Lafayette KT-200 BFO

[k7mdo]

I couldn't help myself at the last ham fest here in Oregon.  30 dollars for what appeared to be a very well built little receiver.  Couldn't wait to get it home to see what I got for the bucks.

It didn't take long to get the cabinet off and notice it was obviously "kit built" but reasonably well done.  Quickly found two connections with no solder, fixed that and cleaned up 40 years of dust, checked the tubes (all original from date of manufacture), put it on the variac and was happy to hear stations on every band.  In fact it is a very good sounding set on AM.

The only problem is its ability to resolve SSB.  The BFO appears to be insufficient. The set only has variable IF which doesn't include the RF amplifier tube and makes it near impossible to tune strong SSB stations.  The BFO seems insufficient.  It is a 6AV6 coupled with a 5 pf cap to the detector.

Do you suppose it would require a modification to include a way to reduce the rf gain tube or would it be possible to increase the BFO injection to solve the problem?

73,Tom

[PA0NVD]

Hi Tom
I remember playing around with this receiver quite some years ago. The low BFO is a design problem of this receiver. I am afraid that you indeed need a mod to get substantial improvement. I tried, but with more coupling, the signals strength starts pulling the BFO frequency, so you need a mod like the addition of a product detector. I once made one using two transistors in a diff.pair, the BFO in the two emitters and the IF in one of the bases. Worked sufficiently, only the IF had to be attenuated. I used HV transistors fed with approx 100 V derived from the HV via a resistor

[W3RSW]

For RF attenuation trial, put a 5 k ohm pot across the antenna input terminal to ground, antenna lead-in to pot wiper.  Manual AVC. Or should I say simply RF VC? 
This I’d done to my BC 348 Q I got from JN with good results from a semi-serious receiver.

[k7mdo]

Thanks to both of you, I'll give the pot a try for sure.

I am of a mind to just relegate the set to AM as it sounds good enough for 75 meter rag chews.

Tom

[WZ1M]

Just being curious, I took my original, kit form, KT-200 down of the shelf and pluged it in. No issue with the BFO at all, works great. CW tone and pitch are good. I bought this as a kid when first started into ham radio.It was my first receiver.
Regards,
TRS
Gary

[AJ1G]

My E. H. Scott SLRM has always had a weak BFO injection level, since, at least  according to the schematic,  there is no direct connection of the BFO stage to the IF or detector stages, just stray  coupling.  I improved the injection level years ago by making a gimmick cap of twisted wire with the leads inserted into the BFO tube's socket at the cathode, and to the grid of the last IF  stage tube socket.  This worked OK, but as with most older receivers, you could not use it with the AVC on.  The BFO signal would swamp the AVC and drive the RF gain way down.

The Scott's low injection level may have something to do with the set being designed to meet the "super" low incidental signal radiation (SLR) requirements invoked by the Navy on these sets for use at sea during WW2.  However, my Scott RCH receiver (also identified as the SLR-12B  when used in the Mackay HF radio systems on Liberty Ships) has a very robust internal BFO injection level.

Lately I have been injecting an external BFO signal into the Scott from either a Rockland 5100 synthesizer or an AN/URM-25D signal generator.  Adjusting the BFO signal level permits using it with the AVC in the receiver on.  The fine resolution of the tuning knob on the URM-25D at  the nominal 455 kHz IF frequency also allows very fine adjustment of the BFO signal.