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K2 IF amplifier output fault

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Alan Ibbetson

K2 IF amplifier output fault

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
[hidden email]

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Roelof Bakker

Re: K2 IF amplifier output fault

Hello Alan,

I had also a problem with low output.
It appeared that I had installed a 1800 pF capacitor instead of 180 pF at
C182.

Have you checked this capacitor?

Best regards,
Roelof Bakker, pa0rdt
Middelburg, Netherlands
JO11tm

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Don Wilhelm-4

Re: K2 IF amplifier output fault

In reply to this post by Alan Ibbetson

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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Alan Ibbetson

Re: K2 IF amplifier output fault

In reply to this post by Roelof Bakker

In message <001601c80917$5639de70$2101a8c0@PCRoelof>, Roelof Bakker
<[hidden email]> writes
>Hello Alan,
>
>I had also a problem with low output.
>It appeared that I had installed a 1800 pF capacitor instead of 180 pF
>at C182.
>
>Have you checked this capacitor?

Roelof,

Thanks for your suggestion. C182 is marked "181" as expected. I removed
it and measured its value. It is 176pF.

So that's not it. Next test is to remove D40 and D41 as suggested by
Don.

--

Cheers,

Alan G3XAQ
[hidden email]

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Alan Ibbetson

Re: K2 IF amplifier output fault

In reply to this post by Don Wilhelm-4

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?
>

Don Wilhelm-4

Re: K2 IF amplifier output fault

Alan,

Great to hear that you have found success.

Yes, I do believe that manual has an omission. It would be difficult to
saturate the IF amp with the RF Gain set to minimum.

I believe you are reading from Appendix E page 12 - while on page 11, it
says to set the RF Gain to minimum (necessary for item 7 on page 12),
but item 8 should include setting the RF gain to 'maximum' if I
understand what is being tested correctly.

The K2 is quiet with no signal - it was designed to be that way. The
other factor is that most receivers have much more reserve audio output
power than the K2, so one cannot 'crank up the audio' and hear a LOT of
noise, especially with no antenna connected. It makes no sense to
listen to the noise floor of the receiver at a loud level anyway. If
you connect an antenna you should hear the band noise above the receiver
noise level (on the HF bands with any decent antenna).

One benefit to all your trouble - at least you know the K2 will stand a
+23 dBm signal at its input with no ill effects!

73,
Don W3FPR

Alan Ibbetson wrote:

> 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!
>

Brett gazdzinski-2

RE: K2 IF amplifier output fault

In reply to this post by Alan Ibbetson

It is a quiet receiver.
The only thing quieter I have run into, that still
hears weak signals, is my homebrew single conversion
receiver, and that is likely not as sensitive as the K2.

On both receivers, I have made the mistake in thinking
the antenna fell down, and gone outside to look.

Now I mostly hang out on 80 and 40 meters, and when its quiet,
it has fooled me more than once into checking the antenna.

Its sort of like the old days before all that noisy high gain IC chips
came out, when you had simple tube type receivers without preamps, when
it was quiet, it was quiet.

Nowadays, when its quiet, its not as noisy...

Brett
N2DTS

> 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
>
>