How does it Work?

 

(1) Mixer

(2) IF amp and BFO

(3) Detector

(4) Audio Amp

(5) Power Supply

 

Ahh the power supply! The section that gets the least respect from anyone!

 

Well here is the one from the CONAR receiver so let's check it out.

120 Volt AC out from the wall outlet goes into capacitor filter bank C7 and C8. Their purpose is to filter out RF, Wireless Intercomms, X10 signals and anything else coming in through the AC power. It goes through switch S2 which is located in back of the Volume control pot. Notice neon bulb LP and it's dropping resistor R17. This neon give a nice warm orange glow that is prettier than any LED. From there it goes into power transformer T1. Power transformers are usually very large but since we only have 4 tubes this one could be mistaken for an audio transformer because it's so small.

From there, it produces two AC voltages. The B+, which is about 110 AC, and the 6.3 AC filament voltage. From there it goes into diode D1. WHAT'S THIS ??? ONE stinkin' diode??? Yep, most of you know where I am going with this... This one diode configuration is known as a half wave rectifier because it only allows half of the AC cycle through. Think of it as a pulsing DC at 30 cycles per second. Trouble is, the time spaces between the pulses (where there is no voltage) are very large. Filter capacitor bank C14a and C14b have to make up for the missing voltage. They don't do a very good job and now you know why there is so much AC hum coming out of the speaker!!! Our first modification will replace D1 with a full wave rectifier.

Notice there are TWO B+ voltages? The first one goes to the audio amplification stage. The second one comes after the voltage dropping resistor R3 and goes to the rest of the stages (Mixer, IF, BFO). Anyone know why? Well, it's because the audio section uses quite a lot of power compared to the rest of the stages.

Big audio signals might drag the voltage down and if it were tied to the rest of the set, would affect performances of these other stages which would affect the audio stages which affect performances of these other stages which would affect the audio stages which affect performances of these other stages which would affect the audio stages which affect performances of these other stages....

Well, you get the idea. It's a type of oscillator known as "motorboating" and sounds like the putt putt of a small engine. If your CONAR has this symptom, check R3 (470 ohms). It might have shorted out, connecting both B+ voltages together. If not, replace R3 with a slightly higher value resistor and see if that solves the problem.

 

Well that's it! You now know everything about how it's supposed to work.

Time to open the case and modify!



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Complete Schematic for the Conar Model 500 Receiver

Conar Transmitter Manual - and - Conar Receiver Manual