My FM converter for tube radios using the dial and tube amplifier

[david winter]

Hi,

I designed my own FM converter for tube radios using the TEA5767 module.
I started from an existing project designed by Hontelé, but rewrote the code to improve the tuning and fix bugs as well.
The card is very small (3cm x 3.5cm) and powers using the 6.3Vac line or a DC source under 16V. It use a small pushbutton to calibrate the dial min/max positions.
It's limited to the europan 75µs / 87.5-108MHz band and works great.
It requires a connection to the variable capacitor, thus loosing the AM bands unless you add your own switching method. But at the same time, you tune using the original dial.
DAB/DAB+ radios can be heard using a (now cheap) handy DAB/DAB+ to FM converter.

[Kentode]

That's very neat David, well done! Would you like to share the schematic diagram and PCB layout?

[bobhowe]

Originally Posted by david winter

Hi,

I designed my own FM converter for tube radios using the TEA5767 module.
I started from an existing project designed by Hontelé, but rewrote the code to improve the tuning and fix bugs as well.
The card is very small (3cm x 3.5cm) and powers using the 6.3Vac line or a DC source under 16V. It use a small pushbutton to calibrate the dial min/max positions.
It's limited to the europan 75µs / 87.5-108MHz band and works great.
It requires a connection to the variable capacitor, thus loosing the AM bands unless you add your own switching method. But at the same time, you tune using the original dial.
DAB/DAB+ radios can be heard using a (now cheap) handy DAB/DAB+ to FM converter.


Attachment 243947

Hi David you have a PM

[agardiner]

Hi David,

Lovely solution. I presume it is audio out, which you then connect to the output amp? Would be great to add a tunable 470KHz AM modulator to send the signal through the sets IF stages.

Anyway, are you planning on making and selling these modules?

[Gabe001]

Very interesting

Can the audio out go to the pick-up, thereby retaining the AM stations with the addition of FM when the radio is switched to the gram setting?

[agardiner]

Originally Posted by Gabe001

Very interesting

Can the audio out go to the pick-up, thereby retaining the AM stations with the addition of FM when the radio is switched to the gram setting?

That would depend on whether you have free contacts on the switch to enable the tuning capacitor to switch between the module and the AM part of the set.

[david winter]

The module requires a connection to the variable capacitor, so you will need to add your own switching method to keep the AM bands.
Modulating the audio output to pass through the AM IF is not recommended, not only because you would loose in sound quality, but mainly because the IF frequency can range from 130KHz to 492KHz.
Sending the audio output to the LF amplifier is the best solution so far.

The boards are available either as one single piece or as a 4-module board to be split like chocolates (very easily done ).

Note that the TEA5767 module is CMOS and sensitive to electrostatic discharges.
It is highly recommended to install it carefully.
Finally, the bottom layer is plain ground, making it easy to fix to the chassis.

[agardiner]

Great! Do you mean completely bare boards, or are they populated except the TEA module? I presume your firmware is in the other little 8 pin IC?

[david winter]

The boards come as shown. The only additional components you may need are a diode and a capacitor to double a too low heater line or a heater line where the secondary winding has its center point grounded.

[Kentode]

So... Are you making the boards available? Will there be a kit?

[Kentode]

David, many thanks for the link you PM'd to me.

I've not yet attempted any smd projects and I'll have to read up on how to work with static sensitive devices.

I can foresee a shopping list of equipment, fortunately I've already got a stereo microscope!

[david winter]

All you need is a ground ring which is basically a conductive ring put on your arm like a watch, and which is connected to the earth through a high resistance of typically 1 megohms.
Once you're protected, all you need is to:
- Fix the module and ground first
- Connect the AC/DC input (make sure your heater winding is grounded: if it has a center tap to the ground, you need to add one diode and one capacitor to form a voltage doubler; I can provide a simple schematic)
- Connect the variable capacitor to the rond tap, and the audio out (use a shielded wire if length is more than a few inches.

To calibrate the module, turn the radio on, make sure you have at least 6Vdc at the DC test point. Turn the dial so that the variable capacitor is at its maximum value and press the yellow button once. Turn the dial full opposite way, and press the yellow button once again. If everything went right, you should hear the FM using a small wire (1 meter is enough in almost every case).

Note: if you install the module in a AC/DC set where there is no mains transformer, or where the chassis can be "hot" (connected to the mains):
- Install the module with ground to chassis as usual
- Add a separate DC power supply of at least 6Vdc (maximum 16Vdc)
- Add a 1M resistor between antenna tap and ground
- Add a 10pF (approximately) capacitor between antenna tape and antenna plug.

I strongly recommend to ensure that the mains switch of the radio is free from corrosion and makes no residual resistance. Add a small 10nF to 50nF anti-spark capacitor (X2 class recommended).

[radiomobile]

Hi David. Very interesting project. Can your converter be used with permeability tuned radios such as car radios?

[david winter]

Originally Posted by radiomobile

Hi David. Very interesting project. Can your converter be used with permeability tuned radios such as car radios?

This only works with radios designed with a variable capacitor.
Lots of car radios use another design with coils (I guess that's what you call permeability tuning) which isn't compatible with my module. In this case I install the Aurora FMC2 module (I'm an Aurora dealer, but only provide the full restoration and mod service as per contract).

[FERNSEH]

Hello David,
question: the existing 0 -500pF tuning capacitor performs the function of tuning through the 88 to 108Mhz FM waveband?

DFWB.

[david winter]

Originally Posted by FERNSEH

Hello David,
question: the existing 0 -500pF tuning capacitor performs the function of tuning through the 88 to 108Mhz FM waveband?

DFWB.

Yes. It works with most values, even slightly lower or higher.
If a trimmer is present on the tuning capacitor, set it to its maximum value so that the tuning starts at 50pF or so.
Nothing else must be connected to the tuning capacitor.
The calibration button will adapt the module to the tuning capacitor values (press once when tuning capacitor is at maximum value, and once at minimal).
Calibration can be done again if required.

[bobhowe]

Hi just bought this FM converter looking forward in to putting this module in my ivory Bush DAC90A with the Ed dinning auto transformer ( ideal as it has a 6.3V tap ) & my UL44 valve conversion

[david winter]

If your transformer has a 6.3V tap with the bottom edge grounded, that's ok.
Never connect the module in parallel to a filament chain in series.
With AC/DC sets I always hide a miniature DC power supply. They cost nothing or almost, and are very easy to use. You may even connect them to the 100/110V tap: they will last longer.
I sometimes salvage the circuit from a USB charger (5V is not enough and I compensate by adding a few loops on the miniature transformer, which easily gives 6V).

[bobhowe]

H David this auto transformer all the voltage taps are bottom edge grounded to Neutral on the sets chassis

[Mr 1936]

Hi David

I seem to be missing something here.

The TEA5767 is a Philips surface mount IC.

I can find a variety of small populated PCBs on sale that could be described as TEA5767 modules.

Have you used one of these commercial ones, and if so which one? Or is this a special/custom design ?