I'm working on K2 S/N 4590 (Rev B pcb). The receiver suffers from the
dreaded "low audio output" fault, meaning it's short on overall gain.
Sensitivity tests show it to be deaf. Although 50uV RMS PD does show
near s9 on the meter (s7 to s9 depending on band), reducing the
generator
output from this -73dBm by 30dB drops the S meter to s1. Another 10dB
reduction in generator level (to -113dBm) makes the signal disappear, so
the MDS is more than 20dB poorer than expected.
I've followed the signal tracing procedure in Appendix E of the manual.
It all
looks OK, with signals within a few dB of the "expected" values, until I
get
to the IF Amp Saturated Output test. There should be up to 300mV of
output with large input signals. No matter what input signal level I
use at the antenna, even levels as high as 1V RMS, I cannot get more
than about 10mV RMS on pin 8 of U12, the output of the MC1350 IF amp
chip. I get D40/D41 going into hard limiting long before I reach 1V
antenna input. The 8v DC supply looks OK; the voltage drop across R112
suggests the chip is drawing 7.3mA. Also, and probably as a consequence
of the low IF output, I can't get more than 5V of AGC at pin 1 of U2 on
the control board, and AGC test 8 says I should get 6.9V.
I've lifted one end of R111, the swamping resistor across L34, and it
measures 5K6 as expected. I've replaced the MC1350 chip and it has made
no difference. The 10mV or so of signal at the output of the chip does
seem to get through the second xtal filter OK and is down converted to
audio by U11 with a voltage conversion gain of 2, as expected.
My test gear is an HP 8640B sig gen, a Boonton 92B RF millivoltmeter,
and a Fluke 8842A multimeter. I have no reason to suspect them of lying.
It looks like there is a fault around the output circuit of the IF
amplifier. I
wonder if anyone else has heard of faults here? I can find nothing on
the
reflector archives. I'm only mildly suspicious of L34, the Toko 4.7uH
output
inductor. Do they ever fail? This one shows a resistance of 0.3 ohms,
measured in-circuit. Is this reasonable? I measure 2450R from U12 pins 1
or 8 to earth.
Unsoldering L34 for examination will be a major effort. Before I embark
on
that journey, I wonder if anyone has any insights? Can someone confirm I
really should be seeing over 150mV of RF at the output of U12 with very
big antenna input signals?
--
Thanks,
Alan G3XAQ
[EMAIL PROTECTED]
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