Elecraft

K2 #7340 PreAmp Problem

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Ray G3XLG

K2 #7340 PreAmp Problem

Hi All

I have been building this K2 taking my time and I thought I was soldering
carefully.

All has gone well so far and I have got as far as the IF amplifier alignment
having completed VFO linearization etc with no problems and all readings
within spec. I followed the instructions for the IF alignment including
putting the PreAmp on but I could not hear the internally generated signal
at 7000kHz and there was only a very low audio output.

With the PreAmp off (or even with the ATT on!) the audio is much louder and
I can then hear that signal and so I adjusted the slug in L34 for best
signal strength with low noise. I then carried out the 40 meter band pass
filter alignment with no problem so I am about to start the next phase -
Assembly, Part111.

Clearly there is something wrong with my PreAmp on 40 meters however. Does
anyone have any ideas of what I should look at or check?

Happy Christmas everyone!

73 Ray, G3XLG

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Ray G3XLG

Don Wilhelm-4

Re: K2 #7340 PreAmp Problem

Ray,

Check every component in the Pre-Amp circuit, including the relay.

Do you have the leads of T6 well stripped and tinned? If so, check to
see that the leads are in the correct holes.
After that look critically at the solder pads for those components
around the preamp circuit - look at the schematic sheet 2 to identify
those components. You should see the solder flow out to an almost
invisible edge on both the solder pad and the component lead. It has
been said that the esolder connection should look like a mountain, but
not a volcano - if there is a ring around the lead or you do not have
concave solder fillets, not enough heat was applied to the connection.

73,
Don W3FPR

On 12/24/2012 3:51 PM, Ray Spreadbury wrote:

> Hi All
>
>
>
> I have been building this K2 taking my time and I thought I was soldering
> carefully.
>
> All has gone well so far and I have got as far as the IF amplifier alignment
> having completed VFO linearization etc with no problems and all readings
> within spec. I followed the instructions for the IF alignment including
> putting the PreAmp on but I could not hear the internally generated signal
> at 7000kHz and there was only a very low audio output.
>
> With the PreAmp off (or even with the ATT on!) the audio is much louder and
> I can then hear that signal and so I adjusted the slug in L34 for best
> signal strength with low noise. I then carried out the 40 meter band pass
> filter alignment with no problem so I am about to start the next phase -
> Assembly, Part111.
>
>
>
> Clearly there is something wrong with my PreAmp on 40 meters however. Does
> anyone have any ideas of what I should look at or check?
>
>
>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

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Ray G3XLG

Re: K2 #7340 PreAmp Problem

Hi Don

Thank you for your reply.
I visually checked the components in the preamp circuit & reflowed the
solder of them all including the relay, T6 & Q21, but still got the same
results as before. The coil T6 wires have been put in the correct holes but
I can't check the wires were tinned correctly without removing it but I
think they were.

However, I did a dc voltage check on the transistor Q21 & found some
anomalies- the E,B & C should be 1.6vdc, 2.3vdc & 13.3vdc but with the
PreAmp OFF they are E 0.00vdc, B 0.00vdc & C 13.36vdc. With the PreAmp
switched on, they are E 0.00vdc, B 0.33vdc & C 13.17vdc. (For completeness I
checked them with the Attenuator switched on & they were the same as with
the PreAmp switched off.)

Does it look to you as though the transistor Q21, a 2N5109 is faulty?

Regards
Ray

-----Original Message-----
From: Don Wilhelm [mailto:[hidden email]]
Sent: 24 December 2012 21:38
To: Ray Spreadbury
Cc: [hidden email]
Subject: Re: [Elecraft] K2 #7340 PreAmp Problem

Ray,

Check every component in the Pre-Amp circuit, including the relay.

Do you have the leads of T6 well stripped and tinned? If so, check to see
that the leads are in the correct holes.
After that look critically at the solder pads for those components around
the preamp circuit - look at the schematic sheet 2 to identify those
components. You should see the solder flow out to an almost invisible edge
on both the solder pad and the component lead. It has been said that the
esolder connection should look like a mountain, but not a volcano - if there
is a ring around the lead or you do not have concave solder fillets, not
enough heat was applied to the connection.

73,
Don W3FPR

>

______________________________________________________________
Elecraft mailing list
Home: http://mailman.qth.net/mailman/listinfo/elecraft
Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
Please help support this email list: http://www.qsl.net/donate.html

Ray G3XLG

Don Wilhelm-4

Re: K2 #7340 PreAmp Problem

Ray,

I would say that Q21 does not have adequate base voltage for it to
conduct. The base voltage should be about 2.3 volts.
Check R72 and R73 and C143 for good soldering and correct values. Check
R75 on the bottom as well.
The base voltage is set by the voltage divider consisting of R72 and R73
and R75.

