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    FM-3 Rebuild


    Posts : 227
    Join date : 2013-02-10

    FM-3 Rebuild Empty FM-3 Rebuild

    Post by Maintarget on Mon Aug 04, 2014 10:11 pm

    I'm getting ready to do a complete rebuild of a Dynaco FM-3 and purchase a new mother board and Cap board upgrade from the FM-3 Zone
    I have surfed the web and read every article I could find on refurbishing the Dynaco FM-3 of particular interest was a thread by Bill Thomas and VanAlstine and many others, Anyone have some real hands on experience or recommendation they would like to share?
    Looking forward to adding to my VTA ST-120
    Much appreciated

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    Join date : 2013-03-11
    Location : California

    FM-3 Rebuild Empty FM-3 is not like a PAS3X

    Post by setu on Wed Aug 06, 2014 9:16 pm

    Hi Maintarget,

    This is my first posting here. I am thankful for this fine Dynaco forum.

    My FM-3 amateur project was previously posted 8/8/2012 on Audio Asylum>Dynaco-doctor
    Yes, Bill Thomas (RIP) was a great friend to us Dynaco folks.
    Time-arithmetic suggests that he was posting & helping while dealing with fatal disease.
    And then his great postings stopped...

    I lack experience with the new combined-circuit board or psu from
    FM-3 zone but I am a very happy previous customer there.
    Perhaps a new combined board avoids many typical FM-3 probs&difficulties?
    I used an SDS psu board----it is a tight fit.

    I only entered the "different" FM-tuner world because of my marginal FM-3 and the
    desire to round-out my Dynaco experience. Reading the Bill Thomas articles
    on PC-7,PC-8,PC-12 boards greatly helped with understanding the FM-3 manual.

    Rebuilding the 3 PC boards is do-able----but it was much more challenging
    than my PAS3X or ST70 projects. I cant easily measure 10.7MHz and the FM-3
    required much more dexterity,patience,calm-attitude,hemostats,magnifier,mirror,understanding...
    More difficult---but still rewarding.

    Some FM-3 rebuild thoughts:
    1) optimize the FM antenna because it sources the totsig
    2) alignment was the last and clearly MOST productive task I did----and much easier than expected
    3) heat dissipation requires 4 taller feet and more chassis vents
    4) the total-signal seems most accessible/familiar for our upgrade in the PC-12 stage
    I was afraid to "improve" components in the high-freq PC-7 or intermed-freq
    PC-8 stages. And how to compensate thermal variation? Unwanted inductance?
    5) tube-socket hygiene is under-estimated improvement here----higher heat seems to
    fuse/oxidize tube-pin "dirt"
    6) time,money,nervous&tedious work on this older less-sensitive FM-tuner was
    questionable(compared to my SONY XDR-F1HD) however I love Dynaco and
    its tube sound---Telefunken 12AX7 in PC-12 of course!
    7) I choose electrolytic caps based on lowest ESR,select tubes with my B&K747B,
    improve Dynaco-specified resistor-tolerances of 5% with <1% matches(Fluke 83)
    Cool I reflow most old solder joints----esp if they seem un-shiny
    9) ensure all AC wiring(including tube-heater wires) is separated from signal wires and/or
    crosses them at right-angle
    10) ensure tube-shield grounding

    FM-3 posts seem rare on this Dynaco forum?

    Best of luck on your rebuild.
    I found very interesting---they call them tunas!

    ps: I wish we fans could buy a black t-shirt(cotton size L)
    with the DYNACO logo on it


    Posts : 227
    Join date : 2013-02-10

    FM-3 Rebuild Empty Re: FM-3 Rebuild

    Post by Maintarget on Wed Aug 06, 2014 10:00 pm


    I'm surprised there isn't more chatter on this site about the FM-3 I know it is not the best tuner ever made, I can't begin to tell you how proud I am to be able to say I built my amp (VTA ST-120) and enjoy it everyday with that said I'm interested in the learning and doing part to be able to say I modified and assembled that tuner.
    FM-3 Zone claims that their main board eliminates 99% of the original wiring and the change to a single ground plane results in a reduction in noise to a "Non-exsitant level" (I guess I'll find out if it's true or not Smile and report back)
    Based on what I have read on line it sounds like it would be wise to add a couple of small computer fans to help extract the heat from the unit.
    My biggest concern is the alignment process.
    Thanks for your response

