Regenerative demodulator.

[FERNSEH]

The attachment shows the circuit of the sound demodulator that is used in the pre-war Baird T23 TV receiver. A similar circuit is to be found in later versions of the T5, the company's first TV receiver.
In the T23 the demodulator is a pentagrid Marconi-Osram MX40. This receiver is a TRF and I found that the Mullard FC4 octode will not work at 41.5Mc/s, the only valve that will work is the MX40.
Grids 1 and 2 operate as a 500Kc/s oscillator in a colpitts circuit. The TV sound RF or IF signal goes to grid 4. The demodulated signal at the anode is rich in RF/IF and 500Kc/s component hence the comprehensive filtering before the audio amplifier.
The circuit is described as a regenerative demodulator. It seems to work quite well in the T23 as long as the right valve is employed.

Any ideas how this circuit works?

DFWB.

[Radio Wrangler]

It looks like a self-oscillating mixer, but the 500kHz tank circuit would have to injection lock its 81st harmonic to the 41.5MHz carrier, thereafter it would mix the modulation sidebands down to audio. There would certainly be a lot of harmonics etc whizzing around.

A pretty marginal bit of design, hence the pot to twiddle until the lock worked, but a masterpiece of minimalism. A PLL synchronous demod done in one bottle!

Definitely NOT the set you want your neighbours to buy

David

[Synchrodyne]

Originally Posted by Radio Wrangler

.. but a masterpiece of minimalism. A PLL synchronous demod done in one bottle!

Indeed! Ersatz, but nonetheless a form of synchrodyne, and quite remarkable all things considered.

And a very early – perhaps the first - commercial application that was evidently missed by Prof. Tucker in his 1954 survey, see: http://www.thevalvepage.com/radtech/synchro/synchro.htm.

That it used a heptode (pentagrid) makes it a step simpler than Tucker’s simple design based upon an ECH35 triode heptode. ( I suppose that the heptode self-oscillating frequency changer’s propensity for pulling would be of diminished concern in this application, where lock-in is the objective.)

Use of an oscillator harmonic makes it more complex in another direction, although not in a component count sense, but one might infer that stable oscillation at 500 kHz was much more readily achievable than at 41.5 MHz. Also, back in those days use of oscillator harmonics was probably not common practice for any purpose, so that was innovative as well. In one sense the valve operation has to be very non-linear to produce an array of harmonics – maybe close to a square wave at oscillator fundamental – but on the other hand it has to be very linear in its 41.5 MHz multiplying action in order to produce undistorted audio.

Cheers,

[Synchrodyne]

A subsequent thought is that if the subject regenerative demodulator worked at the 83rd harmonic of 500 kHz, then it should have worked at the fundamental with an input signal of at 500 kHz or thereabouts, such as at the 450 to 470 kHz AM IF. Thus I wonder if Baird looked at using it in any radio receivers, say superhet with regenerative demodulator? Of course, with variable tuning, the synchrodyne whistle would have been a problem, but not with preset tuning. One can see a possibility that the Baird folks were playing around with this demodulator for radio work, and someone had the idea of a harmonic version for TV sound.

Carrying the self-oscillating heptode theme forward a couple of decades or so, some of the later Eddystone valve receivers (EA12, 830, 850, 940, etc.) had a self-oscillating SSB demodulator built around an EK90 (6BE6) heptode. Essentially it was like a self-oscillating frequency changer except that the oscillation frequency was the same as that of the (missing) SSB carrier so that one of the outputs was baseband audio. Thus it was broadly similar to the Baird TV circuit except that it worked at the fundamental frequency and the oscillator was, nominally at least, unlocked.

Now as thought experiment, what might happen on one of those Eddystone receivers if the SSB function were selected with an incoming AM signal – I think that on some of the models at least, that combination would not have been precluded by the switching options. Would the EK90 oscillator have then locked to the incoming carrier? I think that there would be a good chance of this happening, and if it did, then the EK90 would become a locked-oscillator synchronous demodulator, although maybe not an optimized example. Maybe someone here with a suitable Eddystone receiver would think of converting this thought experiment into a real experiment?

One then wonders whether or not Eddystone thought of this. But whilst the main objective of the SSB function was likely reception of amateur signals with fully suppressed carriers, some of the SSB signals available, such as point-to-point services, would have been of the reduced carrier (-16 or -26 dB) type, so there was a potential primary use in which the Eddystone self-oscillating SSB demodulator would have been in receipt of a carrier input. Conceivably its operation under those conditions was considered; if so injection locking and avoidance of low frequency beats might have been seen as being referable to unlocked operation.

Cheers,

[Neil Purling]

How did radio manufacturers make a short Superhet work in which there's only one IFT (which is associated with the anode of the Mixer)?
Must the IF transformer have a tertiary winding/ tapped secondary to have regeneration at whatever IF frequency is used, or are there other ways to give regenerative detection with a standard IFT?

[Andrew2]

That's quite a find, and the thoughts put forward are interesting. One thing puzzles me - with a 500KHz oscillator generating harmonics into the low VHF region, was the system prone to patterning? I'm amazed with all those harmonics hurtling about that this was not a major headache!

[Synchrodyne]

I'd assume that the harmonics could be problematical. If the 83rd at 41.5 MHz was strong enough to enable lock, then those around 45 MHz surely would also have been quite strong.

Perhaps the unit was well-screened, so as for example to stop harmonics getting into the vision TRF amplifier early enough and at a high enough level to cause problems. As far as I know the Eddystone self-oscillating unit previously mentioned, which did not rely upon harmonics but probably produced them, had its own screening box, which I think is indicative of the nature of the beast.

Cheers,