Chapman/Derritron S6BS FM

[Murphyv310]

Hi.
I've now started to work on the original decoder to modify it for an MC1310P. The original PSU is divided into separate supplies from two isolated secondary windings. One would appear to drive the decoder itself and one for the "stereo bulb". As the MC1310 draws less than 100ma when receiving a stereo broadcast I have retained the Chapman supply but completely re-designed it. It now has silicon full wave rectification, and a 7812 regulator suitably smoothed and filtered.
I'm 75% through making up the decoder itself, done as neatly as possible on a small matrix board. As yet I've not added in a FET for matching as I'd like to try the decoder first to basically see if it is necessary.
One of the annoying things about the tuner is how the switching is done. AM & FM output is switched via the selectivity/FM, all OK if you use the tuner for mono but if you add a decoder then you feed that from a non switched phono MPX output. This makes it rather messy with either an extra input required on the amp or faffing with leads.
My plan is to connect the MPX output to the selectivity/FM switch so the MC1310 will do the combining for AM(mono) and of course for stereo FM, this means I'll not need an extra input on my Stereo 3-3 or have to change leads. The proof of the pudding of course will be in the eating and I'll most likely if needed, fit a FET in the tuner itself or as there is a space for another valve possibly fit an ECC82 cathode follower if needs be.
I'll post up some pictures soon.

[Murphyv310]

Hi.
As can be seen in the photos the progress made so far.
The original decoder board can be seen, it uses four AC126 transistors, 6 diodes and a few tuned circuits. There also were a couple of AC126's in a regulator circuit one of which was blown, I did replace it but there was no results, I decided at this point to head on the direction I'm taking now.
One rather annoying thing is the power switch for the decoder is a slide switch rated at 125vac, this wont be refitted or at least not wired in as I'll add a switched AC out to the tuner.
   
   
   
   

[Murphyv310]

Hi
This is the circuit I'm using on the MC1310P
   

[Geordie McBoyne]

Information and application notes for the MC1310 stereo decoder. The big advantage for this device is that no inductors are required:

https://www.arwill.hu/forras/termek/felv...388611.pdf

Geordie McBoyne.

[Terry]

Have you run out of shillings for the meter, Trevor?

Those pictures look a bit dark and gloomy!

[Murphyv310]

Ha Ha Terry.
Most likely dull as the flash on the phone was on low. Once near completion I'll take some more.

[jimmc101]

Aren't the values given for R5/C8 , R6/C9 those 75uS de-emphasis? 3.9k x 22n = 85.8uS (the datasheet gives 20n)
(The output of the MC1310 is high impedance and assume no load on the  outputs.)


For 50uS 3.3k and 15nF gives 49.5uS  or alternatively 22n and 2.2k gives 48.4uS (but with 3dB lower output).

Jim

[Geordie McBoyne]

FM pre-emphasis: The process of increasing the treble signals is called pre-emphasis, and reducing the in the receiver is called de-emphasis. The rate of pre-emphasis and de-emphasis is expressed as a time constant. It is the time constant of the capacitor-resistor network used to give the required level of change. In the UK, Europe and Australia the time constant is 50 µs, in North America it is 75 µs.
VHF FM receivers have the de-emphasis circuitry - consisting of a simple CR network built into the circuitry immediately after the FM demodulator. In this way it is not incorporated into an amplifier that may be used for other audio sources that would not require the de-emphasis applied to VHF FM transmissions.

Geordie McBoyne.

[jimmc101]

True for a mono receiver, but for a stereo receiver de-emphasis must be applied after the stereo decoder.
Applying de-emphasis before the decoder would kill the pilot tone at 19kHz and the L-R component at 38 to 53kHz.

Jim

[Terry]

Yes, I agree with Jim.

The application notes say: "With desired load resistance choose C2, C3 [your C8, C9] capacitors to provide standard 75jus de-emphasis."

Also: "The curve shown gives absolute maximum load resistance values versus supply voltage used for full-signal handling capability." For the 12V power supply you are using, this is 6k8.

I've calculated a range of values using resistor values from 3k9 to 6k8.

With the exception of the 4k7/10n pairing, I've suggested parallel pairs of capacitors to give the closest result to 50µS.