SX-140 problem

[wa3qns]

My search for a vintage receiver to compliment my 6L6 MOPA ended several months
ago with a Halli SX-140.  Since then I have recapped it, retubed it, put in new rectifiers changed two pots and a few other incidentals. What I have found very troubling is the lack of control over the receiver RF gain.  I am aware this radio "unfortunately" does not use a conventional BFO.  I have increased the value of the RF gain pot with no effect.  It remains impossible to knock down the gain enough to find my 6 watt transmit signal, reducing output power to about 1 watt dosent help either.  I have even removed V1 the RF amp tube to no avail. I'm left with using a small xtal oscillator to find my xmit frequency on this receiver.  Of course this also means using the receiver for a sidetone monitor while transmitting is also out of the question. 
Because my antenna is an end fed 50' wire tuned with an L-network there is some RF
in the shack but would that rule out being able to reduce receiver gain to an
acceptable level?
Thanks
Joe
wa3qns

schematic...............
http://www.manualslib.com/manual/790785/Hallicrafters-Sx-140.html?page=19#manual

[KA2DZT]

Check the voltage on the cathode of the first RF stage.  It should go more positive with decreasing the RF gain pot.  Increasing the value of the pot probably wouldn't make any change.  If it isn't increasing positive, I would check for a shorted cap in the cathode circuit, also check the voltage on the cathode of the first IF stage.  The two cathode circuits are connected at the 10K resistor to ground.  The RF pot puts more of this positive cathode voltage on the cathode of the RF stage.

Fred

I re-read your post, I thought your RF gain pot wasn't working,  I'm not sure what you're trying to do.  But, if you think you're going to leave a receiver on while you're transmitting a signal on the same freq and be able to reduce the received signal to a whisper, that's not going to happen.

[wa3qns]

Tnx Fred,
I should have stated the antenna is switched from the receiver to the transmitter via a manual coax switch which also grounds the receiver antenna input and with the receiver RF gain reduced all the way the receiver still goes nuts and no usable beat note can be heard.  I have used many receivers in this way and never had an issue like this.

I will check the things you noted because even with the lousy BFO injection the RF gain should do more than it is.  Any more ideas on this problem are welcome.  I am not an electronics technician but I can usually bumble my way through things with some help.
73
Joe

[wa3qns]

Fred,
Voltages appear to be normal, would have to replace the caps as I have
no way to test them.  At least AM sounds good and rig stability is good
but this gain problem is beyond me so far.  It's going on the shelf for
awhile.
ciao

[AB2EZ]

Joe

I suspect that RF from your transmitter is coupling into the receiver via the receiver's chassis ground, in conjunction with the AC power cord, the auxiliary control cables (if used), and/or the speaker wires. If you are using headphones, you may also have RF getting into the headphone cable.

What happens if you disconnect the antenna (both conductors) at the 2-terminal antenna connector on the back of the receiver... and also place a short across the receiver's 2-terminal antenna connector? When you transmit... do you still have the same overload problem?

Also, check to see if RF from the transmitter is getting into the AC power cable, or the speaker/headphone cable, or any other cables attached to the back of the SX-140

A ferrite core on each of the two AC input wires may be helpful... if RF getting into the AC power cable is a problem. You can also reduce this by plugging the SX-140 into a power strip with RF isolation.

With respect to the speaker/headphone cable: proper RF bypassing (e.g. a ferrite in series with each of the two wires, and a .01uF capacitor across the pair of wires... on the speaker/headphone side of the ferrites) should work (provided you are not using powered speakers).

Stu

[wa3qns]

Hi Stu,
Antenna shorted at receiver, chokes on AC in, speaker leads and Key, well grounded but
still not enough.  Using little portable for sidetone but switching xtals is a pain. 
Will keep looking for a rig with a more conventional BFO/RF gain set-up.  Damn shame
I didn't keep Drake receivers. Such is life................
ciao
Joe