National HRO-60 Receiver
Performance Test

National HRO-60

National HRO-60

Band MDS Blocking Two-tone D.R.
(20 kHz) (20 kHz)
 
80 meters -142 dBm 121 dB 86 dB
40 meters -142 dBm 114 dB 83 dB
20 meters -139 dBm 111 dB 78 dB

AM Audio S/N: 52 dB




AM Audio Frequency Response:

 
100 Hz 200 Hz 400 Hz 600 Hz 800 Hz 1 kHz 2 kHz 3 kHz 4 kHz 5 kHz 6 kHz
 
+2 dB +2 dB +2 dB +1 dB 0 dB 0 dB -5 dB -16 dB -32 dB -38 dB -44 dB

AM Audio Distortion: stock agc

 
Mod% 100 Hz 200 Hz 400 Hz 600 Hz 800 Hz 1 kHz 2 kHz
 
30% 5.0% 3.5% 2.8% 2.5% 2.2% 2.2% 2.8%
50% 7.1% 4.0% 2.5% 2.0% 2.0% 1.8% 1.8%
70% 7.9% 4.5% 2.8% 2.2% 2.0% 1.8% 1.6%
90% 8.9% 5.0% 3.2% 2.5% 2.5% 2.2% 2.2%
100% 11% 7.9% 5.6% 4.5% 3.5% 2.8% 1.8%

AM Audio Distortion: modified agc

 
Mod% 100 Hz 200 Hz 400 Hz 600 Hz 800 Hz 1 kHz 2 kHz
 
30% 3.2% 3.2% 2.5% 2.5% 2.5% 2.5% 2.8%
50% 2.8% 2.0% 2.0% 2.0% 1.8% 1.8% 2.0%
70% 2.5% 2.0% 2.0% 2.0% 2.0% 1.8% 1.8%
90% 2.5% 2.0% 2.0% 2.0% 2.0% 2.2% 1.8%
100% 5.0% 4.5% 4.5% 4.0% 3.2% 2.5% 1.6%

Notes: Maximum i-f bandwidth is about 4 kHz @ -6 dB. Before running the audio frequency response and audio distortion test the tone control was adjusted for flat response as measured through the auxiliary audio input.

AM distortion numbers show the somewhat typical increase at lower audio frequencies - most often caused by the low audio frequencies riding on the AGC line. Also there is the usual increase in audio distortion as the modulation percentage increases. This is due to the detector's inability to cleanly demodulate the higher amplitude levels although this receiver does pretty well.

Note that there is a second set of audio distortion numbers for 'modified agc'. As a quick test a .1 uF was bridged across the existing .01 uF main AGC capacitor to see if this would improve the distortion numbers...and it did. This was just a quick test to prove that the AGC was a contributing factor to the problem - not to provide the best possible cure. An AMfone forum thread from Steve WA1QIX addressed the ills of AGC and detector stages in tube AM receivers. Steve attacked both the AGC problem as well as the detector's ability to cleanly demodulate modulation levels in excess of 100%...all as an add on 'module' to an existing receiver. Looks like I'll be building one in the near future.