    Posts : 6
    Join date : 2013-03-11
    Location : California

    FM-3 Rebuild Empty FM-3 internal grounding

    Post by setu on Thu Aug 07, 2014 12:47 am


    Zone is good word for the tuner world.
    I found that world to be a very different 'zone' from pre-amps,amps, and other sources.
    The theory/analysis/strategy/vocabulary of FM-3 stages PC-7,PC-8 is unlike
    the PC-12 amp stage(most familiar stage for me). For me those first 2 stages
    were much more about learning than doing.

    Pre/amps seem the business here----rather than FM sources.

    My wired FM-3 is very quiet---perhaps because I tied all 3 boards,tube-shields,
    psu gnd, & other gnds into 1 large star-gnd. And I used a 240W Weller
    solder gun to reflow the ground plane connections of PC-7,PC-8,PC-12 to
    chassis. I imagine the new board's lack of inductive wiring is vg although
    I dont see induced noise as very significant. However the new board should
    immediately replace all the out-of-tolerance 45 year old parts in 1 global step.
    Probably a HUGE increase in reliability?---who wants old carbon resistors.

    The FM-3 manual's alignment instructions do need to be read more than once and
    also review the clarifications already posted elsewhere.
    Some FM-3 alignment thoughts:
    ---DO NOT attempt to adjust factory-adjusted input side of transformer T1 unless
    you have equip to reset it to 10.7MHz
    ----DO NOT force any transformer slugs and only use PLASTIC allen wrench
    ----DO NOT align until unit is fully warmed up
    ----1 adjustment affects previous adjs so you may need to do cycle at least 3 times
    until more improvement is not possible.
    ----you may grin with happiness after you complete the alignment because the
    improved sound will be so obvious and it was not as hard as you expected.
    ----DO NOT ignore the dangers of this LIVE-voltage procedure
    ----you probably cant hurt anything by practice alignment(s) but I cant suggest
    excessive fiddling with these 45 year old tuning transformer slugs either
    ----Dynaco designed this diy procedure very well
    ----alignment is your final electric change & a friend!----it will then optimize each of 3 stages
    that you will have replaced by the new combined board

    I'm interested in hearing how well your new FM-3 board & psu-board work out.
    Best of luck!


    Posts : 48
    Join date : 2012-04-15

    FM-3 Rebuild Empty Re: FM-3 Rebuild

    Post by turbotoy on Thu Aug 07, 2014 8:23 pm

    I fixed up an FM-3 a couple of years ago. I got a barely working one that was very clean and had all the original Dynaco/Telefunken tubes that I could have resold for more than twice what I paid for the tuner. I installed the "full" upgrade kit from FM-3 zone, and the replacement PEC modules (or an equivalent fix) should be consisderd a must. I used the need to do an alignment as an excuse to buy an RF oscillator to have on the bench and *did* adjust T1 to 10.7MHz after doing some checks to ensure that the oscillator was reasonably in calibration. If you don't have that equipment, do not touch the input slug of T1. Actually, now that I think of it, be very careful when moving that transformer to the motherboard.

    The alignment procedure seemed very effective using only the tuning eye per the procedure in the assembly manual. In other words, I never wondered if I was actually doing it right, and this was the first tuner I had aligned, so don't worry much about that. I think the entire tuner design was exceptionally clever for that reason. I did de-solder/re-solder wires while powered (and at high voltage) as the manual suggests is possible during the alignment, but in retrospect I can't believe they printed that, and probably wouldn't recommend it.

