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Author Topic: VFO only tunes 1/3 the way  (Read 3346 times)
KJ6TSX
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Posts: 116




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« on: June 11, 2012, 10:33:53 PM »

I just bought a Kenwood 520 it works! the only real problem i see is the radio goes silent when the VFO is tuned to approximately 300. Does not matter what band at 300 it quits. Is this a comman problem with these old rigs or did I just buy a real boat anchor Grin
Any insight would be appreciated
George
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W4OP
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« Reply #1 on: June 12, 2012, 06:28:11 AM »

It sounds as though the VFO stops oscillating at 300 and up. Find out the actual oscillation range of the VFO and  listen on a general coverage RX or if you have a counter, connect it to the VFO output and see what's going on. Those rigs are easy to work on.

Dale W4OP
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KD0REQ
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Posts: 950




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« Reply #2 on: June 12, 2012, 11:02:55 AM »

I would wager you have a creepy oscillator transistor.
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WD8AJY
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« Reply #3 on: June 12, 2012, 11:50:15 AM »

Check the variable capacitor in the VFO. i have problems with the rotor making interment contact with the case as it rotates. i fixed it by soldering a peace of flexible wire from rotor to the case with a loop around the shaft.
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AA4PB
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« Reply #4 on: June 12, 2012, 11:56:16 AM »

You beat me to it... I was going to say that it has a variable capacitor inside the VFO can and I'll bet there is something wrong with the mechanics of that capacitor.
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KJ6TSX
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« Reply #5 on: June 14, 2012, 06:20:57 PM »

Thanks for the reply's
I pulled the vfo out and cleaned the ground connection were it brushes the rotor and that helped but it is still not right, I could use some more information on installing that wire.
Does the wire just go around the shaft and apply more pressure on the rotor or did you physically solder a wire to the rotor and the case??
Thanks for all the help
George
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W4OP
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« Reply #6 on: June 14, 2012, 06:47:56 PM »

It is also possible that the cap stators/rotors are shorting out. Look at the VFO circuit and determine if you can measure the resistance w/o disconnecting anything (if the VFO main cap is capacity coupled). Other wise you will have to diconnect the rest of the circuit and then make the measurement.

Dale W4OP
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WD8AJY
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« Reply #7 on: June 15, 2012, 08:20:25 AM »

check for shorts first. if there are none take a peace of small braid (i used some from very small coax) and solder it to the copper plate to the brass spring on the back of the cap. make sure it will not bind when the rotor moves. my email address is rlgooseberry@msn.com. 73 Bob
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KJ6TSX
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« Reply #8 on: June 15, 2012, 03:28:54 PM »

Thanks everyone I got it working, someone had used grease at the ground wiper connection. Some fine sandpaper and contact cleaner and it is working perfectly Grin

George
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WX7G
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« Reply #9 on: June 16, 2012, 06:27:48 AM »

This is sort of like "is the glass 1/3 full or 1/3 empty?"
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KJ6TSX
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« Reply #10 on: June 16, 2012, 04:47:22 PM »

all depend which band I am on on 80 meters it's empty 3.5mh plus 300kh is 3.8mh were the usable frequency begins. 40 meters not so bad 7.0mh plus 300Kh is full Roll Eyes
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AC5UP
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« Reply #11 on: June 16, 2012, 05:30:00 PM »

Years ago when analogue FM tuners used variable condensers there was a common issue with older units known as "NPR Syndrome". Back then the first four MC's of the FM band were reserved for college / non-commercial community stations and the bible thumpers hadn't started snapping up allocations. This meant there was a fair amount of open space below "92" and many listeners weren't exactly NPR material. The average tuning condenser rarely approached full mesh. Throw in a touch of oxidation and you ended up with a tuner that didn't work below 92 MC's.

The fix was to either clean the rotor ground contact fingers or exercise the tuning knob with extreme prejudice.    Tongue
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