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K2 signal tracing progress and questions arising

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Paul Fletcher

K2 signal tracing progress and questions arising

Hi all,

Progressing with fault finding the K2 and there are some immediate apparent issues. Firstly there is a lot of loss through the low pass filters on receive (on 30m at least) - 3dB when the actual loss should be negligible. That's obviously one place I need to look. However (weird one this) the output measured from the bandpass filters is higher than the input. On my scope I'm measuring 78mV pk to pk on the input and 135mV pk to pk on the output of the filters at the junction of D6 and D7 which is clearly not what is expected (unless this particular K2 has somehow broken the laws of physics). I'm wondering if it's possible that something in the transmit chain is active? I have checked the 8T and 8R voltages and they are OK. Downstream of D6 losses seem comparable with the Signal tracing checklist until I get to the composite mixer where everything takes a dive down (another area to check).

Any thoughts on the bandpass filter issue?

Cheers,
Paul.

Don Wilhelm-4

Re: K2 signal tracing progress and questions arising

Paul,

My only thought on the bandpass filter "weirdness" is that either the
bandpass filter is not properly terminated at the D6/D7 junction, or
that your 'scope probe is picking up something extraneous - change the
routing of the ground on the 'scope probe and see if there is any change.

Your signal generator level may be too low to exclude extraneous noise
pickup from consideration.
Do you have a QRP transmitter that you can feed into a dummy load and
use a pickup wire or coil positioned to provide input to the K2? You
might try using that as your signal generator (use your 'scope to check
the level into a 50 ohm resistor before connecting to the K2) - the
level of the test generator shown in the K2 manual is to be adjusted to
0.14 volts RMS (about 400 mV peak to peak). The K2 will not be damaged
with that level.

If the signal drops drastically at the mixer, it could be related to the
above, or you may not have adequate oscillator injection from the VFO.
The mixer needs a +7 dBm level at the LO port (500 mV RMS or 1.4 volts
peak to peak) - all the mixer ports are 50 ohms.

73,
Don W3FPR

Paul Fletcher wrote:

> Hi all,
>
> Progressing with fault finding the K2 and there are some immediate apparent
> issues. Firstly there is a lot of loss through the low pass filters on
> receive (on 30m at least) - 3dB when the actual loss should be negligible.
> That's obviously one place I need to look. However (weird one this) the
> output measured from the bandpass filters is higher than the input. On my
> scope I'm measuring 78mV pk to pk on the input and 135mV pk to pk on the
> output of the filters at the junction of D6 and D7 which is clearly not what
> is expected (unless this particular K2 has somehow broken the laws of
> physics). I'm wondering if it's possible that something in the transmit
> chain is active? I have checked the 8T and 8R voltages and they are OK.
> Downstream of D6 losses seem comparable with the Signal tracing checklist
> until I get to the composite mixer where everything takes a dive down
> (another area to check).
>
> Any thoughts on the bandpass filter issue?
>
> Cheers,
> Paul.
>
> ------------------------------------------------------------------------
>
>
> No virus found in this incoming message.
> Checked by AVG - www.avg.com
> Version: 9.0.704 / Virus Database: 270.14.59/2494 - Release Date: 11/10/09 02:38:00
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>

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Paul Fletcher

Re: K2 signal tracing progress and questions arising

Don Wilhelm-4 wrote

Paul,

My only thought on the bandpass filter "weirdness" is that either the
bandpass filter is not properly terminated at the D6/D7 junction, or
that your 'scope probe is picking up something extraneous - change the
routing of the ground on the 'scope probe and see if there is any change.

OK fixed the voltage discrepancy by limiting the bandwidth on the scope (measurement issue). One problem appears to be in the chain up to the receive mixer. I'm losing about half of the signal but I'm not sure if it's the low pass filters or something after the LP filters pulling the signal down. Next steps are to check for ground shorts in the LPF bank. Another check I thought of doing is terminating the LPF (by opening W1) into a 50 ohm resistor and checking that out. If that's OK open W6 and do the same check (to check the T/R switch).

Any other suggestions?

Cheers,
Paul.

Don Wilhelm-4

Re: K2 signal tracing progress and questions arising

Paul,

With an ohmmeter, everything will look shorted in the LPF section. If
you have an antenna analyzer, you can remove the W1 jumper and terminate
the LPF side with a 50 ohm resistor - connect the antenna analyzer to
the antenna jack and scan through the filter passband - you want to do
that for each band. If you see a large SWR, then check the LPF
corresponding to that band.

A drop in signal at the receive output of the LPF could also be caused
by a T/R switch problem.

