James Stanley – The KRC-2 regenerative receiver equipment: a evaluation

Tue 25 April 2023
Tagged: electronics

I just lately learnt about regenerative radio receivers. They
reintroduce a number of the output of an amplifying transistor again into the enter, to get extra acquire.
It is a step up from crystal radios, and never as complicated as
superheterodynes. I purchased a KRC-2
equipment and this submit is my evaluation.

The quick evaluation is that I prefer it and if you wish to construct a regenerative receiver it is best to get it.

It took me about 3 hours to place the equipment collectively. The directions are fairly good and straightforward sufficient to observe, they arrive in a pleasant booklet.

Regeneration board

The primary job is to solder up the “regenerative board”. I used to be shocked to seek out that the copper facet of the PCB is isolation-routed.

I’ve had a go at this myself previously:

And whereas that specific PCB did work, it’s the solely success I’ve had. Each different try has failed in a method or one other, as a result of I used to be
attempting to design to the type of tolerances I anticipate from JLCPCB.

I believe the strategy used within the KRC-2 design is a lot better, as a result of it maximises the width of the traces and the realm for the solder pads.
You might be more likely to provide a profitable PCB with this methodology than if you happen to attempt to lay it out like a standard PCB.
I ponder how a lot hassle it’s to make layouts like this in KiCad…

The opposite facet appears to be screen-printed:

The footprint for the transistor requires the “base” leg to be bent the alternative solution to what you’d usually anticipate, which confused me for a couple of minutes:

It is bent in the direction of the flat facet as a substitute of the rounded facet. That is opposite to each the same old apply and the diagram within the directions:

The coil appears to be wound round a chunk of plastic M10 threaded rod:

I believe it is a good concept, because it offers you good constant spacing between the turns and means you do not want insulated wire.

Case meeting

With the PCB carried out, we transfer on to placing every thing collectively within the case.

I used to be shocked to discover a second PCB already fitted contained in the case:

That is the “oscillator board”. Why is that this ready-made? Why not let me put this one collectively myself?

Most likely as a result of the soldering is way trickier (it makes use of SMD resistors, and has fairly just a few flying components
related to the rotary swap) and by the appears to be like of it the variable capacitor must be trimmed by hand, which presumably is kind of a talented job and cannot be carried out till
the board is assembled. Truthful sufficient.

The KRC-2 appears to be an uncommon regenerative receiver, in that it makes use of a ten.7 MHz oscillator combined with the incoming sign from the antenna, which I have never
seen on every other designs. In line with the documentation, this reduces spurious emissions and makes the regeneration management extra secure. I do not know sufficient about radio to guage
the professionals and cons.

Assembling the case entails attaching the battery holders to the again panel, the “sense” and “regen” knobs and speaker to the entrance panel, and wiring the battery, antenna connection,
regenerative board and oscillator board all collectively.

The speaker is glued to the again of the entrance panel with PVA glue across the fringe of the paper cone. This may not be my first alternative,
though this specific speaker does not seem to have any provision for mounting {hardware}. Possibly you may maintain the again of the
speaker down with tabs towards the metallic? The glue appears to have caught effectively sufficient, however I am unsure whether or not it will rip the paper cone if I ever tried to take it aside.

The final step of the meeting is to solder wires on to the solder tabs on the prime of the oscillator board. The directions ask you to take the highest panel off the case in
order to entry this, however I discovered this to be not possible. You may see right here that the highest panel is glued to the batten, which can also be glued to the facet panel:

I am not even positive how that is meant to have gone collectively. If the highest panel had been detachable when the screws had been eliminated, then it will both take the batten with it or not, and in both
case there can be nothing remaining to affix the facet panel to the highest panel. I ponder if it is a design revision that hasn’t made it into the directions?

Anyway, unable to take away the highest panel, I got here up with an alternate methodology of soldering the wires to the tabs, which was to make use of some tweezer heroics to carry the wires in place and
then attain the soldering iron all the best way in to the nook:

This labored, however after I completed I had a greater concept: I believe if you happen to had been to take away each the entrance and rear panel, you may solder the wires in place on the bench at your leisure,
after which snake one of many panels by means of the housing and screw them each again on from the surface.

Trimming

To trim the regeneration, there’s a 20-turn trimpot on the regenerative board. The directions ask you to set “sense” and “regen” to the center, flip the trimpot
all the best way down, swap to “vary 1” (which I assume is the 1-10 MHz vary), give it 10 seconds to heat up, after which slowly advance the trimpot till a hiss is heard within the speaker.

Effectively I heard a hiss within the speaker even with the trimpot at minimal, and it hardly modified in any respect even once I turned all of it the best way to most.

I believed that in all probability I had made a soldering error, however earlier than taking all of it again aside I performed round with it a bit and labored out that the quiet hiss I had been listening to within the speaker
“does not depend”. The actual hiss that you simply’re searching for is way louder, and also you’re extra prone to discover it if you happen to tune someplace to the center of the frequency vary as a substitute
of on the very backside.

Use

Transferring left to proper:

The “regen” knob is a potentiometer on the regenerative suggestions, to manage the energy of the regeneration. When you set it too low you do not hear something (past
the quiet hiss that’s at all times current). When you strategy the crucial level the hiss will get louder, and past the crucial level it causes whistles and buzzes. You may
tune it simply past the crucial level if you wish to take heed to Morse code or single-sideband, and slightly below the crucial level if you wish to take heed to AM.

The “sense” knob is a potentiometer on the antenna enter. It’s advised that you simply use this knob to manage the audio quantity, nevertheless it looks like the receiver would profit from
a second quantity knob as a result of attenuating the incoming sign does extra than simply make it quieter.

The following knob is labelled “positive” and is used to make positive changes to the frequency after having chosen a frequency with the big knob.

The final knob is a rotary swap that selects between the three totally different frequency ranges, and can also be an influence swap.

The receiver is kind of onerous to tune, however that is positive, that is what makes it enjoyable to make use of! If I simply wished to modify a radio on and take heed to one thing
I’d have purchased a mass-produced one.

It appears to be delicate to how shut my hand is to the left hand facet of the field, which is the place the coil on the regneration board lives. In some configurations you’ll be able to virtually play it like a Theremin.

I anticipated to have the ability to tune some medium-wave stations on the backside finish of the 1-10 MHz vary, however was virtually fully unsuccessful. I did handle
to seek out TalkSport as soon as, nevertheless it was extraordinarily delicate to small actions on the entire knobs. I’ve had most success listening to worldwide stations between 10 MHz and 15 MHz. I do not understand how a lot of this is because of my antenna (I’ve ~5 metres of wire hanging indoors, which is about 1/4 of a wavelength at 14MHz), how a lot is as a result of receiver, and the way a lot is because of ionospheric situations. Sadly the overseas stations principally appear to talk overseas languages, so I battle to know what they’re saying.

I attempted grounding the receiver by means of {the electrical} earth, nevertheless it appeared to introduce a whole lot of interference (uneven noises). For now I’ve obtained the “floor” connection tied to a metallic shelving rack, which appears to be a slight enchancment over nothing in any respect. I intend to interchange the antenna with a wire hanging within the roof ridge within the loft, with a coax working right down to my workplace. I do not know what to do about grounding, if something. The KRC-2 directions embody a diagram that means hammering a spike into the bottom.

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