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Old 27th May 2019, 5:34 pm   #1
johnyjf
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Default Saxon SW Radio does not perform

Hopefully this is the correct forum. I have just built one of these using a home made MW coil rather than the Denco Green Range items recommended. Mine does not include the Power Supply they recommend but a home made choke smoothed one using a 6X5, It is about 110v down on output which is obviously low. I am seeking opinions as to whether it is too low to work before I embark on the task of making a PSU with the correct output voltage In the original cct. (attached) the anode of the 6J5 has 67v on it mine has 37v, Ia being about 2.2 m/a. The Radio does attempt to oscillate when the reaction capacitor has its plates fully meshed but I get no stations even with a 25ft long wire and a decent earth. It also occurs to me that the 2.5mH anode choke may be a little low on value for the MW band??
Thanks in anticipation.
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Old 27th May 2019, 6:09 pm   #2
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Default Re: Saxon SW Radio does not perform

When the circuit is oscillating you should be able to pick it up with another radio (portable transistor type?) to check where you are tuned.
Probably best not to sit right on a broadcast frequency for too long though.
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Old 27th May 2019, 6:14 pm   #3
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Default Re: Saxon SW Radio does not perform

I'd investigate why your HT isn't up to a sensible value - I'd be expecting a simple radio like this to run with at least 200V on the 'yellow' smoothing-capacitor terminal.

Also, what headphones are you using? From the circuit I'd guess it needs old-fashioned high-impedance high-sensitivity balanced-armature types (ex-Governmentr "DHR" for example) - if you're using modern low-sensitivity low-impedance types it will be distinctly 'quiet'.

When you've only got a few millivolts of audio, you need to make sure these are properly matched to your earpiece.

[If low-impedance phones are all you have, try using an old valve-radio output-transformer - or even a 'LT700'-type transistor-radio output transformer - to improve the impedance-matching.
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Old 27th May 2019, 8:18 pm   #4
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Default Re: Saxon SW Radio does not perform

Have you tried reversing the reaction winding?
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Old 27th May 2019, 8:20 pm   #5
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Default Re: Saxon SW Radio does not perform

Some excellent suggestions, I should have thought of the adjacent radio trick, I'll try that Wednesday , my next play day. The head phones are high impedance types, 2-4 kohms. I might try an old valve OPT, I have one somewhere. May also try replacing the 6X5 with a pair of silicon diodes, should boost the HT a bit. Thanks for help.
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Old 27th May 2019, 8:23 pm   #6
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Default Re: Saxon SW Radio does not perform

Quote:
Originally Posted by AC/HL View Post
Have you tried reversing the reaction winding?
Yes I have, its definitely oscillating when the capacitor plates are fully meshed but only then. Move 1/8 turn away and it stops.
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Old 27th May 2019, 8:33 pm   #7
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Default Re: Saxon SW Radio does not perform

2.5mH ought to be enough.

Have you tried using your coil for a crystal set? That would confirm that it is actually tuning to MW. Where did your coil design come from?
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Old 27th May 2019, 8:49 pm   #8
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Default Re: Saxon SW Radio does not perform

Also, what antenna are you using? It's quite possible that the antenna is 'damping' the tuned-circuit excessively (we don't know the ratio of turns of the antenna coil to the tuned-circuit coil you are using).

Try wiring a low-value capacitor - something in the range 10 to 50pF - in series between the primary antenna-winding of the tuned-circuit and the antenna itself.
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Old 28th May 2019, 6:38 pm   #9
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Default Re: Saxon SW Radio does not perform

Some excellent further suggestions, the coil is 80t of 26 swg on a 2" Paxolin former. The aerial winding is 10T bifilar would at one end the reaction winding is 10T bifilar wound at the other. Tuning cap is 300Pf single gang.I may try the crystal set idea, have a high gain twin OC75 amp that will drag noise out of anything so should be simple.
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Old 29th May 2019, 9:41 am   #10
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Default Re: Saxon SW Radio does not perform

Sometimes the component values on the schematics or material lists are printed incorrectly.

What are the resistor values and the HT voltage of the original design?
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Old 29th May 2019, 1:22 pm   #11
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Default Re: Saxon SW Radio does not perform

Thanks for the idea but I don't think there is anything wrong with the cct. If you look at the valve's working point is seems compatible with the circuit values as printed. I'm going to spend some time playing.
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Old 29th May 2019, 2:22 pm   #12
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Default Re: Saxon SW Radio does not perform

Does '10T bifilar wound' mean 20T, or 10T wound with part of the main coil? If the latter, it is possible that the two extra windings are too tightly coupled to the main coil.
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Old 31st May 2019, 5:34 pm   #13
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Default Re: Saxon SW Radio does not perform

Bifilar in this case means 10T wound with part of the main coil. Wouldn't take forever to separate the aerial and reaction windings from the main one. Interesting thought.
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Old 1st Jun 2019, 11:48 am   #14
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Default Re: Saxon SW Radio does not perform

You may need more turns, but more loosely coupled. A 25ft "long wire" is actually quite short for MW and so will have a high impedance. I am going to guess that you need maybe 20-30 turns for the antenna coil, wound next to the tuned coil but not over it. Alternatively, use capacitive coupling.

The reaction winding used to be called a 'tickler' - which I assume means loose coupling. At these frequencies a pentode has huge gain so only a little feedback is needed to get oscillation.
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Old 2nd Jun 2019, 6:33 pm   #15
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Default Re: Saxon SW Radio does not perform

I am halfway through trying the crystal set idea, then I'll add some turns to the aerial coil, may have to do a complete rewind, not the end of the earth, just a little irritating. I won't bifilar wind it if I do.
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