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Radio Road: A Paean to the Dyna FM-3 (Rev 1) by stellavox

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A Paean to the Dyna FM-3 (Rev 1)

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(that's not "pee on")

Who(m?) of you old timers haven't owned at least one piece of Dyna
(Dynaco) gear, or better yet had the unmatched opportunity and
experience of building one? I built a Stereo 70 in college and have
owned at least two more, plus an equal number of PAS preamps, SCA-35
integrated amps and as many of their tuners.

While the amps and preamps, after the inevitable tweeking, gave
excellent account of themselves, I never thought the tuner to be in
the same league as the other tube classics from the likes of Scott,
Fisher and the like. Well I was wrong.

I recently moved from Connecticut and took the opportunity to
downsize, actually selling my primary listening system in the
process. Still recovering from that mistake, I needed an "upstairs
system" to enjoy in the evenings while reading or early mornings
while cruising the "Net".

I happened upon a pair of LS3/5a clones and was initially happy
driving them with an upgraded SCA-35 and a Sony CD player/burner but
longed to get "back on the air". I realized that I was given a Dyna
FM-3, which was sitting in a box along with some other gear needing
repair. OK, let's haul it out, plug it in and see what gives; so the
odyssey began.

During the process of acquiring a manual, I discovered that the
original FM-1 monaural Dynatuner was designed in the late 1950's by
audio legend Stu Hegeman, with help from Dyna's Ed Laurent. Sid Lidz
later designed the remarkably simple, time-division multiplex section
and the resulting stereo tuner was dubbed the FM-3. With its
varieties and the fact that it was available both as a kit and
factory assembled, it could very well be the best selling tube tuner
ever produced.

Per recommendations from an old Audio Dimensions Inc (ADI)
modification article, I first checked the values of the important
resistors and found them to be within tolerance. Resistors R11 and
R12 that drop the screen voltage for the first two tubes on the PC-8
IF board were undersized, and the resulting heat blackened the
phenolic board material. Change these 10K ohm units to 5 watts
minimum and space them off the board. Objectionable hum in the audio
output was a fairly common problem; caused by one or more sections of
the electrolytic filter capacitor C32 drying out - mine was OK
(fortunately).

One of the neat features of the tuner's design was that you could
align it using only the built in tuning-eye tube and the signal from
a strong FM station. The multiplex section could be aligned in the
same manner using the 19Khz pilot signal from a station broadcasting
in stereo. I tweaked the alignment following the instructions then
checked it again using my Radiometer RF sweep and Stereo Multiplex
generators. I couldn't measurably improve upon any previous
adjustments and consider this a REAL tribute to the expertise and
thoughtfulness of the designers. They allowed kit builders to
accomplish excellent results with NO test equipment required!

At this stage of the game I hooked the FM-3 to my "upstairs" system
and found as I had remembered, that the sound was Hi-Fi - OK but not
great. This I had anticipated after reading another modification
proposed in a 15 year-old Audio Amateur magazine article.
The "culprit" masking truly good audio presentation lies in the
encapsulated modules that follow the multiplex decoder. These
provide the de-emphasis of the transmitted audio signal while
filtering out high frequency artifacts from the demodulation
process. Apart from using questionable ceramic capacitors in the
module, a MUCH better design was proposed by David Berning. Unsolder
the old identical 3 terminal modules labeled PEC-1 (left channel) and
PEC–2 (right channel) and replace them with low-pass filter networks
that I will try to describe in the following paragraph.

Each PEC module's input is connected to pin 1, with pin 2 connected
to ground, and the output taken from pin 3. Replace this with a
network consisting of quantity 3, 10Kohm resistors (5% or better, ¼
watt or larger) in series. Connect this 3-resistor, series
combination between pins 1 and 3. Now take quantity 3, 510picofarad
(5% or better) polystyrene or equal caps, connect one side of each
cap to ground (pin 2) and the other side of the first cap to the
junction of the 1st and 2nd resistors; the other side of the second
cap to the junction of the 2nd and 3rd resistor and the other side of
the third cap to the end of the last resistor (also connected to the
output pin 3). Finally, put a 100picofarad cap in parallel with the
3-resistor string, i.e. from terminals 1 to 3. That's it. I kludged
the network from junkbox parts and put it under the chassis where you
won't see the "mess" (don't look behind that curtain). You can mount
the parts on a perf-board if visuals matter. O yeah, I also suggest
replacing C26, the 10nanofarad ceramic coupling capacitor at the
output of the discriminator with a better type, say mylar.

This new network literally transforms the sound and after a week of
listening I still can't believe how satisfying it is. As is pointed
out in the FM-3 review elsewhere on this site, the tuner has decent
but not killer RF performance. Sensitivity is very good but
selectivity isn't and mine does suffer from intermod effects. On the
other hand, I haven't found much quality FM in Oneonta, NY.

After living with the modded tuner for a few weeks I've found that the audio output was way too high and turning down the tuners volume control introduced hum into the signal, so I bypassed the control and changed the circuit of the audio output tube (V72) to a cathode follower. The new circuit is identical to that of V7A except with a 1K ohm grid resistor for R85/86. The audio output comes from the cathode (pin 3 or 8) through a 2 mF, 50 volt mylar cap. The cathode follower has much less gain than the stock output stage, but there is still plenty and the lower output impedance of the cathode follower can easily drive a transistor preamp/amp, whereas the stock circuit wants to see a high impedance tube preamp.

Thank you Stu, Ed, Sid and David for helping me continue to derive
pleasure well after you have moved on. Your product efforts certainly
deserve to be included in the "High End" pantheon.


CHEERS

Charles King


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Topic - A Paean to the Dyna FM-3 (Rev 1) - stellavox 05:07:39 02/23/06 ( 5)