Ramsey FM110b Stereo Transmitter review

[Novaro]

I would like to make a small review of my current low power fm transmitter i use to broadcast audio around my home and a little more!

You know Ramsey Kits? Was very popular during the last decades for their kits. Their popular transmitters were FM10,FM25,FM35,FM100 all of them based on Rohm IC's.

Their quality was indeed good for what they are especially the FM100 one still sells on eBay for 300 and above dollars and they dissapear only a couple of hours after the listings. Nostalgia? Are they really rare? So good? Or they are just the all in one package that occasional users demand for their broadcasting hobbie.

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The Ramsey FM110b

The company closed and the owner of the company made a new design. This time there is no IC fm chip but dedicated Stereo Encoder , VCO , PLL circuits again in all in one package with a mixer for audio and MIC inputs. The device was called FM110b and it was sold for small community Radio Stations on Carribean islands.

Stereo Encoder
The stereo encoder is based on 74hc74, MC1496 and five LS404 op-amps. A simple analog design to generate the pilot and subcarrier signals. There is also a simple MPX clipper using two pairs of 1N4148 diodes so the modulation cannot exceed the 75khz limit.

VCO
The VCO is based on four bb639 varicaps and a coax line as a resonator.

PLL
PLL is around the MC145170D2 and for the modulation takes care a BS170 or 2n7000 FETs.

RF Amplifier
The RF amplifier stage consists of a BFG591 that drives an RD15HVF final transistor for a total power of 15 Watts. There are on board jumbers that can be used to bypass the above units and broadcast on low power with only 100mw of power.



More to come (internal photos, schematics, videos , audio performance)...

[sinus trouble]

Nice Novarro!

That has definately moved on from the Ramsey kits i remember from the late 90s! They was all over the internet at that time!

Infact! I was gonna buy one at the time? But i opted for the NRG in the end for that exact reason, The IC that you mentioned!

Not at all to do with the Ramsey engineering quality!

It will be interesting to see the internals!

[Novaro]

This is the main board

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[Novaro]

Users Manual

Includes a description of unit operation and a schmematic diagram of the whole circuit.

[Albert H]

Another pretty typical John Ramsey design. The stereo coder is (slightly) interesting, in that he's using filtered square waves for the 38kHz sub-carrier and the 19 kHz pilot. The resonant filters using the 22mH chokes both calculate to be centred slightly below the correct frequencies.

He goes to a lot of trouble to filter the audio on the way in, but then doesn't bother with the other harmonic products made in the 1496 mixer!

The clipper is just hacking the composite multiplex (including the pilot) into square waves, and there's no proper filtering to clean up the mess made by the clipper! If there's the slightest hint of clipping, it will appear as spurs either side of the carrier.

The VCO design will be microphonic - bang the box or the coax resonator, and it will be heard on air! The amplifier design is nothing revolutionary, and the "Low" setting will be nearly room-filling signal strength, and "High" is far too much for most Hobby Broadcasters - 15 Watts will get you noticed, and the spurs and harmonics from this thing won't endear the Authorities to you! The RF filtering is inadequate, and cannot be optimal across the whole 20 MHz of Band II - if it works at 108 MHz, it'll be useless at 88 MHz, and if it's set for the lower part of the band, you're going to waste several of your expensively generated Watts if you operate higher up the band!

There's no protection against high SWR, so killing the FET will be easy (if you didn't already destroy it with static during construction).

There's no provision for actual alignment of deviation, so you're very unlikely to get it right just "by ear". Also, the modulation sensitivity will vary across the band......

This is NOT a viable solution for hobby broadcasters. Professional?

[Bton-FM]

The VCO design will be microphonic - bang the box or the coax resonator, and it will be heard on air!

What is it about the design of the Modulator that means it will be microphonic?

[Albert H]

Try it, and you'll see. The coax (unless it's a type I've never seen before) will vary in capacitance as it's moved, so vibration turns up in the mod! We had an exciter design that had a similar resonator, and we could hear the footsteps of the baldy men coming to take our rig off the air! I reverted to nice solid Toko S18s for VCOs after that!

[jvok]

Tbh I've found tokos are microphonic too. Maybe not as bad as coax but you can still hear it if you tap on the board. Hand wound coils like in the nrg rigs are bad too. Smd chip coils might be best but then they usually have low q.

[Novaro]

Another pretty typical John Ramsey design. The stereo coder is (slightly) interesting, in that he's using filtered square waves for the 38kHz sub-carrier and the 19 kHz pilot. The resonant filters using the 22mH chokes both calculate to be centred slightly below the correct frequencies.

He goes to a lot of trouble to filter the audio on the way in, but then doesn't bother with the other harmonic products made in the 1496 mixer!

The clipper is just hacking the composite multiplex (including the pilot) into square waves, and there's no proper filtering to clean up the mess made by the clipper! If there's the slightest hint of clipping, it will appear as spurs either side of the carrier.

The VCO design will be microphonic - bang the box or the coax resonator, and it will be heard on air! The amplifier design is nothing revolutionary, and the "Low" setting will be nearly room-filling signal strength, and "High" is far too much for most Hobby Broadcasters - 15 Watts will get you noticed, and the spurs and harmonics from this thing won't endear the Authorities to you! The RF filtering is inadequate, and cannot be optimal across the whole 20 MHz of Band II - if it works at 108 MHz, it'll be useless at 88 MHz, and if it's set for the lower part of the band, you're going to waste several of your expensively generated Watts if you operate higher up the band!

