Bush DAC90A with FM.

[Geordie McBoyne]

Another modification to the AM-FM switch.
The AM local oscillator is shut down when the receiver is switched to FM.
This is implemented by shorting the HT supplied to the local oscillator and screen grids of the UCH42 and UF41 valves to chassis via a 1.5Kohm resistor.
A rather crude method to kill the local oscillator but no other alternative could be found because of the limited number of function positions on the MW/LW/FM switch. An alternative is supply the local oscillator with HT from the reservoir capacitor via a 47Kohm resistor, but in practice the reduction of voltage on the sub HT rail is only a small figure.
I find the FM module is sensitive to a low temperature environment. I'll move the receiver to somewhere a bit warmer than the workshop and report back how it performs.

Geordie McBoyne.

[Geordie McBoyne]

In fact the module has given up working altogether. Just a hiss. The triangular waveform still present on the tuning capacitor terminal.
Not much can be done now except replace the FM module.

Geordie McBoyne.

[bobhowe]

Hi David what a shame is it worth to get the modified module that David Winter has made kind regards Bob

[Geordie McBoyne]

Hi Bob,
          that's what I'll do, but will leave it until next year.  Been too much work done to this set to give up on it.

Geordie McBoyne.

[bobhowe]

Hi David us from the North dont give up that easy I would wait till at least Royal Mail have finished their strike action before you order one as it will certainly get lost unless you get it sent by DHL they are very good I got a 3 foot sodium light bulb sent to me and it arrived very quickly and was not damaged . My module arrived last year very quickly the longest part was from it arriving in the UK to get to me the French postage side was very quick kind regards Bob

[Geordie McBoyne]

It might be a good idea to insert a buffer stage between the audio output from the FM module and wavechange switch. With the module output disconnected and the free end of the audio lead connected to an oscilloscope it was found a pulse of 2volts or thereabouts appeared when the wavechange switch was turned between LW and FM. In the DAC90A the bottom end of the volume control is returned to the cathode of the UBC41 audio amplifier. Much better to return the volume control earthy end to chassis and fit cathode bias components to V3, the UBC41.
One of the modifications suggested by Kalee20?

Geordie Boyne.

[bobhowe]

Hi David is it worth to fit a miniature 1to1 isolating transformer about the physical size of a speaker transformer in a pocket transistor radio between the buffer stage as well ? kind regards Bob

[Geordie McBoyne]

Hi Bob, there might be a problem with impedance matching. A simple buffer stage can take the form of an emitter follower employing a BC548 transistor.

Geordie McBoyne.

[Murphyv310]

Hi.
I find these modules extremely fussy, I've done two of them for non techy collectors. You "must" make sure there is NO other connection to the tuning cap other than the module, don't even consider running through an old paxolin wavechange switch as even a couple of volts of leakage will fry the module, if not straight away it will given time.
Some sets audio stage won't have sufficient gain either, a midget audio op tx wired in reverse works beautifully though.
PS the modded module isn't really that much better. I did an honest and fair review on my channel and Mr Winter wasn't happy!