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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 17th Jan 2012, 12:41 pm   #1
GMB
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Default "Het" (BFO) not as expected

I'm just restoring an AN/APR4 search receiver.

The "HET" switch might be expected to operate a BFO but on checking it out I see that although the feature is for similar effect, there is no BFO at all but rather it appears to feed audio into the last IF to modulate it.

I haven't seen this done before and was wondering if any other receivers do this instead of the traditional BFO to make CW audible?

I can see that with a 30MHz IF and 38-1000MHz tuning range using 1940's technology a conventional BFO might not really work so well.

I have yet to see how well this works out as the circuit is quite broken at present.
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Old 18th Jan 2012, 1:32 am   #2
Dave757
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Default Re: "Het" (BFO) not as expected

Quote:
Originally Posted by GMB View Post
I'm just restoring an AN/APR4 search receiver.

The "HET" switch might be expected to operate a BFO but on checking it out I see that although the feature is for similar effect, there is no BFO at all but rather it appears to feed audio into the last IF to modulate it.

Hi GMB,

I've just had a look at the circuit, and the Het switch seems to function as follows:
In the Off position Het switch S-103 completes the earth circuit to meter M-101.
In the On position the earth to the meter is removed and the meter goes to earth via a 2.7K resistor R102-12 (perhaps this is to desensitise the meter when the BFO is in use). At the same time, C108-1, the Cathode decoupler is bypassed by 100 Ohm resistor R122-2. Does this perhaps make valve V101-7 self oscillate and provide a BFO type action?

Kind regards

Dave
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Old 18th Jan 2012, 10:44 am   #3
GMB
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Default Re: "Het" (BFO) not as expected

V-101-7 looks like a standard phase-shift audio sine-wave oscillator to me and you will see it connected to the last IF where its signal is in place of the AVC line, thus effecting AM modulation of anything going through that.

The change to the meter circuit is subtracting a small offset which I guess is to allow for the fact that the audio signal will itself come through the IF stage and so would add to the meter reading.

I won't be able to see this working for a while as I have run out of replacement capacitors. Worse, I just discovered that the dreaded "Micamold" capacitors are generally in a worse state than might have been expected. It seems that "Micamold" is the "Hunts" of the US war era.
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