Introduction
This is the second
commercially
made Fremodyne receiver I've acquired; the first being a Howard 474.
If you haven't come
from the Fremodyne
page, I suggest you visit there first for an introduction to this unique
method of VHF FM reception and for further description of this tuner. You
should also check the circuit and service manual here.
The Meck FM converter is like other FM converters of the time in that it is really just an FM tuner with an audio output. It does not actually convert the FM to AM (like the later FM converters used with AM car radios). This audio output is several hundred millivolts and is meant to be fed into the audio stage of an existing radio which typically consists of a triode voltage amplifier and pentode output stage.
I acquired this set in May 2005 from this site. And like the Howard 474 I would have never found one without the internet.
The
Circuit
Full circuit and service manual here
If you look at this circuit and compare
to the other Fremodynes, you'll notice the transformer coupled audio output.
Apart from this, the circuit is pretty much stock standard. However, instead
of using a 12AT7, a 14F8 is used. This is just the Loctal equivalent. Loctal
valves were developed by Philco for mobile use, to eliminate the possibility
of valves falling out under constant vibration. The base resembles that
of an octal valve but is metal and has a groove which engages in a spring
clip in the socket.
The pins are like later miniature valves
in that they go straight through the glass.
A 35W4 is used as a rectifier, and like
other U.S made radios of the time uses a line cord resistor for the heater
dropper. Again, as was typical of U.S design, this tuner has a live chassis
with no power transformer.
Cost cutting.
The tuner was in
excellent physical condition as I received it. In fact I doubt it has had
very much use at all. The first strange thing I noticed was the back was
held on by a couple of spring clips which push into mouldings in the bakelite
cabinet. What was even more strange was the chassis was not actually secured
to the cabinet. The only thing that prevents the chassis sliding out of
the cabinet is the knobs. To stop the chassis lifting up is a single 1/4"
Spintite screw through the cardboard back. Just as well that the chassis
is
very light! It was obvious this was John Meck's attempt at a penny pinching
design to get FM at a minimal cost. There is no dial lamp and instead of
four feet on the bottom of the cabinet, there's three. The dial itself
is interesting. The tuning condenser is fitted with a conventional dial
drum meant to be rotated by dial cord attached thereto. Not here there
isn't...instead a rubber grommet slid over the tuning spindle forms a crude
friction drive by being pressed against the dial drum. It would appear
that I could easily convert to a cord type dial drive if this rubber grommet
deteriorated. The final bizarre thing is the three terminals are not labelled.
How is anyone meant to know what they're for? Only by opening the tuner
and knowing the Fremodyne circuit did I work it out.
Getting it going.
Once out of the
cabinet I could see that a previous owner had done some work. The dual
B+ filter condenser had been removed and replaced with separate electrolytics.
The 8uF condenser in the waveshaping circuit had also been replaced with
a 15uF.
There was a 430
ohm 20W resistor which had one of its leads detached, probably due to being
posted half way around the world. A closer look revealed that the mains
lead was of the line cord resistor type. With this foreign resistor in
place it would appear that the resistor part of the line cord had gone
o/c. Indeed it measured thus.
The other components
were all original, including two paper condensers.
Above the chassis, the audio output transformer is clearly visible
on the left. The chassis is made of aluminium.
I powered up from one of my 240-110V transformers
and got the typical rushing sound of a superregenerative receiver. With
the "line cord antenna" already connected, the usual amount of stations
could be tuned in. I was using my bench amplifier which is a simple two
valve circuit with a 6M5 pentode and 6X4 rectifier feeding an 8" speaker.
The sound quality left a lot to be desired.
For a start, the SCA/stereo subcarrier interference was plainly obvious,
with a constant whistle in the background and distorted sound. As well,
the sound seemed to lack any bass content. Not how a Fremodyne should sound!
Improving the performance
The quench frequency was in dire need of
adjustment to start with. I found it to be about 20Kc/s. Nice 1000c/s beat
with the pilot tone there! First thing was to try replacing the two paper
condensers (.005 and .002uF). This improved things somewhat. The distortion
was improved but the whistle was still there. As mentioned in the Fremodyne
article, the quench frequency is controlled by the 150K and .005uF in the
detector circuit. Experimenting with various resistors showed that paralleling
a 560K across the 150K (118K) got rid of the whistle to a large degree,
with the quench now at around 33Kc/s.
With my outdoor 5 element Exastereo FM
yagi now providing the signal, it was at last behaving like other Fremodynes.
I had to realign the local oscillator
as 2GO on 107.7Mc/s was not receivable. This was easy and the dial was
accurate again.
Under the chassis. Note the 430 ohm heater dropper near the front.
The RF and local oscillator trimmer condensers can be seen behind the mains
switch.
