Heathkit Mohawk LSB/USB

[KJ7USA]

I recently purchased a Mohawk for AM use and have noticed that when switching between LSB/USB while in the AM mode, I have to retune to peak the signal. There is also a significant drop in signal strength in USB compared to LSB. Is there an adjustment I can make to make so I don't need to retune between LSB/USB? Also, is there another adjustment I can make to increase the gain on USB? I have peaked the 50 KC/1682 KC transformers for maximum s meter reading like the manual states. I do not have the temporary mixer and oscillator trimmers to perform the rest of the alignment. I used my TS-590 calibrated to WWV to zero beat the BFO and calibrator between LSB/USB and that's about it.  I don't have any test equipment. Thank you in advance!

[nq5t]

I recently purchased a Mohawk for AM use and have noticed that when switching between LSB/USB while in the AM mode, I have to retune to peak the signal.

It depends on how you are tuning the signal.  If you tune an AM signal in for zero beat with the BFO ON (and the BFO is centered properly) -- then you will be tuning to select the sideband you have set on the selector switch.  Turn off the BFO for normal AM listening.  You can then change sidebands with the USB/LSB switch and get the other sideband.  How well this works depends on a number of factors.  Later Hallicrafters radios did the same "selectable sideband" thing on AM, and it doesn't really work perfectly, IMO, and is mostly worthless.

I'll guess that what you are doing is tuning in the signal like you would any AM signal -- BFO off, tune until the signal peaks and sounds "tuned in".  By tuning that way, you are centering the AM signal (it's carrier) in the passband.  It doesn't really matter which sideband you have the switch set to, you are getting as much of the AM signal, BOTH sidebands, that will fit in the passband.  So if you've tuned this way, by virtue of the way the IF frequency conversion circuits work, when you switch to the other sideband, you will have to retune to "center" the signal again.  

So I suggest just pick a sideband and leave the switch there.  Tune in the AM signal.  You can then adjust the TUNING, up or down to shift the signal in the passband and get one (or mostly one) sideband.  Shifting this way can help eliminate interference if any, and also by getting mostly one sideband in the passband you get maximum fidelity.  It's just less complicated than trying to tune the signal perfectly (and on the correct side of the carrier) so that you can use the mode switch to change sidebands directly on AM.

As to the difference in signal levels when you switch sidebands, I'd suggest swapping out the 12AT7 1632-1732 kHz crystal oscillator tube as a first step.  It could be that one half the tube is weak, and that the injection levels from one of the two oscillators is significantly lower than the other.  The only thing that actually changes when you change sidebands is which half of that tube (and thus which of the two oscillators) is running.  Other things could affect it -- a low activity crystal, component value shifts, even dirty switch contact, etc., but the tube is a good first start if you don't have any test gear to check voltages, etc.

Grant NQ5T

[WQ9E]

Grant did an excellent job of explaining how to properly tune these selectable sideband receivers on AM and the same applies to the Hallicrafters SX-96 and later family of receivers.

One other possibility is that one or both of the second conversion crystal oscillators have drifted from the original frequency.  The crystal frequencies are supposed to be 1632 and 1732 Khz. and the second IF is aligned to 50.4 Khz. (half of the frequency difference between the two crystals plus a slight offset (400 hertz) to provide desired sideband selectivity); the first IF is aligned at the midpoint between these two crystals.  Check the oscillator frequencies using either a frequency counter or a very accurate digital receiver carefully tuned to zero beat (check the test receiver against WWV first to confirm accuracy). If one or both crystals have drifted then you can either replace the crystal (expensive) or compensate for the frequency change during alignment.  

For example assume the 1632 crystal is now actually oscillating at 1630 Khz while the 1732 Khz unit remains on its proper frequency.  Then the absolute difference is 102 Khz. instead of 100 so diving by 2 gives you a new target frequency for second IF alignment of 51 Khz. instead of 50 Khz, note that aligning the IF center response for 400 hz above calculated frequency still applies to provide the desired asymmetrical response with respect to center frequency.  The first IF would be aligned to 1681 although in practice it is broad enough that generally such a small change makes no significant difference.  Of course the BFO would now be aligned to 51 Khz. instead of 50 Khz. at zero beat.

[KJ7USA]

Wow! Thanks Grant and Rodger for the excellent suggestions. I'll start off by ordering the 12AT7 tube and start from there. I do have a multimeter to check voltages and resistance, so I will also do that. I am new to AM and have so much to learn. I am glad to have this resource.

73,
John KJ7USA

[WQ9E]

John,

You are very welcome. 

One common mistake made by new users of the RX-1 is to run the IF gain control at full gain.  This will result in a noisy receiver which distorts and overloads easily.  Many receivers of this era have an internal "gain set" control that is adjusted during alignment but Heathkit had faith (probably too much) in their users and provided this control on the front panel so it could be adjusted for optimal operation under differing conditions.  Unfortunately many people turn it to max gain which is NEVER the proper setting in a correctly operating RX-1, gain in the IF strip will be excessive.

With AGC on and RF gain set to max adjust your RX-1 IF gain so that its 80 meter no signal S meter reading is roughly equal to what your other receivers show and this should be a reasonable setting for all bands.

[KJ7USA]

Thanks Rodger. I noticed the distortion with IF gain up to high. The audio sounds much better with it backed down around the 3 or 12 o'clock position.