    How does it sound? On strong signals it is quite impressive for what it is. Friends have looked at me rather shocked when they realized that we were listening to FM. Honestly, the noise floor is the least of its issues, mine is very quiet. It really does need a decent rooftop antenna to pefrom best, ideally with a rotor as it is not a very selective tuner. I am curious about how yours will turn out using lots of fresh components on the motherboard, as that can only improve things.

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    Join date : 2011-05-09
    Age : 52
    Location : Texas

    FM-3 Rebuild Empty Re: FM-3 Rebuild

    Post by denny9167 on Fri Aug 08, 2014 5:14 pm

    I've rebuilt several FM-3's over the last couple of years, and designed my own PCB's , it really is a remarkable design, especially for the DIY'er. It's not the best tuner overall, but what it lacks, it makes up for in sound, simply amazing sound!! Ground plane, ground plane, the most important factor in determining the performance, if the ground planes have good contact with the chassis you are over halfway there in alignment. T-1 is important as well, and the alignment screws on the variable capacitor are essential in grabbing those stations. Good luck!!

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    Join date : 2013-02-10

    FM-3 Rebuild Empty Re: FM-3 Rebuild

    Post by Maintarget on Fri Aug 08, 2014 9:30 pm

    I bought two very nice original working tuners off evilbay so worse case scenario I got two shots at getting it right Smile
    Denny what kinds of changes/upgrades if any did you incorporate into your board design?  
    I really appreciate the advice and look forward to the build.

    Posts : 151
    Join date : 2011-05-09
    Age : 52
    Location : Texas

    FM-3 Rebuild Empty Re: FM-3 Rebuild

    Post by denny9167 on Sun Aug 10, 2014 8:21 am

    Just a few component changes,one was higher wattage plate resistors to both V2 and V3, updated diode bridge in multiplexer circuit. I also added dip switches to both PC-8 and PC-12 to aid in the alignment process. I haven't ordered any more boards in a couple of years, because I've been busy working on other projects. New boards really make a difference in performance. If I get some free time, I'm going to rebuild a couple. I still have several FM-3's that are just sitting in storage that are waiting to be rebuilt, I do have one that is still nearly all original, will probably not do anything to it, still works great.

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    FM-3 Rebuild Empty FM-3 Rebuild(2-cautions)

    Post by setu on Sun Aug 10, 2014 3:38 pm

    I mentioned resoldering the ground planes of PC-7,PC-8 boards to
    the FM-3 chassis with a bigger soldering gun (eg Weller D550 240/325W) to better ensure ground.
    I just noticed in my notes that the FM-3 is a cadmium-plated chassis---thus fumes
    might be toxic at higher chassis-soldering temp---or maybe not?
    I realize chassis-resoldering is not yet at hard-soldering temperature
    and I have sometimes foolishly ignored lead fumes...
    However it might be smarter to resolder FM-3 chassis-grounds outside?

    Before working on unit I discharge PSU capacitors.
    I do this via an diy XMAS light between PC-7(#9) to ground for B+1
    and PC-7(#12) to ground for B+2.



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    Join date : 2014-08-18

    FM-3 Rebuild Empty Just Finished FM-3 Alignment

    Post by tgarson on Mon Aug 18, 2014 2:44 am

    I just completed service and alignment of a factory built FM-3 for a client (along with the PAS-3X and Mark III pair that he inherited with it).

    All pieces were in excellent condition, obviously well cared for, and turned out as good as it gets.

    The FM-3 presented several issues that might not be apparent to even many experienced techs.

    First my opinion:
    A properly working all original FM-3 will not benefit from part subbing, "upgrading", or any other sort of mods which don't substantially change its fundamental design, subsequently making it not an FM-3.

    The  FM-3:
    The FM-3 is (and its predecessors, FM-1, etc.) fairly unique in that it was designed from the very outset to be assembled from kit form, and acceptably field aligned with minimal or even no electronics tools other than a VTVM.

