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Low BFO Output

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Tedd Wong

Low BFO Output

First some history. During the BFO test, I discovered my BFO range was low. My upper frequency was 4917.55 and lower frequency was 4914.26. After much consultation with Gary, I tried the various tricks such as padding C174 and C173, adding caps to the junction of X3 and X4, and even adding turns to L33. After much prodding I finally got the range from 4917.55 to 4912.60. So far so good. I also got my filters aligned with Spectrogram which proved to be very easy. Then I noticed my receiver sensitivity appeared to be very low. (Even worse than my ICOM 706. Now that's bad!) Out came the XG2 to perform some MDS tests. Following the directions from the XG2 manual for setting up the K2, inputting 1 uV, and with the AF gain at maximum, I measure .009 volts at the speaker terminals. With no input I can't even get a reading. (Kinda hard to divide by zero.) This led me to the signal tracing portion on the manual. I got as far as the BFO output measurement and there I r!
an into the problem of a very low BFO output to U6 pin 1. The manual states .20 to .70 Vrms and I get .05 Vrms with my RF probe. Could my super stiff crystals be the cause of such low output? Perhaps I tried bending the crystals too much? Would a low BFO output cause such low receiver sensitivity? Are there some other components to suspect other than the crystals? Incidently Elecraft is sending me another set of BFO crystals, gratis, to hopefully cure my BFO range problem without padding. Thanks for reading. Merry Christmas and Happy New Year to all Elecrafters.

Tedd K6OI
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Don Wilhelm-3

RE: Low BFO Output

Tedd,

Yes, a low BFO voltage would cause poor receive sensitivity. I doubt that
pulling the crystals caused the problem, but the new crystals may fix it.
While you are removing things, you may want to arbitrarily replace the NE612
at U6 just in case.

I recently repaired a K2 with a similar problem with one of the K2s I built
last May - I can't say just what the problem was (most likely the crystals),
but this one had failed before and replacing the NE612 appeared to correct
it the first time, but after having a similar failure a second time, I
replaced the crystals, varactors, inductor and the NE612 - I do hope and
trust that it will stay fixed now (no, this was not a solder connection
problem).

73,
Don W3FPR

> -----Original Message-----
>
> First some history. During the BFO test, I discovered my BFO
> range was low. My upper frequency was 4917.55 and lower
> frequency was 4914.26. After much consultation with Gary, I
> tried the various tricks such as padding C174 and C173, adding
> caps to the junction of X3 and X4, and even adding turns to L33.
> After much prodding I finally got the range from 4917.55 to
> 4912.60. So far so good. I also got my filters aligned with
> Spectrogram which proved to be very easy. Then I noticed my
> receiver sensitivity appeared to be very low. (Even worse than
> my ICOM 706. Now that's bad!) Out came the XG2 to perform some
> MDS tests. Following the directions from the XG2 manual for
> setting up the K2, inputting 1 uV, and with the AF gain at
> maximum, I measure .009 volts at the speaker terminals. With no
> input I can't even get a reading. (Kinda hard to divide by
> zero.) This led me to the signal tracing portion on the manual.
> I got as far as the BFO output measurement and there I r!
> an into the problem of a very low BFO output to U6 pin 1. The
> manual states .20 to .70 Vrms and I get .05 Vrms with my RF
> probe. Could my super stiff crystals be the cause of such low
> output? Perhaps I tried bending the crystals too much? Would a
> low BFO output cause such low receiver sensitivity? Are there
> some other components to suspect other than the crystals?
> Incidently Elecraft is sending me another set of BFO crystals,
> gratis, to hopefully cure my BFO range problem without padding.
> Thanks for reading. Merry Christmas and Happy New Year to all
> Elecrafters.
>
> Tedd K6OI
>

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Tedd Wong

RE: Low BFO Output

In reply to this post by Tedd Wong

All problems associated with the K2 BFO have been solved! The new crystals from Elecraft solved the low BFO range and low BFO output problem. I'm not sure if this is significant but I noticed the markings on the new crystals are slightly different than the ones I replaced. I now get a BFO range of 4911.55 to 4917.27 kHz without having to bend the crystals and the output to the pin 6 of mixer U11 is around .500 Vrms.

Also previously I was not able to get enough audio at the internal speaker connections or the headphone jack to measure the MDS. I mistakenly blamed it on low receiver sensitivity. It turned out to be bad solder joints at the headphone jack. Apparently the headphone jack pins don't protrude very far through the PC board and this resulted in flaky headphone jack solder joints. Since the audio is routed through these joints to P5, the audio at the internal speaker was low as well. Using the XG2, I now measure an MDS of -135.6 dbm.

A big thanks to all on the reflector who helped, especially Don, W3FPR and Gary @ Elecraft.

Tedd K6OI

>
> From: "Don Wilhelm" <[hidden email]>
> Date: 2005/12/24 Sat PM 01:43:39 PST> To: "Tedd Wong" <[hidden email]>, <[hidden email]>
> Subject: RE: [Elecraft] Low BFO Output
>
> Tedd,
>
> Yes, a low BFO voltage would cause poor receive sensitivity. I doubt that
> pulling the crystals caused the problem, but the new crystals may fix it.
> While you are removing things, you may want to arbitrarily replace the NE612
> at U6 just in case.
>
> I recently repaired a K2 with a similar problem with one of the K2s I built
> last May - I can't say just what the problem was (most likely the crystals),
> but this one had failed before and replacing the NE612 appeared to correct
> it the first time, but after having a similar failure a second time, I
> replaced the crystals, varactors, inductor and the NE612 - I do hope and
> trust that it will stay fixed now (no, this was not a solder connection
> problem).
>
> 73,
> Don W3FPR>
> > -----Original Message-----
> >
> > First some history. During the BFO test, I discovered my BFO> > range was low. My upper frequency was 4917.55 and lower
> > frequency was 4914.26. After much consultation with Gary, I
> > tried the various tricks such as padding C174 and C173, adding
> > caps to the junction of X3 and X4, and even adding turns to L33.
> > After much prodding I finally got the range from 4917.55 to
> > 4912.60. So far so good. I also got my filters aligned with
> > Spectrogram which proved to be very easy. Then I noticed my
> > receiver sensitivity appeared to be very low. (Even worse than
> > my ICOM 706. Now that's bad!) Out came the XG2 to perform some
> > MDS tests. Following the directions from the XG2 manual for
> > setting up the K2, inputting 1 uV, and with the AF gain at
> > maximum, I measure .009 volts at the speaker terminals. With no
> > input I can't even get a reading. (Kinda hard to divide by
> > zero.) This led me to the signal tracing portion on the manual.
> > I got as far as the BFO output measurement and there I r!
> > an into the problem of a very low BFO output to U6 pin 1. The
> > manual states .20 to .70 Vrms and I get .05 Vrms with my RF
> > probe. Could my super stiff crystals be the cause of such low
> > output? Perhaps I tried bending the crystals too much? Would a
> > low BFO output cause such low receiver sensitivity? Are there
> > some other components to suspect other than the crystals?
> > Incidently Elecraft is sending me another set of BFO crystals,
> > gratis, to hopefully cure my BFO range problem without padding.
> > Thanks for reading. Merry Christmas and Happy New Year to all
> > Elecrafters.
> >
> > Tedd K6OI> >
>