73,
Don W3FPR

On 12/25/2012 11:28 AM, Ray Spreadbury wrote:

> Hi Don
>
> Thank you for your reply.
> I visually checked the components in the preamp circuit & reflowed the
> solder of them all including the relay, T6 & Q21, but still got the same
> results as before. The coil T6 wires have been put in the correct holes but
> I can't check the wires were tinned correctly without removing it but I
> think they were.
>
> However, I did a dc voltage check on the transistor Q21 & found some
> anomalies- the E,B & C should be 1.6vdc, 2.3vdc & 13.3vdc but with the
> PreAmp OFF they are E 0.00vdc, B 0.00vdc & C 13.36vdc. With the PreAmp
> switched on, they are E 0.00vdc, B 0.33vdc & C 13.17vdc. (For completeness I
> checked them with the Attenuator switched on & they were the same as with
> the PreAmp switched off.)
>
> Does it look to you as though the transistor Q21, a 2N5109 is faulty?
>
> Regards
> Ray
>
> -----Original Message-----
> From: Don Wilhelm [mailto:[hidden email]]
> Sent: 24 December 2012 21:38
> To: Ray Spreadbury
> Cc: [hidden email]
> Subject: Re: [Elecraft] K2 #7340 PreAmp Problem
>
> Ray,
>
> Check every component in the Pre-Amp circuit, including the relay.
>
> Do you have the leads of T6 well stripped and tinned? If so, check to see
> that the leads are in the correct holes.
> After that look critically at the solder pads for those components around
> the preamp circuit - look at the schematic sheet 2 to identify those
> components. You should see the solder flow out to an almost invisible edge
> on both the solder pad and the component lead. It has been said that the
> esolder connection should look like a mountain, but not a volcano - if there
> is a ring around the lead or you do not have concave solder fillets, not
> enough heat was applied to the connection.
>
> 73,
> Don W3FPR
>
> ______________________________________________________________
> Elecraft mailing list
> Home: http://mailman.qth.net/mailman/listinfo/elecraft
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:[hidden email]
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

Ray G3XLG

Re: K2 #7340 PreAmp Problem

In reply to this post by Ray G3XLG

Hi Don

I did a check on the voltage divider resisters as you suggested & found that
R75 read 82 ohms rather than the expected 680 ohms! I had accidentally put
in an 82 ohm resister (R94! From the buffer circuit) and put the R75 680 ohm
resister in the R94 position - they are consecutively fitted on page 59 on
the bottom of the board and whilst I check the resistance before fitting I
must have mixed them up!

I switched them round and the PreAmp now works as it should. The dc volts
are now E 1.51vdc, B 2.22vdc & C 12.89vdc.

Thank you so much for your help Don, and I feel I've really learnt something
as well!

73 Ray G3XLG

-----Original Message-----
From: Don Wilhelm [mailto:[hidden email]]
Sent: 25 December 2012 16:48
To: Ray Spreadbury
Subject: Re: [Elecraft] K2 #7340 PreAmp Problem

Ray,

I would say that Q21 does not have adequate base voltage for it to conduct.
The base voltage should be about 2.3 volts.
Check R72 and R73 and C143 for good soldering and correct values. Check
R75 on the bottom as well.
The base voltage is set by the voltage divider consisting of R72 and R73 and
R75.

73,
Don W3FPR

On 12/25/2012 11:28 AM, Ray Spreadbury wrote:

> Hi Don
>
> Thank you for your reply.
> I visually checked the components in the preamp circuit & reflowed the
> solder of them all including the relay, T6 & Q21, but still got the
> same results as before. The coil T6 wires have been put in the correct
> holes but I can't check the wires were tinned correctly without
> removing it but I think they were.
>
> However, I did a dc voltage check on the transistor Q21 & found some
> anomalies- the E,B & C should be 1.6vdc, 2.3vdc & 13.3vdc but with the
> PreAmp OFF they are E 0.00vdc, B 0.00vdc & C 13.36vdc. With the PreAmp
> switched on, they are E 0.00vdc, B 0.33vdc & C 13.17vdc. (For
> completeness I checked them with the Attenuator switched on & they
> were the same as with the PreAmp switched off.)
>
> Does it look to you as though the transistor Q21, a 2N5109 is faulty?
>
> Regards
> Ray
>
> -----Original Message-----
> From: Don Wilhelm [mailto:[hidden email]]
> Sent: 24 December 2012 21:38
> To: Ray Spreadbury
> Cc: [hidden email]
> Subject: Re: [Elecraft] K2 #7340 PreAmp Problem
>
> Ray,
>
> Check every component in the Pre-Amp circuit, including the relay.
>
> Do you have the leads of T6 well stripped and tinned? If so, check to
> see that the leads are in the correct holes.
> After that look critically at the solder pads for those components
> around the preamp circuit - look at the schematic sheet 2 to identify
> those components. You should see the solder flow out to an almost
> invisible edge on both the solder pad and the component lead. It has
> been said that the esolder connection should look like a mountain, but
> not a volcano - if there is a ring around the lead or you do not have
> concave solder fillets, not enough heat was applied to the connection.
>
> 73,
> Don W3FPR
>
>

Ray G3XLG

Don Wilhelm-4

Re: K2 #7340 PreAmp Problem

Ray,

It is great to hear that you now have it working properly. Good find.

73,
Don W3FPR

On 12/26/2012 6:30 AM, Ray Spreadbury wrote:

> Hi Don
>
> I did a check on the voltage divider resisters as you suggested & found that
> R75 read 82 ohms rather than the expected 680 ohms! I had accidentally put
> in an 82 ohm resister (R94! From the buffer circuit) and put the R75 680 ohm
> resister in the R94 position - they are consecutively fitted on page 59 on
> the bottom of the board and whilst I check the resistance before fitting I
> must have mixed them up!
>
> I switched them round and the PreAmp now works as it should. The dc volts
> are now E 1.51vdc, B 2.22vdc & C 12.89vdc.
>
> Thank you so much for your help Don, and I feel I've really learnt something
> as well!
>
> 73 Ray G3XLG
>
>