There's no protection against high SWR, so killing the FET will be easy (if you didn't already destroy it with static during construction).

There's no provision for actual alignment of deviation, so you're very unlikely to get it right just "by ear". Also, the modulation sensitivity will vary across the band......

This is NOT a viable solution for hobby broadcasters. Professional?

Thank you mr Albert for this great technical review.

The cable is RG188 and yes when you touch acts like microphone. When not it does not seem to affect audio. But again just by ear perception. I removed already the fan so no obvious vibrations.

I also removed the clipper because i used at times software or hardware solutions for peak control.
One thing i have to say is that i believe every design had to have a place in the market of Hobby Broadcasters. Not all people like to be heard as the best in town.

I am on the minimalist side of things. I like this device as an all in one solution. It was a good try for the time (2007) not the best could happen as per your comments on circuit operation and thank you for that because i undestood the circuit even by a little.

In the Low setting i can pickup the station about 2km in a car at certain places where optical contact with the antenna is better. I have connected the whip rf out to the PL259 using a small coax.

Finally i think what John Ramsey did with this unit and also the FM100 was clever. To put in a small unit an audio mixer with line and mic inputs , a stereo encoder , a VCO, an amplifier and a nice interface to select frequency etc or guess your deviation using the VU meters.

Thank you very much for this detailed post.

[Bton-FM]

Tbh I've found tokos are microphonic too. Maybe not as bad as coax but you can still hear it if you tap on the board. Hand wound coils like in the nrg rigs are bad too. Smd chip coils might be best but then they usually have low q.

So are all oscillators microphonic to a degree? What do you use to get around this problem?

I've been experimenting with an Elenos oscillator (uses a length of coax and loads of varicaps) recently - I found that the coax is microphonic but not as bad as a hand wound coil. With a hand wound coil in the same circuit you get a horrible loud ringing sound when you hit the board.

I stripped off the sheath of the coax and soldered the braid directly to the ground plane. This significantly reduced microphony - if you tap on the board you can hear it but its very quiet and a whole lot better than having the coax floating in the wind. I doubt it would give you any issues.

I quite like the coax approach since the inductance doesn't really change no matter the shape of the coax. The down side is that using 10 varicaps is expensive for a pirate rig.

What I want to try next time I get some PCBs made is potting in the VCO coil in epoxy resin.

[Albert H]

Bton-FM - try using reverse-biased green LEDs as varicaps. They're cheap enough, and the range of capacitance is pretty good. I've found that green ones show the most capacitance, and have the biggest linear range. I found that it was necessary to shield the LEDs from light - especially fluorescents and LEDs as these cause hum! I used to paint the LEDs with a dollop of black paint.

[Bton-FM]

Bton-FM - try using reverse-biased green LEDs as varicaps. They're cheap enough, and the range of capacitance is pretty good. I've found that green ones show the most capacitance, and have the biggest linear range. I found that it was necessary to shield the LEDs from light - especially fluorescents and LEDs as these cause hum! I used to paint the LEDs with a dollop of black paint.

How noisy are they aside from being sensitive to light? Bare in mind this oscillator uses 10 varicap diodes.

What's the reason that most commercial transmitters use this topology? Is it because the coax resonator doesn't require adjustment?

[jvok]

So are all oscillators microphonic to a degree? What do you use to get around this problem?

One trick that works well is sealing the coils in wax. Tune them up then melt a bit of wax over them. Once its set it holds the turns tight and kills the microphonics dead. Itll change the tuning slightly but no big deal in a wideband vco.

Only thing I can think off thats not microphonic at all is a dds which is what a lot of the pros are using now but not so good for pirate rigs, still too expensive.

But at the end of the day it does it really matter if your rigs a little bit microphonic? hopefully its not somewhere were anybodies gonna be touching it anyway.

[Albert H]

How noisy are they aside from being sensitive to light? Bear in mind this oscillator uses 10 varicap diodes.

What's the reason that most commercial transmitters use this topology? Is it because the coax resonator doesn't require adjustment?

That's part of the reason. The other one is that inductors are getting scarce and expensive! The suggestion of putting wax over your resonator is a good one too - it's frequently done in commercial gear.

Green LEDs aren't noisier than most varicaps. I used BB409s in one exciter, and they were horrible for noise.....

All I can say is "Give 'em a try - if one brand of LED doesn't work for you, another might!"

I also use Kingbright bidirectional Green LEDs as clippers, because they conduct just as you get to PPM 6!

[Novaro]

I now have a short circuit somewhere.

I believe near the power supply. The first part shorted seems the 7805 out and ground are sorted. After that the main cap 3300uf also sorted. I found a semi burned cap near the final transistor. I will try to find a strategy.
I removed some parts. I dont believe i have a chance fixing it.

Suddenly after a cleaning with WD40 on a toothbrush for better photos!! And then replacing the main cap with a new one with proper connections to the underside of the board because old was just fitted over the solder pads.

The board was already in bad condition during the years (12) due to me trying to find something that was wrong in the stereo encoder part.

Sad for me was a friend but i cannot help anymore i think.

[Novaro]

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