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Title: National HRO 5TA1 TX Mute Post by: N6YW on November 14, 2014, 12:25:15 AM Greetings
There have been some discussions regarding the scheme that the HRO 5TA1 used for TX stby as being not especially good for keeping the RX stable because it removes the B+ from the circuits during TX. I reasoned that a possible work around would be to add a relay that opens the cathode ground path on V1 at R27 that would essentially turn the tube off. I suppose grounding the grid would work just as well, or would it? Does anyone care to address this idea? I included a photo of the circuit to give you a graphic location. While I am finishing the restoration of my SX-28 for the BC-610-E, I would like to use the HRO. I love that radio and for the 40's period match up, it's a great combination. Thanks for looking. Title: Re: National HRO 5TA1 TX Mute Post by: WQ9E on November 14, 2014, 04:34:06 AM The system I prefer for muting uses the existing RF gain control bus. For the HRO-5 lift the center tap of the RF gain control (R1) from ground and place either a fixed resistor of 10K or better a variable resistor of 10K between the center tap and ground. The T/R relay shorts this added resistor for receive but during transmit the gain is reduced to standby conditions. By using a variable resistor in place of the fixed resistor you can have variable gain during standby to use the receiver as a monitor for CW (or phone using headphones).
If you just open the cathode of the first RF stage you will probably still have a fair amount of signal coming through the receiver unless the antenna input is also shorted and you have the receiver case "cold" for RF, otherwise enough signal from the transmitter to be a problem will make it into the second RF and mixer stages to then be amplified as usual. Title: Re: National HRO 5TA1 TX Mute Post by: N6YW on November 19, 2014, 08:13:30 PM Thank you AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
I would have answered sooner but I was out of town. That sounds easy enough. I failed to mention that in my scheme, the antenna input would also be shorted to ground because I would be using a DPDT relay. In addition to this wonderful rig, I also picked up a very rare Hammarlund SP-10 in good condition with the power supply. An electronic restoration is planned for the near future, so look for my posts. I'll let you know how the HRO mod goes. I am sure it will work nicely. Thanks again. 73 de Billy N6YW |