TECHNICAL SUPPLEMENT FOR THE SR-160 & SR-500 by WDØGOF August 2010
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LOW LEVEL AUDIO CONSTANT IN RECEIVER
Over the years I have seen and added comments to a lot of posts concerning gain in the mic amp of the SR-
150, 160 and 500. Some of these have involved placing a cathode bypass capacitor on the 2nd mic amp. I’ll
have to admit that I to thought this was a cleaver idea, but not any more. This is not a good idea. Recent
discoveries indicate it should not be done. The problem is there in all three rigs but was noticed first in the
SR-150. In the RX mode with the audio gain turned fully CCW there will be a faint 2500 Hz to 4500 Hz tone in
the speaker. When measured, it will run from 10 to 150mw. I have had four 150’s recently with this problem.
Needless to say the first time I noted the problem it was quite puzzling. When I pulled the 2nd 1650 I.F. tube
which killed all the Rx noise I found that the level of the objectionable tone was not changed by the setting of
the AF Gain control. Now how exactly I ended up in the VOX circuit I can’t quite remember. But I noted that if
I turned the VOX gain control all the way down the tone went away. I connected the scope to the plate of
V18B the second mic amp, and to my surprise I found a 75v peak to peak audio oscillation. I also noted at
that time a 10uf/50v capacitor had been added from the cathode of V18B to gnd. When I removed the
capacitor the oscillation stopped. I have run into oscillation problems on 4 rigs in recent months and in each
case a bypass capacitor has been added to the cathode of the second mic amp. In one case I did not have a
tone in the RX but was unable to properly set up the VOX gain and delay. The oscillation, on this one turned
out to be about 18 kHz, which is out side the range of my old ears and the response of T5. So the tone did
not appear in the speaker.
Why was it noticed in the 150 receiver audio and not the 160 and 500? Well the 150 has VOX and the VOX
ckt provided the feedback path to the audio output. Due to low level of the signal in the speaker I believe the
feedback path is from the mic amp to the VOX ckt then thru cr5, r98, c117 and directly into the secondary of
T5.
The oscillation is there in the SR-160 and SR-500 rigs. The only indication you get is the carrier balance does
not quite meet spec and receivers on the other end report a low level squeal or tone in your audio.
I have come to the conclusion that the un-bypassed cathode resistors in the mic amp ckts do serve the
intended purpose of the original design. That is, to degenerate the high frequency response of the mic gain
train, and that’s a good thing. The original design is good and if all the “parts” are good it will work well. One
of the most miss-matched parts is the microphone. The 150, 160 and the 500 all want to see a high Z
dynamic mic with an output in the range of 5 to 10 millivolts. High Z dynamic microphones in 1963, in the
published words of Hallicrafters were in the order of 600-1200 ohms. Ceramic and xtal mics have far too
much output in the 3 KHz to 4 KHz range with the level at 500 Hz usually 3 to 8db lower. Since the audio
response of the SSB circuitry is from around 500 to 2700 HZ, depending on the rig, putting a mic on the radio
that over drives the system in the 3 Khz to 4 Khz range does nothing but screw up the spectrum.
So in conclusion if you cannot drive SSB to full power check your mic or fix your broken radio. But don’t
compromise the design by adding bypass caps.
Best regards, Walt, WDØGOF