    In nearly all FM-tuners, the double tuned (slugs top and bottom) Intermediate Frequency (IF) transformers are stagger tuned with one coil tuned slightly above and the other slightly below 10.7 MHz. The resulting passband shows up on a spectrum analyzer, or 'scope display of a dual sweep alignment signal, as a flat topped "peak" with the sides of the peak just above and below 75kHz out from 10.7MHz. The flat top width being the required 150kHz bandwidth for quality FM Broadcast reception. Each coil inside of each transformer has a fairly high "Q", determined so that maximum out of band slope is achieved with minimal "dip" (ripple) in the filters center region (10.7MHz).

    The FM-3 is not designed to operate correctly with its filters set up stagger tuned. All of the filters that make up the coils in its IF transformers are relatively low Q. The FM-3 gets its selectivity  from having all filters tuned to center (10.7 MHz). There is enough gain in the last two stages to achieve limiting with any reasonably decent input signal. The limiting "flattens" the top of IF passband so that there is minimal amplitude variation of the signal going into the detector. Further, the FM-3 detector was changed to a ratio detector from the discriminator originally used, even though the manual still calls it a discriminator. The output from a ratio detector is much less effected by amplitude variation (passband ripple) than a discriminator.

    Before any other work is performed, use a good quality contact cleaner to thoroughly clean the ball bearings and brass contact plates between the sections of the tuning condenser. It won't hurt to dribble solvent down inside the brass planetary tuner drive tube and then force some light grease into it using compressed air.

    Due to age, ANY FM-3 will need all transformers aligned, including TR1, because of component value drift. This can be accomplished by either having a precise 10.7 MHz sine wave signal source with which to directly drive TR-1, or having a non precision (but stable!) FM band signal source (use NO modulation) and using a quality frequency counter to make certain your local oscillator is tuned so that 10.7MHz (+- a few percent is OK) is being produced at the mixer tube plate.

    Once you are certain that you are actually driving the IF strip with 10.7 MHz, tune all of the transformers for peak reading on the FM-3s signal strength meter. Keep in mind that this is an iterative process. Several passes of adjustment will be required to get all transformers optimally peaked. Keep the input signal level low enough so that the bars of the meter never quite touch.

    Once the IF is peaked, including the BOTTOM slug of the detector transformer, use a 'scope or DC voltmeter to adjust the top slug of the detector transformer for zero Volts d.c. at the junction of R25 and R26.

    Follow the manual for adjusting the RF section.

    This should get your (otherwise properly working) FM-3 into a pretty good tuning and sounding state. If you want to gild the lily, you will need at least a low distortion FM signal generator with the ability to attenuate its output so that you can feed only about 5 uVolts into the antenna input of the FM-3. With modulation of the generator set to +-75kHz and output set to 5uVolts into the antenna input, you will hear a bit of distortion in even a well aligned tuner when listening to the tuners output. You can slowly and carefully adjust the IF and detector coils and hear noise level and distortion varying. With a bit of experience, you can achieve best signal to noise and least distortion by simply listening to the tuners output. Of course, you can also use a distortion analyzer to quantify the process, but it would not yield significantly better results.

    Thomas Garson
    Aural Technology, Ashland, OR

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    FM-3 Rebuild Empty Re: FM-3 Rebuild

    Post by Dynalover on Sun Sep 07, 2014 7:44 pm

    When they are setup right, they are total "ear candy", believe me. Nobody would go through all the trouble of rebuilding them otherwise. The soundstage is downright spooky. They are very sensitive, their chief limitation would be selectivity in a crowded market.

    You've gotten some good advice, can't really add much but reiterate that it's best of course to start out with a low mileage example. The Epperson upgrade kits are excellent value. I elected to use a can cap replacement for the electrolytic section. Just make sure you are completely done with everything - parts replacement, tubes, etc, before attempting any alignment. It would be best to re-align the bottom of T1 to 10.7 if possible, this came prealigned from the factory.. The idea to remember though, is that if you re-align everything each time you upgrade a component, eventually this will tend to loosen the cores up in the transformers. Be sure to remove the "gimmick" capacitor before alignment of the discriminator section and then reinstall.

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