Posted by
Alan Ibbetson on
Oct 08, 2007; 9:45pm
URL: http://elecraft.85.s1.nabble.com/K2-IF-amplifier-output-fault-tp454319p454321.html
Don et al,
I think I may have made forward progress.
With or without D40/D41 I cannot get more than about 10mV at the output
of the MC1350 IF amp when performing the IF Amp Saturated Output test.
This is with the RF gain at minimum and as much signal into the antenna
jack as my sig gen can produce (3V RMS PD).
However, why would Elecraft have me do this test with the RF gain at
minimum? We are trying to produce as much output as possible. So I set
the RF gain to maximum and now there is 280mV of signal at the output of
the IF amp, which is what the manual predicts. Although there is no
figure quoted, I find I need 1mV from the sig gen to produce the 150mV
for the next test (2nd crystal filter loss measurement).
The receiver goes on to pass all the subsequent tests. Having peaked the
input filters and re-run the BFO calibration I measure the MDS in 700Hz
to be -132dBm with the preselector off and -138dBm with it on. This is
within experimental error of the specs in the manual.
So, the receiver isn't deaf. However, it still sounds very quiet to me.
7mV of audio hiss at full volume (yes, within spec) is just not loud at
all.
Sorry to have wasted everyone's time, although I do think the manual
either misprints "minimum RF gain" when it means maximum, or they really
do expect you to pump much more than 3V (+23dBm) into the poor thing!
73,
Alan G3XAQ
In message <
[hidden email]>, Don Wilhelm
<
[hidden email]> writes
>Alan,
>
>Those levels listed in the manual were developed before the Strong
>Signal Handling diodes D40 and D41 were added to the design. Lift one
>end of both diodes and re-test the Saturated IF Amplifier test - the IF
>amp may not saturate fully with the diodes in place. While that may
>not be the full answer to your problem, it may provide some additional
>insight.
>
>Do you have your BFOs adjusted properly for the signal pitch you are
>attempting to receive?
>Do you have the KDSP2 installed? If so, temporarily remove it and
>replace with jumpers in the headers to substitute for R18 and R19.
>
>Another side note - your HP6840 can inject a 4913 kHz signal directly
>into the IF amp if you remove the SSB board (or in the W3 jumper
>position if the KSB2 is not installed). That may or not be helpful,
>but it can give another data point.
>
>73,
>Don W3FPR
>
>Alan Ibbetson wrote:
>> 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?
>
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