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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 9th Sep 2017, 3:51 pm   #1
dtvmcdonald
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Default RA17C12 peculiarity

My Racal RA17C12 whose restoration I described in an earlier thread has a peculiarity.

When first turned on the 1st VFO tuning is odd. It has three peaks, the central one being strongest. After 30 minutes this is gone and it has only one peak, with the width being roughly the same as the distance between the original two outer peaks. The center frequency does not change noticeably.

BUT ... if I raise the line voltage from 121 to 126 for just one minute right after turnon, the three peaks are gone and it stays that way when I set the voltage back to 121. (The power supply is fixed to provide the nominal filament and HV voltages at 121).

The 37.5 and 40 MHz filters were carefully realigned as both had some bad mica caps in them (despite having caps that look like 1970s vintage US ones).

Any ideas what is causing this harmless behavior? A valve or resistor heating up??
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Old 9th Sep 2017, 11:33 pm   #2
Skywave
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Default Re: RA17C12 peculiarity

Check for the correct heater rail: 6.3 v.a.c. For a convenient measurement point, the dial light will do.

Al.
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Old 10th Sep 2017, 2:43 am   #3
dtvmcdonald
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Default Re: RA17C12 peculiarity

As I said, the heaters are spot on at 6.3 and the B+ also spot on, at 121 volts AC input.
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Old 10th Sep 2017, 7:56 am   #4
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Default Re: RA17C12 peculiarity

I'd rig it up to plot those filters again and watch their shapes after turning it on from fully cold. I wonder if some capacitor has a big tempco and you've adjusted for it at working temp?

My second guess would be looking for sidebands on the LO.

David
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