The Amīcaflex 4 transistor reflex radio

[Amie]

Hi Trevor.

No problems! Its all relavent, its a reflex design after all!

You've already noted the wire (capacitive) regen works on the HF end of the MW if you connect a gimmick between the top of the tuned winding and the collector of your 114 you can adjust the feedback, in reality it will only need a pF or two.

The bottom of the tuned circuit needs to be at circuit ground.... And also the coupling winding needs to be giving phase reversal.

One more thing that can be done with the circuit to make an improvement is once everything is phased right, to orientate the choke near the ground end of the tuning coil, to give feedback also, this can be balanced with the capacitive feedback with the gimmick to provide pretty good level sensitivity across the whole band.

How the Amīcaflex works basically.

From great Gran's place in southern Nottinghamshire in the evening, the set can get the R Scotland and MW R4 transmission, plus Manx, Ulster, and loads of Spanish stations....

Currently with a friend replacing a dropper in a DAC 90, so I'd better get back to that!

Amie.

[Murphyv310]

Thanks Amie.
Looking at the Micromatic circuit we see that to keep the component count down even TR2 uses simple dc feedback bias between base & collector. C5 too is providing AC negative feedback at higher frequencies so giving some bass boost, great for those toppy piezo earpieces.
Don't forget at that time there was a lot of experimentation to get the best results no PC programs to aid design and circuit parameters.
The way L1 and L2 are wound is unusual, it helps with matching and the Q of the the two windings I believe.
I'll need to get back to the rod pentode radio over the winter but sadly I need to use decent magnifier these days, too many years fixing radios and TVs with all this close work.

PS Manx coming in quite well on the little set, doubt I'll get Caroline on it but it's quite receivable here so long as selectivity is good.

[Radio Fixer]

Well done, excellent effort.

Dont think diodes D5, 6, 7, 8 and were mentioned? Is this just a regulator for volts to the RF stage? 6 to 8 have arrows that to me indicates they are emitting something so assume they are LEDs but not drawn as I know them. Seem upside down and why 3? Thought it was bad practice to parallel LEDs without individual feed resistors.

Gary

[Kalee20]

Thought it was bad practice to parallel LEDs without individual feed resistors.

It is generally, yes.

But - I've done it myself successfully, to illuminate tuning scales and spread the light (as is the case here)! The turn-on 'knee' is soft enough to make it work with small LED's at low currents.

Running LED's at significant power dissipations, there will be heat generated and as hotter LED's have lower forward voltage drops, a LED in a parallel array could easily get to hog most of the current - and fail.

Here, there's going to be minuscule temperature rise owing to the low currents (look at the resistor values) and so diode ohmic resistance is going to make everything come nice.

And... They are drawn right-way round, and the symbol is (almost) what I use myself, in fact it's a bit more commonplace than my own symbol!

[Radio Fixer]

Thanks.

Ah! Yes! thinking of Zeners for the drwg.

Gary