PULSE COUNTING FM TUNER

This is my construction of a "pulse-counting detector FM receiver" as described at:
Ref #1) cool386 website.
Ref #2) phils valve radio site.
I combined from the design's as shown on both site's, but I deleted the Audio O/P stage, so it is basically then a tuner (program source) which can feed my valve based Audio Amp'
The above references give a good account of the theory/operation of the pulse counting detector etc, so no need for repetition here.
Original design is credited (with others) to a 1956 Wireless World article by M G Scroggie

"Naked" type construction with valves protuding thru top-cover.
Main chassis hinges open to allow easy access to wiring.
My fabrication uses almost all recycled componentry.
Metal case is ex tower PC power supply box. (nickel-plated steel)
Mains transformer is ex color TV 240v : 110v + 6.3v (fils)
Tuning gang is ex colour TV UHF manual tuner: 3 x 13pF gang with 20:1 reduction, anti-backlash gears tuning mechanism, only one 13 pF section being used.
Valves ex B/W TV: 6ES8, 6BX6, 6AL5
It's good to hold-on to all this "old junk" you see!!
After some experimentation; I used a 6ES8 (6DJ8 also worked well) high slope 12.5 mA/V twin-triode,
autodyne freq' convertor {from phils valve radio (Ref #2)} and
the 6BX6 based I.F. amp {from cool386 - build Nr 4 (Ref #1)}
- one design merit, being; the absence of "hard to get components"
I didn't have any success with the 6BL8 (pentode) electron coupled oscillator (ECO)- wouldn't "hoot" at all, may have been due to the nature of the RF chokes I had available ? (6BL8 oscillator graphic [below] refers)
and also cool386's recommendations about The RF chokes.
However, the 6ES8 triode Local oscillator (ECO with Hartley Osc type tapped coil inductor) worked fine.
I also used a 30pF Philips Beehive trimmer as the coupling cap' b/w G-G input buffer and Mixer/L.O.
This has to be backed well off (low pF) for best operation. (schematic below)
A rough paper dial-scale was produced by comparing audio with another FM radio.
An artworked dial is then made using the PC/printer from the scanned rough 'original' calibration.
Valve HT voltage is obtained from a silicon diode voltage-doubler, and thence dropped through a 3K3 5W resistor to give 200v filtered DC, however
(Phils valve site - design, uses only 25V as HT voltage rail!)

I had not previously encountered this type of circuit, and held some disbelief as to how well it might work - but having now built an example; I am well pleased with the performance!
This circuit acheives a lot from a design of minimal complexity i.e.

1) Combined L.O. and mixer which uses the one single LC circuit as both oscillator tank and RF input cct tuning.

2) Low I.F. passband 20KHz - 250KHz (WBFM broadcast standard is +/- 75KHz max carrier deviation)
which uses no coils/inductances but the reactances of the coupling capacitors and/or valve input capacitance with fixed resistors to determine upper/lower I.F. passband limits.
Although this means an inferior image rejection for a super-hetrodyne type receiver, and each broadcast signal can be tuned-in at two adjacent 'spots' on the dial
(being from high or low side L.O. injection); FM bcst stations are chanellized more than 2 x 250KHz apart so reception of two broadcast stations (the desired and the image) doesn't occur.

3) Use of pulse counting "detector" which uses the differentiating property of a (small 47pF) capacitor to convert from an FM signal to an AM equivalent; i.e. voltage o/p (amplitude mod') is directly proportional to rate of change (frequency mod') of input signal (1st derivative)
Once again: no coils, aligned/tuned ccts needed!
The low value 4K7 load resistor also aids the "differentation" process.
A simple RC filter is next and also provides "de-emphasis"
Link to excellent "Cathoderay" article about pre-emphasis/de-emphasis in FM broadcasting

4) Recovered Audio is substantial and will feed high/med Z headphones directly, and almost overloads my valve amp' input (volume control is barely off the stop)

Tuning does require a delicate action but once "on-station" the Audio is generally acceptable.

Click on any photo's for a larger labelled view.



Link: Solid-State PULSE COUNTING FM RADIO 1966~2016
Armstrong Micro FM {construction kit}