FM de-emphasis network - UK Vintage Radio Repair and Restoration Discussion Forum

Boulevardier · 27th Jan 2020, 7:40 pm

Can anyone explain how this circuit (Grundig Elite-Boy 500) does the FM de-emphasis? Surely it must be done between the output of the ratio detector (junction of C526 & R532) and the FM/AM IF input switching to the audio amplifier just after MP11. There are only three components between those two points – a 1.8KΩ resistor, a 4.7 nF capacitor, and a 220 pF capacitor. An online calculator gives a time-constant of only 8.5 µs for that network. Surely there must be something else going on…

Mike

kalee20 · 28th Jan 2020, 8:05 pm

I agree!

May be a typo... 1.8k perhaps is actually 18k in circuit?

Boulevardier · 28th Jan 2020, 9:19 pm

Thanks Kalee. I have checked the value in the set, and it's clearly marked as 1.8K, 5%. It measures 1.776K (only about 1.5% off spec). I've also measured the capacitor (the decimal point isn't that clear) and it's marked 4,700, and measures 4,574pF (2.75% low). So component values seem to be on-spec. Haven't checked the 200pF as that value is trivial in relation to the tolerances.

I'm still puzzled, and wonder if Papa Grundig just meant it to be that way. There is some sibilance on audio, which doesn't start to disappear until a 10nF is substituted for the 4.7nF. The ratio detector circuit isn't one I've seen before- no third winding. Perhaps the unknown circuit impedances and inductances are significant enough to explain the peculiar component values in the de-emphasis circuit.

Mike

Synchrodyne · 28th Jan 2020, 9:20 pm

I think that one also needs to consider the load into which the demodulator and de-emphasis look when FM is switched in. That load – presumably essentially resistive - is effectively in parallel with the de-emphasis capacitor. Depending upon its value, it may be the dominant factor in setting the de-emphasis time constant, rather than the combination of the demodulator output resistor that is in series with the de-emphasis capacitor(s). De-emphasis circuits can be hard to figure out at times – they are not always simply a low impedance source feeding a simple R-C network that in turn is looking into a (relatively) very high impedance that has but negligible effect. I discovered that aeons ago when trying to figure out the de-emphasis circuit of the Quad FM tuner. (In that case the load presented by the control unit was part of it.)

Cheers,

28th Jan 2020, 8:05 pm	#2
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,082	Re: FM de-emphasis network I agree! May be a typo... 1.8k perhaps is actually 18k in circuit?

28th Jan 2020, 9:19 pm	#3
Boulevardier Octode Join Date: Sep 2017 Location: Bristol, UK. Posts: 1,654	Re: FM de-emphasis network Thanks Kalee. I have checked the value in the set, and it's clearly marked as 1.8K, 5%. It measures 1.776K (only about 1.5% off spec). I've also measured the capacitor (the decimal point isn't that clear) and it's marked 4,700, and measures 4,574pF (2.75% low). So component values seem to be on-spec. Haven't checked the 200pF as that value is trivial in relation to the tolerances. I'm still puzzled, and wonder if Papa Grundig just meant it to be that way. There is some sibilance on audio, which doesn't start to disappear until a 10nF is substituted for the 4.7nF. The ratio detector circuit isn't one I've seen before- no third winding. Perhaps the unknown circuit impedances and inductances are significant enough to explain the peculiar component values in the de-emphasis circuit. Mike

28th Jan 2020, 9:20 pm	#4
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	Re: FM de-emphasis network I think that one also needs to consider the load into which the demodulator and de-emphasis look when FM is switched in. That load – presumably essentially resistive - is effectively in parallel with the de-emphasis capacitor. Depending upon its value, it may be the dominant factor in setting the de-emphasis time constant, rather than the combination of the demodulator output resistor that is in series with the de-emphasis capacitor(s). De-emphasis circuits can be hard to figure out at times – they are not always simply a low impedance source feeding a simple R-C network that in turn is looking into a (relatively) very high impedance that has but negligible effect. I discovered that aeons ago when trying to figure out the de-emphasis circuit of the Quad FM tuner. (In that case the load presented by the control unit was part of it.) Cheers,