73,
Don W3FPR

Paul Fletcher wrote:

>
> Don Wilhelm-4 wrote:
>
>> Paul,
>>
>> My only thought on the bandpass filter "weirdness" is that either the
>> bandpass filter is not properly terminated at the D6/D7 junction, or
>> that your 'scope probe is picking up something extraneous - change the
>> routing of the ground on the 'scope probe and see if there is any change.
>>
>>
>
> OK fixed the voltage discrepancy by limiting the bandwidth on the scope
> (measurement issue). One problem appears to be in the chain up to the
> receive mixer. I'm losing about half of the signal but I'm not sure if it's
> the low pass filters or something after the LP filters pulling the signal
> down. Next steps are to check for ground shorts in the LPF bank. Another
> check I thought of doing is terminating the LPF (by opening W1) into a 50
> ohm resistor and checking that out. If that's OK open W6 and do the same
> check (to check the T/R switch).
>
> Any other suggestions?
>
> Cheers,
> Paul.
>

Paul Fletcher

Re: K2 signal tracing progress and questions arising

Don Wilhelm-4 wrote

A drop in signal at the receive output of the LPF could also be caused
by a T/R switch problem.

Update so far. Having terminated the LPF's in a 50 ohm resistor I can definitely confirm all of the LPF's are fine. I measured swr and monitored in / out on the scope and loss is negligible. As soon as I reinstate W1 the output of the LPF is pulled low (50% of level when terminated with 50 ohms). I guess the next step is to do the same check with the chain broken after the T/R switch and progressively work down the chain. The is no ground short on the input to the band pass filters at DC. I can't directly terminate the T/R switch at W6 because of the DC bias so what value blocking cap should I use?

Any other thoughts appreciated.

73 Paul M1PAF

Don Wilhelm-4

Re: K2 signal tracing progress and questions arising

Paul,

Rather than trying to do your proposed test, I would suggest you look
carefully at the T/R switch area first. Typical problems are diodes
oriented wrong (check against the Parts Placement diagram in the back of
the manual). A broken lead (or unsoldered, poorly tinned, etc.) of RFC3
could cause similar problems - be certain RFC3 is wound on a ferrite
core (dark gray) and not one of the black cores.

73,
Don W3FPR

Paul Fletcher wrote:

>
> Don Wilhelm-4 wrote:
>
>> A drop in signal at the receive output of the LPF could also be caused
>> by a T/R switch problem.
>>
>>
>
> Update so far. Having terminated the LPF's in a 50 ohm resistor I can
> definitely confirm all of the LPF's are fine. I measured swr and monitored
> in / out on the scope and loss is negligible. As soon as I reinstate W1 the
> output of the LPF is pulled low (50% of level when terminated with 50 ohms).
> I guess the next step is to do the same check with the chain broken after
> the T/R switch and progressively work down the chain. The is no ground short
> on the input to the band pass filters at DC. I can't directly terminate the
> T/R switch at W6 because of the DC bias so what value blocking cap should I
> use?
>
> Any other thoughts appreciated.
>
> 73 Paul M1PAF
>
> ------------------------------------------------------------------------
>
>
> No virus found in this incoming message.
> Checked by AVG - www.avg.com
> Version: 9.0.709 / Virus Database: 270.14.76/2517 - Release Date: 11/21/09 02:47:00
>
>

Paul Fletcher

Re: K2 signal tracing progress and questions arising

Don Wilhelm-4 wrote

Paul,

Rather than trying to do your proposed test, I would suggest you look
carefully at the T/R switch area first. Typical problems are diodes
oriented wrong (check against the Parts Placement diagram in the back of
the manual). A broken lead (or unsoldered, poorly tinned, etc.) of RFC3
could cause similar problems - be certain RFC3 is wound on a ferrite
core (dark gray) and not one of the black cores.

73,
Don W3FPR

OK - have done the checks and re-soldered a few connections and I'm now happy that the attenuation in the pre-mixer stages is OK. I've progressed through the receive chain and the only obvious thing is the audio amp that is causing problems. The signal out of the amp (with or without the speaker connected) is not significantly higher than the signal in (in fact it's actually attenuated if I compare the signal across PD1 and PD2 and the output). I've replaced the LM380 and both mute FET's to no avail. There are no shorts on the output and the sidetone is healthy (same output as input measured on the bypass pin) and I'm completely flummoxed. I've checked out the AF gain control and it varies from 5k (max volume) to a few ohms (min volume) and I've confirmed this measurement right up to the chip.

According to the signal tracing fault finding I should be able to get 1.6V RMS out of the audio amp but no chance - nowhere near.

Help!

Cheers,
Paul.

Don Wilhelm-4

Re: K2 signal tracing progress and questions arising

Paul,

Do you have a sufficient signal level to give you 1.6 volts out of the
AF amplifier?
The impedance of the amplifier's input is considerably higher than the
impedance connected to the output, so the AC voltages may actually be
similar in and out. Can you peg the S-meter with your signal generator?

