TECHNICAL SUPPLEMENT FOR THE SR-160 & SR-500 by WDØGOF August 2010
17
SR-160 / SR-500
RECEIVER FAULT ISOLATION
EVOLVED BY WDØGOF
EQUIPMENT REQUIRED
:
HF RF signal generator capable of .5 microvolts to 200 millivolts.
Audio output meter (similar to General Radio 1840A).
Scope 10 MHZ or better. 1:1 and 10:1 probes or switchable probe.
Audio oscillator.
PROCEDURE
: There is not anything inherently clever about this process. What it does offer is signal injection
levels and output results gathered from dozens of tests performed on both the 160 and the 500.
For those who wish
to expound on the evil effects of probe loading and ckt miss-matches, I concede you are correct, it does happen.
But
the process works, it is repeatable and simple to follow.
STEP 1
: We will start in the middle of the receiver. Connect the audio output meter to the speaker jack on the front
of the rig. Tune the RF generator to 5200 kHz. Turn the rf and af gain on the rig to max. Disable the AVC by
grounding the junction of R65 and R67 using a clip lead. Connect the 10:1 probe to the rf generator and inject 200
microvolts at the output of FL-1. Tune the frequency of the generator for max audio output. Audio output should be
.5 watt. If this test is good then the back half of the receiver is ok go to STEP 6 if not go to STEP 2. Before you
precede to STEP 6 temporarily un-ground the AVC. If the audio reduces to little or no output you most likely have
an AVC problem, the fault will most likely be in V10.
STEP 2
: Using the 1:1 probe inject 4 millivolts of 5200 kHz into pin 2 of V4. Retune the RF gen. for max audio
out. If you get ½ watt or better of audio out then the problem is in the first IF amp V3. If not go to STEP 3.
STEP 3
: Using the 1:1 probe inject 0.1 volts of 5200 kHz into pin 7 of V5. Retune the RF gen. for max audio. If
you get ½ watt or better of audio out then the problem is in the second IF amp V4. If not go the STEP 4.
STEP 4
: Using a tee connector connect the audio oscillator and the 1:1 probe to the scope. Set the audio oscillator
to 1khz. Connect the probe to pin 2 of V5 and adjust the injection level to .3 Vpp on the scope. . If you get ½ watt
or better of audio out then the problem is in the product detector V5A. If not go to STEP 5.
STEP 5
: Move the 1:1 probe to pin1 of V6 and increase the audio signal to 12 Vpp. If you get ½ watt or better of
audio out then the problem is in first audio amp V5B. If not the problem is in the audio output stage V6.
STEP 6
: Tune the rig and the RF signal generator to 3900 khz. With the 1:1 probe connected to the generator
inject 150 microvolts into pin 7 of V2. Adjust the generator or the rig for max audio output. If you get ½ watt of
audio out go to STEP 7. If not the problem is in the receiver mixer V2.
HINT FOR LATER If you have good audio in this
step you can test the xtal filter bandpass tuning. Connect a freq counter and the audio out put meter to the audio output. Adjust your rf
generator for a1000Hz tone and note the audio output level. Now adjust for a 500Hz tone the level should not drop more than 3db. Now
adjust for a 2900Hz tone the output should not drop more 3db from the 1000Hz ref. If it is ok then you will not have to worry about it later.
STEP 7
: Reduce the generator output to 7.5 microvolts and move the 10:1 probe to pin 1 of V1. If you get 1 watt
of audio output then the problem is in S1, L1, K2 or associated circuitry. If not, then V1 the receiver RF amp is the
likely problem stage.
NOTE
: If the receiver is working properly a 1 microvolt signal at the antenna input will produce a minimum of ½
watt of audio output. Once properly aligned ½ watt output can be achieved with from 0.4 to 0.8 uv input.
