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Old 18th Aug 2017, 9:34 pm   #1
Ricardo
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Default National HRO CW Oscillator (BFO) control

I have an issue with the CW Oscillator (BFO) control in my HRO.

I can rotate the variable capacitor continuously well over 360 degrees, albeit with resistance felt when it pushes past the associated toggle switch and turns it on and off.

I believe that this continuous rotation should be prevented by a pin projecting from near the front of the shaft hitting a tab from a ring washer.

The tab that you can see projecting from the ring washer is no longer rigid (it is very loose and will snap off soon) and the pin pushes past it very easily.

I can probably find a similar ring washer with a single tab, but my question is would this washer originally have had more than one such tab limiting the rotation of the control in both directions?

I'd be grateful if someone with an HRO with a properly functioning CW Oscillator could take a look at their radio and tell me whether this ring washer should have one or more such tabs?
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Old 18th Aug 2017, 10:09 pm   #2
Bazz4CQJ
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Default Re: National HRO CW Oscillator (BFO) control

I'm just looking at mine; this has been modified at some time to deal with just this problem. The washer in question definately looks like it was hand-made and instead of having two lugs (like yours must have had originally) it has one continuous big lug, which must be nearly 10mm wide. This very strong single lug, limits the rotation to ~290°.

Never noticed that before!

B
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Old 19th Aug 2017, 8:15 am   #3
a223bfg
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Default Re: National HRO CW Oscillator (BFO) control

Hi Ricardo,

The attached pictures show the arrangments on two of my HRO's, one has the large tab as described by Bazz, the other has at some point had the ceramic drilled and a small bolt fitted to act as a stop.

Phil
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Old 20th Aug 2017, 9:27 am   #4
Ricardo
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Default Re: National HRO CW Oscillator (BFO) control

Bazz and Phil,

Many thanks for the information and the photos.

The bolt as the end stop to prevent further anti-clockwise rotation after the OFF position was an interesting idea, but I decided go with the single long tab solution. It did however take me three attempts to get it right!

For my first attempt I fashioned a washer out of a flat piece of aluminium which already had a suitable diameter hole in it. I bent a wide (flat) tab on the side OK, but it really needed to be curved for the pin to hit the ends and not the sides. In trying to do this I completely distorted the ring itself.

The next version was far more successful. I cut the bottom off of an old metal can capacitor of the right diameter and used that to make my washer with a long curved tab. The only minor issue with this one was that the tab was too short.

So I made another capacitor based washer this time taking care not to cut the tab part too short.

As it happened the diameter of the second capacitor was slightly less than the first so I have used both of my home made washers one inside the other to give some additional strength at the anti-clockwise (OFF) end of the control.

My control is now limited to 10 on the dial in the clockwise direction corresponding to maximum capacitance. At zero on the dial the BFO is still ON and the capacitor is at its minimum position. Further anti-clockwise rotation to the OFF position on the control switches it off and the tab stops it rotating any further in that direction.

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