Re: {Collins} R388 TR Relay

I did a bit of digging into the military documentation (details upon request) and found the following:

As mentioned, some R-388's have a BREAK-IN switch on the front panel. This switch is wired in series with the relay coil, and simply disables the relay when in the OFF position.

The muting relay normally disables the antenna input, and removes B+ from the 3rd IF stage. HOWEVER, some radios were supplied with a jumper joining the "hot" terminals of C214A and C214B, effectively bypassing one set of contacts on the relay, so the 3rd IF stage remains active. They don't state the purpose of this, but it would definitely produce sidetone in the presence of a strong signal.

It seems we have examples of both situations here.

My R388 has the switch, but not the jumper. I've used it as a second receiver with my KWM-2A, and the muting is 100% effective in this situation. For details of this setup, go to and click on TECHNICAL NOTES.


On 2013-07-23 12:49 AM, Richard Knoppow wrote:
FWIW, I also checked for residual signal with the power switch in standby but the mute off. Nothing. I think the antenna part of the mute relay is to protect the input of the receiver in case something is connected wrong. I suppose its possible that these receivers were sometimes used with antennas separate from the transmitter and no T-R relay. The input signal I used from the GDO is extremely strong so I think I would have heard it if there was any leakage at all. About diversity work: Some 51J series receivers have the necessary connections for diversity but many do not. Diversity requires that there be an external connection to the detector diode so that the two or three receivers can be connected to a common diode load. Usually the AVC is also brought out. The simplest type of diversity system uses the output voltage from the diode getting the strongest signal to reverse bias the others. The transition is fairly sharp. Combining the AVC tends to keep all receivers operating in their linear ranges. Some more elaborate combiners use the IF output. Receivers used for more modern diversity reception generally have some means of operating the local oscillators and BFO (when used) from an external source.

Richard Knoppow
Los Angeles

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