Weyrad Superhet Circuit

[Neil Purling]

I got a set of Weyrad coils long ago, comprising:'CTM1' Aerial & HF, plus another box marked 'COM 1' containing 2 more coils.
The COM1 coils are oscillator coils.
I have had a anode-bend TRF made from the Aerial & HF coils but now im going to do a proper job and make my first superhet.
Thanks to Ed Dinning I have some IF transformers.
Has anyone got any old papers that came with Weyrad coils that had an example circuit?
The oscillator coil has two windings, one having about twice the turns of the other. I am hoping that if anyone has any old plans the wiring of the mixer stage will be more fully explained.

[Roy]

I have a Weyrad leaflet covering type "H" and "K" coils - I don't know if these are anything like yours (maybe similar coils on a different style of former?). It includes a superhet circuit using 6K7 and 6K8, and 500pF tuning gang.

I used a NOS Weyrad coil pack for the AM section of my AM/FM tuner, and it works well. This leaflet was in with the coil pack - I wasn't sure if that was a packing error, or the leaflet referred to the type of coils used to build the pack. Either way, it wasn't much use, and I had to work out the coil pack connections by tracing out the circuit.

I'll scan it for you if nothing more promising turns up.

Roy

[Neil Purling]

My coils are air-cored, wheras the at least some of the Weyrad 'H' range had ferrite cores on brass adjuster rods.
For the Weyrad air-core coils I have I believe there would be a trimmer on the aerial side and both a trimmer and a padder on the oscillator side.
Repanco iassued a diagram with their SH4 coil set that shows the padder to be a pre-set variable capacitor of some type.
I think this adjustable padder is for better local osc tracking at the LF end of the band.
I am going to need to know the right values for the trimmers and variable padder.
You will see the image of the local oscillator coil attached to this post.
Which would be the grid winding & which is the anode winding?
The windings are clearly of different size. I have no idea where each winding should be connected.

[Roy]

Sorry Neil, your Weyrad coils are completely different, so my data sheet won't be much use. Unless someone comes up with the correct data, you will probably have to do rather more experimenting than you had originally planned!

Roy

[Neil Purling]

Does anyone else have any ideas, pleae!

[Ed_Dinning]

Hi Neil, try a google and look for theb vintageradio.me.uk site
This has a lot of Radio Constructor data as well as Home radio of Mitcham. There is a data sheet on they for some Weyrad coils.
They look pretty close to Wearite P types .

Ed

[Neil Purling]

On the oscillator coil I have the tags are marked with paint. I know what end of what winding terminates at each tag. I still don't know which is the grid & which is the anode winding. That's not a big problem.
I need to find the right values for the trimmers and the padder. It is only one MW band & there's no dial to worry about.
Everything after the frequency changer is a piece of cake.

[Neil Purling]

Speaking of the Web site: http://vintageradio.me.uk/info/repancosh4.htm
This is the circuit I followed. However the coils I used were by Weyrad: An Aerial & Oscillator set for MW only

The IF strip is fine The frequency changer does not want to play. There is no 'hiss' through the speaker.
It is a ECH35. Heater is aglow & there is HT. Should you be able to look at the local oscillator by a 'scope probe on pin 6 of the valve, the anode of the triode section?
If I am not doing that right, where should I look?
I have never created a valve superhet from scratch before. The oscillator coil is correct & the grid and anode of the triode use the correct windings of the coil.
One of the greater unknowns is the value of the padder. It is a compression type preset, but will it stop the local osc completely if it is the wrong value?
I thought it was just there for creating the frequency difference, so you ultimately get a beat freq at the IF.

The Repanco coils were created on polystyrene formers of about half the diameter of the coils I used. I do not know it the Repanco SH4 coil set had ferrite slugs.
That Web site doesnt have any more details of the circuit.
I looked at the old copy of Radio for Boys. That had a mains superhet in it.
Although it used a Osmor coil pack I have been able to see that there were not any mistakes in the Repanco diagram.

[G6Tanuki]

Is the local-oscillator actually oscillating? Easy way to check this is to get another 'external aerial' type medium-wave radio you know works, tune it to the HF end of its range (1300KHz) and fit it with a short aerial-wire (2 or 3 feet - I usually use a spare test-meter lead).

