Finally got back to the HQ-145 project. Borrowed a 100 MHz oscilloscope
to measure the 1st LO injection level at V2-1.
Band: 4-10 Mc, tuned to 10 Mc: 20 Vpp (LO freq = 10.455 Mc)
Band: 10-30 Mc, tuned to 10 Mc: 0.9 Vpp (LO freq = 13.035 Mc)
That's a big difference!

Measured at several freqs in the 10-30 Mc range:
10 Mc 0.9 Vpp
11 Mc 3.3 Vpp
12 Mc 4.7 Vpp
13 Mc 6.1 Vpp
15 Mc 8.3 Vpp
20 Mc 13.0 Vpp
25 Mc 14.4 Vpp
30 Mc 13.7 Vpp

So, clearly the 1st LO amplitude is very low at the bottom of the 10-30
Mc range. The 1st mixer 6BE6 seems to need at least 5 Vpp of LO in order
for the receiver to have decent sensitivity. I suspect the LO coil tap
point is less than optimal for the low-freq end of the band as the
result of some sort of design compromise. (Better-quality receivers like
the HQ-180 divide the HF band coverage into smaller ranges so the RF and
LO circuits can be more optimized.)

I also happen to have an HQ-100 which is very similar to the HQ-145
except it is single conversion throughout. The difference in sensitivity
at 10 Mc on the two band settings was much smaller than on the HQ-145,
but still noticeable. I pulled the 6C4 tube from the HQ-100 and tried it
in the HQ-145, but the LO wouldn't even oscillate unless tuned to 13 Mc
or higher! That was odd.

I couldn't measure the LO level in the HQ-100 because oscillation would
cease whenever the oscilloscope probe was touched to the 1st mixer. LO
is pulled from the grid of the 6C4 in the HQ-100, whereas it's pulled
from the cathode of the 6C4 in the HQ-145. This might be why the scope
probe kills the LO in the HQ-100. Other than the pull-off point, the LO
circuit appears to be the same for the two receivers.

Well, this has been an interesting exercise. I don't think I'll do
anything further about the HQ-145 except maybe try to get a N.O.S. 6C4
and see if it makes any difference on the 10-30 Mc range.

73,
Joe K9LY


In article ,
Darrell wrote:

This is typical of many receivers. It is generally caused by variations
in local oscillator injection to the mixer. As you change the amount of
capacitance with the tuning capacitor, you change the amount of feedback
in the oscillator. Most oscillator circuits suffer from this.

73,
Darrell, WA5VGO