Bush DAC90A with FM.

[Geordie McBoyne]

This project started last year but stalled because it was believed the David Winter FM module had been destroyed as a result of the AM local oscillator remaining active on FM.
Without doubt, feedback from the local oscillator may have caused the demise of the FM module.
Irrespective of which waveband is selected high levels of oscillator voltage is present, >4volts peak to peak.
In fact, there is nothing to be gained by shutting down the local oscillator on FM. The AM oscillator voltages are still present at the FM tuning capacitor terminal of the module.
The clever feature of the FM module is that the existing AM tuning capacitor can be employed.
Certainly, it was a mistake using the oscillator tuning capacitor, the aerial tuning capacitor should be considered.
The use of a separate tuning capacitor for FM was considered but there is insufficient space on the DAC90A chassis to accommodate one.
A viable solution is employ a vari-cap diode to do the FM tuning. A 100Kohm potentiometer will be required to control the 0 to 9volts for the var-cap diode.
The BB510 diode was made for AM radios, this component has a capacitance of 30 to 500pF.
But let's see if the aerial tuning capacitor works first.

Geordie McBoyne.

[bobhowe]

Glad to see you have restarted this project will be following this with interest kind regards Bob

[peter scott]

The aerial section sounds like a nice solution if the chip works with that range pF.

Peter

[Murphyv310]

Hi
David Winter has clearly stated that the tuning cap needs to be fully isolated from the existing circuitry, which I personally have confirmed. The module is extremely sensitive to stray RF I've found and my one blew when transmitting on HF with just 20 watts and very little stray RF enters into the shack. I gave up on them anyway as I don't like the way they work.
Incidentally I have a Siemens radio that has FM using an ECH42, I see no reason why a UCH42 wouldn't work and slope detection, perhaps give it some thought.
Varicap diodes could well work in the David Winter module, perhaps worth a try, just make sure the supply is properly filtered to the diodes and there is no RF leakage on long wires.

PS. I didn't use the oscillator section of the tuning cap just the aerial tuning section but again all existing wiring must be disconnected or switched out.

[Geordie McBoyne]

Taking no chances this time.
Before refitting the replacement FM module all the connections to the unit were checked for any unwanted waveforms that would potentially destroy another one.
The aerial tuning capacitor is completely free of any unwanted signals.
Module refitted, 9volt battery connected. After the set warmed up I was greeted with plenty hiss. Tuning the module to find stations it was found it performed better at the HF end.
The 'scope shows the waveform with the tuning capacitor set to 101.3Mhz, receiving ClassicFM.

Next stage. Make a power supplier for the module. Current consumption is 50mA from a 9volt supply.

Geordie McBoyne.

[Murphyv310]

Hi
On AC sets if one side of the heaters are grounded the "live" side can be half wave rectified and then filtered with a 4700uf 16v cap and dropped via a suitable resistor for 9v then smoothed with a 1000uf in parallel with a 0.1uf ceramic bead, regulation is unnecessary as the module has built in regulation and further filtering. One I fitted in a Pye set is still going strong although the ownernever tunes off Radio Scotland.

[Geordie McBoyne]

The module in my set was fitted a few weeks ago and was working well supplied from a PP3 battery.
Yesterday, the power supply components were fitted. Components used are a NOS RS components 6V + 6V mains transformer and a very compact voltage regulator module. The module employs the PJ317 regulator IC which is good for 1.5amps so it has an easy time supplying power to the David Winter FM module. Only 50mA @ 10volts.
The receiver seemed to be working well on all three wavebands with little or no interference from the module on the medium waveband.
It is possible to power down the module's power supply by accessing the "V adjust" pin 1 of the PJ317.
The two 6volt windings will provide power for the dial lamps.
Well, that was yesterday. I was convinced there was no interference, but today when tuning in to TalkSport on MW there was a considerable amount of interference coming from the module.
No problems on LW.
The FM module must be shut down on MW.
This was achieved by reducing the module power supply to 3volts when switched to MW. The local oscillator waveband switch has a spare position on MW which permitted a 470ohm resistor to be connected to the "volts adjust" pin on the regulator IC. See attached circuit.

Geordie McBoyne.

[bobhowe]

Hi David you have done a great job in making this work on FM and AM and LW this be done on a Philco Peoples set or an Ekco AD series radio ? makes them very usable now . kind regards Bob

[Geordie McBoyne]

Hi Bob,
installing the FM module in the 444 Peoples Set would be a simple task. Like the DAC90A, the wavechange switch can be replaced with the three position Lorlin four pole switch. Lessons learnt from the DAC90A job will apply to the Philco.
It's becoming all too sure that the AM local oscillator should be shut down on FM.
Some modification to how the HT is supplied to the LO is being looked into.
Need to do some study into the Ekco AD65 type of receivers to find out a viable method to do the FM conversion.

Geordie McBoyne.

[bobhowe]

Hi David will have to look out for a reasonably priced 444 peoples set the Ekco AD65 is way out my price range I have seen them for sale at £1800 kind regards Bob