Audio quality
At this point, the tuner was performing
as well as it could. Unfortunately the frequency response of the audio
output is poor. No doubt this is to do with the transformer output. I've
not seen this in any other Fremodyne circuit. It is possible that the DC
flowing through the primary winding is responsible to the bass lacking
sound. Also, there is no actual de-emphasis network which in the other
Fremodynes is performed by a 100K and .001uF. Surprisingly, there was relatively
little quench signal in the output when checked on the CRO. It is possible
that having a DC cathode load of much less than the original 22K has upset
operating conditions also.
So why is this transformer used?
Well if you look at the circuit
this converter originally used, you'll see that it wasn't.
My guess is that due to the live chassis
design that there were problems with users receiving electric shocks or
having problems with ground loops. Given that this tuner is an exercise
in cost cutting I'm surprised they went this far; a transformer is much
more expensive than a resistor and condenser.
The Heathkit Fremodyne FM Tuner, the FM1,
has a power transformer to provide isolation and would not have these disadvantages.
Safety. What safety?!
While I expected
it, it was hard to accept staring straight at me, the mains is directly
connected to the chassis which is fully exposed for the user to touch.
I am used to live chassis equipment, and would say I am one of very few
Australians not deterred by it.
But normally such
equipment has the chassis enclosed with no user touchable parts.
This delightful
little tuner, however, is a handfull of death when plugged into a typical
U.S power point. I just can't believe the back of the chassis is fully
exposed for the user to touch.
Look at the exposed screw in the back, the rivets securing the three
way terminal strip and the exposed chassis beneath the cord entries. These
parts are connected directly to the power line! Note also the absence of
labelling as to what the three terminals are for. From left to right they
are Earth, Aerial, and Power line aerial.
Not only that, the
single screw that holds the back to the chassis is not isolated. Here we
have 120V at the user's fingertips, just waiting for the user to be playing
around with the aerial or audio connections with tuner plugged in. This
little bundle of cuteness has its electrified behind ready for a mind blowing
surprise regardless of whether the tuner has its power switch on or off
too. If it's "on" then the chassis is connected directly. In the "off"
position, the low resistance of the heater circuit will ensure more than
enough current for a quick letting go, and the utterance of words like
"Shit/fuck! What was that?". And with a weak heart there could well be
one less FM listener.
True, if the chassis
is connected to the earthed side of the mains it is relatively safe, but
what about the 50% of times it isn't? And because the switch is in the
chassis side of the line, it means the chassis floats at 120V when
the converter is turned off.
At least the aerial
connections are isolated.
Instructions found folded up inside my 3rd Meck converter. "This
is a superheterodyne converter"...that's naughty! Reading that, one might
think of several IF stages, a limiter, and ratio detector. Superhet it
may be, but there's no getting away from the super regenerative detector
with inferior sensitivity, distortion, and noisy reception.
The 2nd and
3rd Meck converters.
Since acquiring
this converter, two more have appeared. No.2 looks like is has actually
been used a fair bit. This and No.3 are of what seems to be the later version
with resistance coupled audio output instead of the transformer. The chassis
in this version is actually isolated, although by a .2uF condenser. However,
such isolation is defeated and the aerial becomes a death trap when terminals
2 and 3 are joined as per the instructions.
I restored set No.
2 as per the first one, except there was a slight problem with the heater
supply. With the line cord resistor open in many places it was impossible
to use. As the heater current is 300mA in the 7F8/6H6 version, a resistor
under the chassis to take its place would be unacceptable with the heat
produced. So, I found a small 120 to 12V transformer which fitted in nicely
and solved the problem.
As for set No.3,
it looks virtually unused. Seeing as it is in such pristine condition,
I decided not to restore it. Much to my surprise, I found the instructions
had been folded up and stuffed into the back. Incidentally, this must be
even later that No.2, as they've actually put a screw in to secure the
chassis to the cabinet. Of course the line cord resistor is open in this
one too so I've not bothered to test it.
The
circuit for this version is here. Note that there is a mistake in the
circuit. There should be a .2uF condenser between B- and chassis, and the
earthy side of the audio output is taken from the chassis.
Conclusion
It has been since
1987 that I've wanted one of these having seen the initial mention in the
January 1948 "Electronics" magazine article, and then the "Radio-Craft"
December 1947 article. It was a surprise too when this one turned up for
sale on the internet, as I was looking for something unrelated. It's a
very nice looking unit too which is the sort of thing that should be on
display.
The live chassis
issue is not a problem here as I have plenty of double wound transformers
to plug it in to. Using an autotransformer to power this set is not acceptable.
Sensitivity is much
the same as other Fremodynes. It's good to see also that the layout is
not as critical as I thought, with some component leads left longer than
they should be. But I tell you what, those cone shaped knobs are certainly
a deterrent to the most ardent of knob twiddlers...just as well as I leave
it on 2WS most of the time.
Sound quality is
the only let down, due to the transformer coupling in the first converter
but for non critical listening applications it is ok. The later converter
design has somewhat better sound quality.
They're a set I'm
glad to have in my collection.
cablehack@yahoo.com
