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1-4 of 4 messages
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Page 1 of 1
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Problem with NC-98 VFO
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by N5KBP on October 13, 2009
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I have been working on an old National NC-98 for a couple of weeks. I thought I had it complete until I attempted to align the 14 to 40 mhz band. The 6c4 osc tube will stop working below 16 mhz on this band. It works FB on all others. I have changed the tube out twice and checked voltages and resister values. I am at a loss. Any ideas?
Marty
N5KBP
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RE: Problem with NC-98 VFO
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by KA5N on October 13, 2009
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I would try C43 the feedback capacitor for 6C4 which may have changed value. Next the bypass caps at the lower ends (ground ends) of the oscillator coils. After that, any component in this elderly receiver is likely to have failed. Look for bad solder connections etc.
Good luck
Allen
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RE: Problem with NC-98 VFO
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by N2EY on October 13, 2009
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What KA5N wrote is all good. But also check the voltages at the 6C4 socket and the value of the various carbon resistors.
Some styles of vintage carbon-composition resistors will slowly drift upwards in value over time until they are way off tolerance. Often the piece of equipment using them still works - sort of - but not well.
73 de Jim, N2EY
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RE: Problem with NC-98 VFO
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by W5RKL on October 18, 2009
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N5KBP said:
"I have been working on an old National NC-98 for a couple of weeks. I thought I had it complete until I attempted to align the 14 to 40 mhz band. The 6c4 osc tube will stop working below 16 mhz on this band. It works FB on all others. I have changed the tube out twice and checked voltages and resister values. I am at a loss. Any ideas?
Marty
N5KB"
Marty, it appears the local oscillator for band D (14Mhz to 40Mhz) is failing, more than likely due to L8, C42, or both, being improperly adjusted.
I assume you have have cleaned the band switch wafers with contact cleaner. If, however, you have not, I recommend doing that before doing anything else. It may be a dirty band switch wafer for band "D" that is causing the oscillator for band "D" to fail. If cleaning the band switch wafers does not fix the problem, proceed to the next step of adjusting band "D" local oscillator coil and capacitor (L8 and C42).
Set the band switch to band "D" (14Mhz to 40Mhz) and tune the receiver until the oscillator stops oscillating. At this point, stop tuning the receiver's main tuning knob.
Now, adjusting L8 and/or C42 in either direction, clockwise or counter clockwise, should result in the oscillator start to oscillate again. At that point, perform the alignment steps outlined in the manual.
If you do not have the NC-98 manual you can download it at the following website link:
http://bama.edebris.com/manuals/national/nc98/
The alignment begins on page 8.
73's
Mike W5RKL
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