Dangle this aerial-wire near the local-oscillator of your problem-radio, then tune the problem-radio around the 800-900KHz range.

You should at some point hear a 'swish' or whistle as your problem-radio's l.o. tunes across the frequency your other radio's tuned to.

[ronbryan]

Neil

Although the Repanco SH4 coils had threaded formers, they were not fitted with ferrite slugs.

A negative voltage on the grid of the oscillator triode is another indication that the valve is oscillating at some frequency or other.

Ron

[Neil Purling]

I tried the trick with another radio, it was only a Selga 404??
Anyway, no whistle or his through the speakers & I set the IF strip last night 4**Khz in the Aerial terminal produces the 1Khz annoyance from the speaker.
Should you be able to look at the oscillator with a 'scope if it is running?
I figured the oscillator would run, even if the padder were of the wrong value & I could detect some evidence of life with my 'scope probe.

[Sideband]

You need to use a 10:1 scope probe otherwise you may stop the oscillator when you connect the probe. Obviously you then need to set the scope input to x10 to compensate.

Don't forget that you need to connect the oscillator coil in the correct phase otherwise it won't oscillate. Try reversing the connections to one coil first and see if it plays games then.


Rich.

[Neil Purling]

I'll try that with the coil connections.
Has anyone more ideas I can try at the same time?

[Neil Purling]

One lingering doubt: Have I the right windings? How does one deduce which is the anode winding & which is the grid winding?
The oscillator coil is shown in post #3. I want to eliminate the chance I am using the wrong windings.

You aren't going to here anything from the receiver if the local osc were running but miles out of alignment, are you?

[G8HQP Dave]

As Ron says, check the oscillator grid voltage. To reduce disturbance, connect via a 1M resistor i.e. tack solder a 1M to the grid tag, then measure the DC voltage from the other end of the 1M to chassis. You should see a negative value of some volts if the oscillator is working. The voltage may change as you rotate the tuning capacitor.

[Neil Purling]

Measuring the grid voltage is the only way of knowing for sure?

[Neil Purling]

I took my DMM, figuring that has a better ohms/volt figure AND the reccomended 1M resistor. I get +1.78V. That's not good.
I assume the grid is positive because it derives from the fact g3 in the Hexode is positive from the 47K connected to HT and it is internally connected to g1 of the triode.
I may sound like a total . I have not made a valve superhet before & I just knew the mixer/osc would have me wanting to go do this:

[ronbryan]

Neil

I think the Repanco drawing of the 6K8 is a bit misleading. The triode oscillator grid (pin 5) is actually internally connected to grid 1 of the hexode mixer, not as shown in the Repanco drawing, where it appears to go to pin 4. So there is no HT applied to the oscillator grid via 47k as you suggest.

Grids 2 and 4 (pin 4) are internally connected together and are fed with a decoupled screen supply (47k and 0.1uF). Grid 3 is the top cap, to which the signal is applied. The hexode anode is pin 3 and the triode anode is pin 6. The cathode is pin 8.

The convention is that the hexode grids are numbered from 1 starting from the cathode end of the valve.

The oscillator grid (pin 5) should not be positive if the oscillator is working correctly.

Ron

[Neil Purling]

If you look at the National Valve Museum pages their listings for the ECH35 and the 6K8 have different pin arrangements. Is that a mistake?
I have tried [B]both[B] types with no difference being observed.
Where might the positive voltage on the grid of the triode be from then?

[ronbryan]

In the 6K8 the oscillator grid is coupled to the mixer via grid 1, with the aerial signal input on grid 3 see http://www.radiomuseum.org/tubes/tube_6k8.html.

With the ECH35 the oscilator grid is coupled to the mixer via grid 3 - see http://www.radiomuseum.org/tubes/tube_ech35.html

If the oscillator is running correctly, the positive tips of the (quite large) oscillations at the grid are clamped approximately to the cathode voltage. Consequently the average grid voltage is negative. If the valve is not oscillating, the grid will assume a voltage near to the cathode bis voltage (i.e. slightly positive if the valve is conducting).

Have you tried reversing the connections of just one of the oscillator coil windings as Sideband suggested earlier. The oscillator coil has to generate positive feedback in order to initiate oscillation, so the reversal proposed is a test to confirm that the correct phasing has been selected.

Ron