Most problems with "low audio" are actually caused earlier in the
receive chain. Check to see if you have at least 200 mV peak to peak
(70 mV RMS) of BFO at RF Board U11 pin 6. The diodes (D40 and D41)
across the input of the IF amplifier will clip at a level lower than the
"Saturated" test, so that item in the Signal Tracing section is no
longer possible to do.

What is the input vs. output from the IF amplifier U12. If there is not
a significant increase, you may have a bad IF amplifier or T7 may be
mounted backwards (red winding goes closest to U12), or the AGC voltage
to the IF amplifier may be too high (check U12 pin 5, it should be no
higher than 3.9 volts).

73,
Don W3FPR

Paul Fletcher wrote:

>
>
> OK - have done the checks and re-soldered a few connections and I'm now
> happy that the attenuation in the pre-mixer stages is OK. I've progressed
> through the receive chain and the only obvious thing is the audio amp that
> is causing problems. The signal out of the amp (with or without the speaker
> connected) is not significantly higher than the signal in (in fact it's
> actually attenuated if I compare the signal across PD1 and PD2 and the
> output). I've replaced the LM380 and both mute FET's to no avail. There are
> no shorts on the output and the sidetone is healthy (same output as input
> measured on the bypass pin) and I'm completely flummoxed. I've checked out
> the AF gain control and it varies from 5k (max volume) to a few ohms (min
> volume) and I've confirmed this measurement right up to the chip.
>
> According to the signal tracing fault finding I should be able to get 1.6V
> RMS out of the audio amp but no chance - nowhere near.
>
> Help!
>
> Cheers,
> Paul.
>
>

Paul Fletcher

Re: K2 signal tracing progress and questions arising

Hi Don (again!)

I was following the steps in the signal tracing checklist that say with 25mV
in to the AF amp I should get 1.6V out with the speaker disconnected and I
don't. I've checked the signal path from the audio amp to the speaker
connector and there are no shorts or open circuits.

The BFO is around 1V pk pk so that looks healthy as do the other
oscillators. The post mixer amp is working OK as are the pre-amp and
attenuator. Loss through the CW filter checks out also. That leaves the IF
amp, the detector and the AF amp. I can peg the S meter with the generator.
AGC voltage with no signal is set at 3.8V (this is a rev A K2 remember).
Something is definitely wrong somewhere though as a 50uV signal is quiet
even with AGC off. One thing I have noticed with AGC off is that if I set
the RF gain so that the IF chain is just out of clipping there is still low
audio (if that helps)

Lots of head scratching going on here.

Cheers,
Paul M1PAF
----- Original Message -----
From: "Don Wilhelm" <[hidden email]>
To: "Paul Fletcher" <[hidden email]>
Cc: <[hidden email]>
Sent: Thursday, November 26, 2009 12:05 AM
Subject: Re: [Elecraft] [K2] K2 signal tracing progress and questions
arising

> Paul,
>
> Do you have a sufficient signal level to give you 1.6 volts out of the AF
> amplifier?
> The impedance of the amplifier's input is considerably higher than the
> impedance connected to the output, so the AC voltages may actually be
> similar in and out. Can you peg the S-meter with your signal generator?
>
> Most problems with "low audio" are actually caused earlier in the receive
> chain. Check to see if you have at least 200 mV peak to peak (70 mV RMS)
> of BFO at RF Board U11 pin 6. The diodes (D40 and D41) across the input
> of the IF amplifier will clip at a level lower than the "Saturated" test,
> so that item in the Signal Tracing section is no longer possible to do.
>
> What is the input vs. output from the IF amplifier U12. If there is not a
> significant increase, you may have a bad IF amplifier or T7 may be mounted
> backwards (red winding goes closest to U12), or the AGC voltage to the IF
> amplifier may be too high (check U12 pin 5, it should be no higher than
> 3.9 volts).
>
> 73,
> Don W3FPR
>
> Paul Fletcher wrote:
>>
>> OK - have done the checks and re-soldered a few connections and I'm now
>> happy that the attenuation in the pre-mixer stages is OK. I've progressed
>> through the receive chain and the only obvious thing is the audio amp
>> that
>> is causing problems. The signal out of the amp (with or without the
>> speaker
>> connected) is not significantly higher than the signal in (in fact it's
>> actually attenuated if I compare the signal across PD1 and PD2 and the
>> output). I've replaced the LM380 and both mute FET's to no avail. There
>> are
>> no shorts on the output and the sidetone is healthy (same output as input
>> measured on the bypass pin) and I'm completely flummoxed. I've checked
>> out
>> the AF gain control and it varies from 5k (max volume) to a few ohms (min
>> volume) and I've confirmed this measurement right up to the chip.
>>
>> According to the signal tracing fault finding I should be able to get
>> 1.6V
>> RMS out of the audio amp but no chance - nowhere near.
>>
>> Help!
>>
>> Cheers,
>> Paul.
>>