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Help - Working on an old Japanese RF Oscillator

Deltatango

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Got an old 9v battery RF oscillator from my early days (40+ years ago) which "covered up to 30MHz" at the time I became aware 24MHz was its max, because it would go into some very low frequency oscillation and then start to go into reverse during the final gang rotation.

It is an Armstrong based oscillator, and I heard then some HAM operators were trying to see why, though never found out any result.

I have tried many normal things like transistor change, bias resistor variation and capacitor changes to no avail, then tried upping its sniffer winding from 2 to 4 turns but it remains steadfast on about 24+ MHz (which is double the minimum range on that position) so are we fighting this in some way. I enclose a circuit which helps but note 100k bias resistor is now a 6k8 original in this unit as well as 0.01's are 0.05uf. Thinking about lowering the sniffer winding to 1 turn, also its layout is spread about with a old style wafer switch of 34mm, I expected inter-coupling issues and tried screening so am a bit puzzled.

Any insights into this would be welcome............

David
 

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start to go into reverse during the final gang rotation.
I guess you mean there's a point where the tuner brings in lesser frequencies while the label says higher frequencies?

I have a 55-year-old radio that does the same thing. At first the tuned frequencies correlated pretty much to the labelled frequencies. As years went by I've fiddled with the variable inductors in the belief I could make the radio stations line up with the printed labels. Now it works less well than new. (It's AM-FM-SW1-SW2.)

When I look at the tuning mechanism there's a small plastic box with semi-circular leaves inside. Tuning the unit causes leaves to rotate thus they overlap a little or a lot.

The leaves must be variable capacitors. So when I examine closely, the range of leaf overlap does not appear consistent with the tuning range. I wonder if it's caused by my attempts to 'fix' the tuning, or whether the leaves became worn, or the spacers in between became worn, or if grease or foreign matter developed and altered the tuner's characteristics.
 
To some degree this is so, it reaches around 24MHz then goes into some low frequency pulse state, recovers and continues in reverse (frequency reduces) as its tuning gang is advanced.

I think they stretched (or tried) to make a dial look neat by having a full 180deg, rather than stopping with a short scale like I have seen with some vintage machines. I may be wrong but the first bit of tune is parallel followed by a series type at the end for its last 6MHz.

Having tried many ideas and variations to bias, transistor type, capacitor values etc my next move is to create a copy of that relevant coil and remove a turn or two in a hope to raise its top frequency ( always a way to lower it easily). It was these units came on the scene 40/50 years ago via magazines of that time and not long before this issue surfaced, I heard some Ham radio operators became interested then to solve it, sadly never heard any result.

Thanks for your insight.

David :)
 
Continuing on with my testing, in general it likes any RF pnp germanium transistor all giving almost equal results with minor variations of bias, put shielding in places to see any changes with no luck.

Have been trying coil changes with up to two more or two less turns in a bid of seeing reactions, so now will try a larger gauge with correct windings.

I wonder if putting its sniffer winding at the same level as the main coil rather than over it, any thoughts please :unsure: .

Thanks from David
 


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