Meck FM Converter CX-500


Introduction
This was the second commercially made Fremodyne receiver I 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.

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.


The Meck FM converter is announced in September 1947.

I acquired the first of my Meck FM converters in May 2005 from this site.

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 Sylvania 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.

Taken from the Howard Sams manual, this shows two more variants of the Meck circuit. A fourth variant incorporates an additional 14F8 performing as an AM modulator, thus allowing the demodulated signal to be fed into an AM radio.

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 stabilising 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. 430 ohms suits 110V mains, but as I have 240V to 120V transformers to run my U.S equipment, this resistor will need to be increased to 480 ohms to keep the valve heaters within ratings.
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 240-120V 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 can be controlled by the 150K in the stabilising circuit, within limits. 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 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 should. 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. This is quite likely as the cathode stabilising circuit would no longer operating as intended. What should have been done is to retain the 22K and capacitively feed the transformer primary.
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.
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.



The Meck collection expands.


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.

Since acquiring the first converter, more have appeared. No.2 looks like is has actually been used a fair bit. This and No.3 & 4 are of what seems to be the earlier 6H6/7F8 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. The audio output circuit uses the normal Hazeltine circuit with the 22K cathode resistor and 100K + .001uF de-emphasis/quench filter network. This results in a vast improvement over the transformer coupled model, and sound quality is good. With my outdoor aerial connected, it is a really nice tuner to use and listen to.
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.
Set No.4 is also a virtually unused example, and again, the instructions came with it. Not surprisingly the line cord resistor was open so haven't tested this one either. I don't have plans to restore it either, as two working units are sufficient for now.

By the time Meck no. 5 and 6 came along, I'd started to lose track of their individual history. One came from Andy Mitz who authored this once well known FM radio site. He was selling some of his collection.

Line Cord Resistor for the Meck.
Around this time, I had developed a method for constructing replacement line cord resistors. Having so many Mecks, and now Emerson CF255's, there was an incentive to work something out once and for all. The scheme involves electric blanket heating cable, and a shoe lace outer covering. You can see how it's done here.
The first line cord went on Meck no. 5 and was a complete success.



Obsessed with Meck FM Converter? The first six in my collection..



Audar FMC-12 FM converter.

This is a rebadge of the Meck converter in a leatherette covered wooden case. It uses the transformer coupled output.
 


Strange choice of IF. 4th harmonic falls on 93Mc/s, preventing any stations on or near this frequency being received.

The Output Transformer...



The white Meck.


This one, the 7th in my collection, came by a rather round about means. An acquaintance in the HRSA had expressed interest in the Fremodyne, and I'd seen this one appear on ebay. As I'd just acquired my 6th recently, I wasn't desperate for another, so I let him bid on it.
Quite some time went by and he'd decided the performance was just too poor in his area, so wondered if I'd be interested in it. How could I say no that?

Apart from the white, or more correctly, ivory, cabinet, the most interesting thing was the line cord resistor was not open circuit. In fact, it was in very good condition with no fraying of the fabric outer insulation, and the rubber inner conductors were in good condition.

The cabinet back screw, the terminal strip mounting screws, and the chassis are connected directly to the mains.

The unit had been recapped and some resistors replaced, saving me some work. And, cosmetically the whole thing was in excellent condition.
This model is like my first, with the 150mA heater valves, 14F8 and 35W4. It also has an output transformer. Electrically, it is the same as the Audar circuit above.
That the performance was poor was an understatement. It could only receive a few stations, and very poorly at that. First thing to do was check the alignment, and this was way out. I peaked the IF coil at 21.75Mc/s, and then set the local oscillator and RF trimmers at 105Mc/s. This made a huge difference, and sensitivity was now normal.

However, there was a persistent whistle with distortion. Often there is some evidence of a whistle which I've mentioned many times before. It's the result of the quench frequency beating with the stereo pilot tone at 19Kc/s, and other audio components present. Normally, a slight tweak of the 150K reduces it to an acceptable level by suitably raising the quench frequency. But here, it needed more than a slight tweak; quite a reduction in resistor value was needed. This suggested some other fault.
The quench frequency is determined mainly by the 1500 ohm cathode resistor and associated .0025uF condenser. And it was here I found the problem. Instead of .0025uF was .0033uF. This could only decrease the quench frequency. So, I replaced it with .0022uF and an immediate improvement was evident.
At this point it was behaving like any other Fremodyne. Due to the stereo subcarrier beat, the quench frequency did have to be raised slightly and paralleling a 680K across the 150K did the trick.


Under the chassis. The white ceramic 5W resistor is to allow for 120V operation instead of 110V without over running the valve heaters.

The final thing to do was check the heater voltages. These were running a bit high, and doing some calculations revealed that the line cord resistor value (425 ohms) was suited for 110V mains. All my stepdown transformers provide closer to 120V (this is now the official U.S. mains voltage anyway), so I simply added a 56R 5W resistor in series to bring the voltage down slightly.

Regarding earlier notes comparing the different audio output circuits; transformer or resistance coupled, I did some experiments. One thing is confirmed, and that is the transformer output provides lesser quality audio. There is less bass response, and intermodulation distortion (that is, the quench beating with audio components) is more evident.
However, it appears the transformer has a step up ratio, despite the above note concerning the Audar converter, and without actually measuring it, audio was about twice the level as the resistance coupled output.
I did investigate what effect the low primary resistance had compared to the 22K resistor in the standard circuit and it seemed to be unnoticeable.
Interestingly, a pair of high impedance headphones in place of the 22K cathode resistor, or transformer, provides quite good volume.



What became of the Meck?
Like electrostatically deflected televisions, the Fremodyne was a short lived fad. By 1951 Heathkit was no longer promoting the design, and from the following advertisement from Radio Electronics, May 1951, it seems Meck thought the same.


Concord Radio appeared to be a place where manufacturers got rid of excess or unwanted stock. Note that the FM converter in the ad has no cabinet and no dial pointer. It would appear that Meck were keeping these items for their midget AM radio which used the same cabinet.

Ebay April 2009
I wonder who it was that paid the excessive price of $103 for one of these thinking they were getting a miniature valve FM radio, which is what it was incorrectly described as in the eBay ad. Maybe the lucky(?) purchaser got a surprise when he took the back off and saw no loudspeaker and only two valves. I wonder if he's had a shock touching the chassis or terminals yet. The odd thing was the seller claimed it "hummed" (how it did this with no power transformer or speaker has me intrigued).

The CY500
This version of the Meck contains an RF oscillator and modulator, using another 14F8, to provide an AM output in the medium wave band. In this sense, it is a true FM to AM converter as no audio input is needed on the receiver.

Performance of the Meck.
Sensitivity is much the same as other Fremodynes with 30uV providing quite a readable but noisy signal, and 100uV making it quiet. 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.
Sound quality is the only let down with the transformer coupled model, but for non critical listening applications it is ok. The resistance coupled audio version is much better and an example of a good Fremodyne receiver.



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