Howes DcRx

[Malcolm G6ANZ]

A long time ago I bought a Howes DcRx 80M receiver. I never completed its construction and now I decided it was time to finish it.
When I came to complete it I found that one of the coils L2 was missing,presumed lost. The instructions for winding the coils are minimal only showing a rough drawing and no indication of the turns required. By comparing the drawn number of turns with the actual number on the supplied coils I would ,probably the correct number. I found a toroid former in my spares drawer and made the coil.
The receiver appears to work in as much as it will give and audio output when a signal is supplied from a sig gen but it is very insensitive.To resolve any audio it needs at least 40uV direct onto the aerial. My old Codar CR70 can resolve down to 3uV.
The oscillator is on the correct frequency (3.5MHz) as measured with a frequency counter.
So, are these receivers really deaf or am I missing something in the home wound coil ?

[G6Tanuki]

The material used for the toroid could be important: if it's a toroid originally designed for something like RFI suppression it could be deliberately RF-lossy!

[Malcolm G6ANZ]

Thanks for the suggestion. I'll try winding on a different former and see(hear) what happens

[G6Tanuki]

Ideally if you're winding a toroid for this kond of balanced-demodulator the secondary winding should be 'bifilar'.

Get 2 lengths of enamelled wire, and twist them together (like the old figure-of-8 flex used for table lamps etc) then use this to make the secondary winding.

Then use your meter to identify the start and finish ends of the 2 windings.

Join the finish of one winding to the start of the other. This is now your centre-tap where C1 and R1 join. The 2 free ends go to the gates of the FETs.

[Radio Wrangler]

If you get stuck, I've got a bagful of little RF toroids, ready wound for that sort of mixer application. It was part of the loads of stuff sold off at the winding up of KW.

David

[Malcolm G6ANZ]

Thanks for the help. I made another coil using just a plastic former and the sensitivity has got better. Its now down to 30uV. Still not good enough but proof that the toroid is the wrong type.
David if you can spare a couple of your toroids that would be great. I will then wind one as suggested by G6Tanuki.
Malcolm

[Radio Wrangler]

PM me an address and I'll bung a couple in a jiffoid bag.

They're ready wound and ought to just work for you.

FOC and don't worry about postage.

David

[John KC0G]

The coils for these radios were supplied pre-wound. See the manual at: http://techdoc.kvindesland.no/radio/...9153822219.pdf

George Dobbs, G3RJV wrote a review in the May 1984 issue of Short Wave Magazine. Unfortunately, he deliberately omitted any details of the coils and PCB layout.

73 John

[Malcolm G6ANZ]

I have read the article by Rev Dobbs. It doesn't say very much about the receiver itself such as sensitivity or coil data.
I tried winding the missing coil using the drawing in the instruction manual, I don't know how accurate the drawing is in relation to reality. But since the coils were supplied ready wound the drawing didn't need to be accurate.

[Radio Wrangler]

Looking at the circuit diagram, L1 is the resonator, it needs to have a controlled inductance

CV1 resonates it in the wanted band and acts to an extent as a preselector.

L2 is used as a simple transformer driven by a link winding on L1. L2 should have a somewhat higher inductance than the link winding on L1, otherwise it will pull the tuning of the RF filter and spoil sensitivity.

L1 would have been one of the Micrometals inc. dust iron cores. -2 mix if red, -6 mix if yellow. In that era, they got called Amidon cores after a distributor who popularised them in American QRP circles.
L2, would be fine as an RF ferrite material, but it could have been dust iron with more turns.

David

[Malcolm G6ANZ]

Success!! I remade the coil L2 using G6TANUKI's suggestion of a bifilar winding. After a bit of fiddling I heard an amateur from Harwich. Not bad seeing as I'm in Bristol.
It works but there is still a problem. There is a lot of mains hum present with the aerial connected, the amount of hum depends a lot on the positioning of the wires going to the tuning and preselector capacitors. Also the toroid is hanging in mid air and is sensitive to movement.
There is no hum without an aerial or when connected to the sig gen. The sensitivity is now down to approx 2uV.
David has kindly offered me a prewound coil that should suit. I'll fit it and let you know the results.

Malcolm

[Radio Wrangler]

Hum is the absolute bugbear of DC receivers.

The obvious thing that everyone goes looking for are hum loops and power supply ripple.

The subtle one is positively evil:

As the mains alternates, the bridge rectifier in the power supply turns on at the peaks, and off between them. The RF impedance of the thing is different between conducting and non-conducting and it looks like a difference in earthing impedance at 100Hz. So you get a modulation hum appearing on all RF signals!

David

[Jon_G4MDC]

So is picking up your own local oscillator. Here is why the position of the wires matters so much.

It needs to be put in a metal box with aerial input at one end, filtered PSU input and audio output at the other. Maybe with a screening wall in between them about where the mixer stage is.

[Radio Wrangler]

The rectifier thing can also modulate the pickup of your LO.
You soon get paranoid about screening and isolation.

David

[Malcolm G6ANZ]

The toroids from Radio Wrangler arrived this morning. Having sorted out the windings I fitted one in the vacant place. It worked as it should. Sensitivity is now around 1uV EMF. By this mean that with a 1uV signal from the sig gen I can resolve a hetrodyne as I tune through the frequency. By re-routing the leads to the variable capacirors I managed to remove the hum and the set is now quiet,although it is microphonic. I will try to cure this by glueing down the wires and coils.
Thanks to all.

Malcolm

[Jon_G4MDC]

Good to hear of progress!
Now you should start on the filtering and screening previously mentioned.

[Malcolm G6ANZ]

I have not had and experience with direct conversion receivers before this attempt. So could you point me in the general direction of what to screen and where to put the screens. As i said I have eliminated the hum by re-routing the variable capacitor wires and adding extra decoupling. The receiver seems to be stable and sensitive.

[Jon_G4MDC]

Put simply you need the oscillator energy to be confined in a screened enclosure with the only output being to the mixer input.

The power lines should be filtered so that oscillator energy can't get out that way.

If you don't then the oscillator "masks" the signals you do want to receive producing a variable amount of DC in the mixer output proportional to how much LO was picked up and in what phase. By dressing the leads as you describe you have probably stabilised that but it won't have gone away.

How to do this varies with different designs and I am not familiar with the layout of the Howes.

When I built my own I started with the LO in a box made of PCB. Tuning capacitor was mounted inside with the tuning shaft through an earthed bush (Jackson C804). Power was fed by feedthroughs of 1nF with 10nF ceramic in parallel mounted on very short leads, ferrite beads both side. RF Output was by coax cable to the mixer.

Maybe you can construct a tin plate box around the oscillator section?

[Malcolm G6ANZ]

I've ordered some brass shim so I can make a screening shield for the oscillator area. I have already added extra decoupling to the supply to the oscillator section.

[Aub]

I always felt a bit annoyed when designs for simple receivers seemed to all start using toroids in the early eighties. I much preferred the good old Aladdin style formers.

Cheers

Aub