[MRCA] Russian US-P Receiver

[Meir WF2U]

Ray, 

The US-P receiver sensitivity is an issue in the original configuration. The sensitivity is limited as a way to implement a crude noise limiter which is operating all the time in both AM and CW. This is done by biasing the cathodes of the dual-diode detector/noise limiter tube 6H6 (6X6 in Cyrillic) to 3 volts by tying the cathodes to the cathode of the audio output 6K7 tube which is at 3 volts due to the cathode resistor. This means that the signal going into the detector has to be above the 3 volt threshold in order to be detected. Modify the circuit by removing both cathodes of the 6X6 from the cathode of the 6K7, tie them together to the ground. This increases the sensitivity to normal for this kind of tube line-up.

73, Meir WF2U
Landrum, SC

On September 8, 2015 2:19:36 PM EDT, Ray Fantini <RAFANTINI at salisbury.edu> wrote:
>I have a Russian US-P receiver that I received from the Ukraine the
>other day. The US-P is a small eight tube superhet that was used as
>both a field radio in the RSB luggable radio set or in various
>aircraft, manufactured during the war till the mid-fifties. The
>receiver uses all 6K7 tubes with one for the first RF, Oscillator,
>first and second IF and audio amplifier with the only other two tubes
>being a 6A7 as a mixer and 6H6 as a detector and AVC. There is also a
>6K7 for the BFO.
>If anything this radio is something like the tube lineup and design of
>an ARC-5 style receiver except unlike the ARC-5 this radio has five
>bands and covers 150 KC to 12 MC.
>The radio had been the victim of much eastern bloc hacking and after
>restoring much of the circuit to its original condition now it's
>working but have been having issues with low sensitivity. It requires
>around two to three hundred microvolts before you start to get solid
>static free copy and two to four mill volts before there is any AVC
>action. Although this works fairly well on the AM broadcast band and on
>high powered shortwave broadcast stations it still leaves a lot to be
>desired. I have checked all the bypass capacitors and screen and plate
>voltages, cathode bias resistors and the AVC bus for any leakage but
>everything checks good. Did pull all the tubes and test them in my
>TV-10 and replaced two of the 6K7 tubes with US versions of that tube
>but going to assume a Russian 6K7 is the same as a US 6K7. All the
>alignment looks just about right but still stumped by the lack of
>sensitivity. The IF has two stages and operates at 110 KC and I think
>that's where I am not seeing enough gain. The radio works ok on the
>local AM broadcast stations and sound surprisingly good for its age. I
>built  an external power supply for the receiver that has an additional
>audio stage built around a 6AQ5 so it can drive a speaker as opposed to
>the original sets audio output that was designed around a high
>impedance headphone system. Do plan to have this at Gilbert if anyone
>wants to see it in person and if anyone has any speculation of where
>the additional gain is let me know.
>
>Ray F/KA3EKH
>
>
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