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Old 18th Jun 2012, 1:35 pm   #1
Malcolm G6ANZ
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Join Date: Feb 2006
Location: Bristol, UK.
Posts: 1,030
Default NAAFI receiver problem

Greetings all, I’m not sure if this should be under the military section or not. Anyway, I have been given a NAAFI type C receiver to get going. It is without its PSU and I’m going to make a mains power supply for it. So at the moment I’m powering it from a Radford Labpack. The heaters from the 6v3 supply and the ht from the packs ht via a dropper resistor to bring the volts down to 250v from 350v. The current drawn is approx. 60 mA. The audio stage works well. But there is no sign of any stations. The local oscillator is running at the right frequency as proved by a radio tuned to twice the IF frequency (930 KHz). If I inject a 465 KHz modulated signal into the detector stage a demodulated tone can be heard. The aerial tuning peaks nicely on the grid of the frequency changer but there is no signal on the anode, as observed by a scope. Looking at the IF cans it appears that they may have been twiddled as the sealing wax is disturbed. Even if its way off tune surely there ought to be something at the anode.

So is 250v too high and the bias way off or is the fault more subtle? I have changed the wax caps and some high value resistors. I’m at work at the moment so cant tell you ay oif the voltages around the valves. If neded I can do that tonight.

Malcolm

Last edited by Dave Moll; 18th Jun 2012 at 1:54 pm. Reason: font normalised and thread title corrected
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