it is difficult to hear any stations.  

    I am trying to troubleshoot this issue and wanted to ask where I should start first?  

    I realize that this is NOT tube gear but thought that several of the members here might own this particular Dynaco unit.  Thank you for the help.

    GP49

    Posts : 781
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: FM-5 Tuner Weak & In/Out Receive

    Post by GP49 on Wed Dec 21, 2016 10:40 am

    First VERY sketchy guess would put the problem in the RF amplifier, the first active stage that raises the raw antenna signal.  It is inside the sealed front end module and not considered serviceable.  Second guess would be in the IF section, a series of gain stages that operate at 10.7 MHz to raise the signal level to where the detectir can convert it to usable audio, and select one channel from adjacent ones.

    Problem is not going to be in the detector, audio or multiplex sections.  The signal strength meter and center tune light operate prior to them.

    There were two major versions of the FM-5; the IF stage was revised during production. I have had to replace uA703 gain stage devices in the IF on FM-5s and on many other tuners and receivers that used them...they were not the most reliable ICs.  But even if a uA703 is a potential problem, you need to trace the problem to a particular device first, if indeed a uA703 is at fault.  Generally they were not intermittent; nearly all were go/no-go.

    This kind of problem, when it is happening, is pretty easy for a good tech with proper test bench gear...a suitable RF generator and oscilloscope...to trace.  Mine isn't even set up any more, it's in the garage!  For an end user I suggest checking for poor solder connections especially if the FM-5 was built from a kit.  The FM-5 kit was supplied with the front end and circuit boards factory-assembled.  The kit builder did mechanical assembly and interwiring.
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    Peter W.

    Posts : 497
    Join date : 2016-08-07
    Location : Melrose Park, PA

    Re: FM-5 Tuner Weak & In/Out Receive

    Post by Peter W. on Wed Dec 21, 2016 11:07 am

    8xlaxx wrote:Since I'm waiting on an attenuator to install for the pre-amp (separate thread),...

    I recently acquired a Dynaco FM-5.  It powers on and I can hear stations with an antenna connected but even with the strongest of stations close by my home, the signal is still very weak.  The blue strength bulbs will light up and the Dyna-Tune function works.  Sometimes the signal will pop out then back in and it is loud with meter deflection, then all of a sudden it drops out again and is difficult to hear the strongest of stations again.  Currently it is difficult to hear any stations.  

    I am trying to troubleshoot this issue and wanted to ask where I should start first?  

    I realize that this is NOT tube gear but thought that several of the members here might own this particular Dynaco unit.  Thank you for the help.

    I keep both an FM5 and an AF6 that I assembled from kits. The FM5 I have came (unlabeled) with a factory-supplied Dolby board, and a *mimeographed*!! instruction sheet on how to install it. I purchased the kit at the Dynaco factory in Blackwood, NJ only weeks before it closed, for pretty much pennies on the dollar. The AF6 I purchase un-assembled at a yard-sale for pocket change. Back in the day, Dynaco was the Rodney Dangerfield of audio - and go NO respect. 90% of the problems such as you are experiencing are, in my experience, mechanical in nature, not electronic.

    So, the first thing you want to do is clean all the switches, with special reference to the Dynatune/Stereo/Mono switch. Everything goes through that switch, so if it is dirty or discontinuous, you will get the symptoms you describe. Next, clean the VC control. Dynaco used the *CHEAPEST* controls they could find at the lowest possible prices. If you have a pre-amp, try jumping out the VC and see if that brings back your volume.

    Lastly, wiggle all the connections to the boards - I have come across more than a few cold solders, even no-solders on these boards. I use a thin wooden coffee-stirrer to poke around as the connections are at the bottom of the standing boards, not conveniently on top.

    If none of the above is of any help, Gene's advice is excellent and would be your next natural step.

    Lastly, if you do find out it is the front end, they are commonly available on eBay - the chance you take is that it is no better than yours.

    GP49

    Posts : 781
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: FM-5 Tuner Weak & In/Out Receive

    Post by GP49 on Wed Dec 21, 2016 8:24 pm

    As for the front end...

    I have an AF-6 which, if it sits for months without use, oscillates and "whistles" at the low end of the FM band. The easy but temporary cure is to run the tuning up and down over that range a few dozen times. If I used it regularly, the problem would never show up. As it is, I have no desire to open up the front end to apply contact cleaner where it probably would do some good: on the grounding wipers of the tuning capacitor. This almost always would fix the same symptoms on Japanese receivers...Pioneers seemed to suffer the most from it...but on those, the capacitor was exposed and easy to reach.

    Right now I am using an FM-5 with an EICO HFT-94 tube-type AM tuner. Maybe should put the AF-6 back again?

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