K2 RF Board resistance check failure. U6 Pin 8

[Wiley Kling]

I am building a recently purchased K2, serial # 7313.  In the past few years
I have built 6 or 8 solid state kits, including a K1, without a hitch.
However, I am now having a problem that is really making me feel like a
"newbie".  

 

I am on page 62 of the manual and things have gone swimmingly until now.
All of the resistance checks (RF Board) are fine except for U6 pin 8 which
is supposed to read >100 ohms.  My reading is 38.3 ohms and I cannot locate
the problem. I have searched the Elecraft Reflector and many folks have
suffered the same problem.  Problem is, I can't find a reference as to how
anyone resolved the issue.  I have done the following things:

 

1. Reversed the leads on my DMM.
2. Performed the resistance checks with a second DMM.  Same results.
3. Checked polarities on all devices.
4. Spent 2 full days looking for cold or bridged solder joints.
5. I have not "lifted" and checked the resistance of all resistors as I
am in the habit of testing them before installing.
6. I have checked the schematics and have given special attention to
the soldering and polarity of all "8B" components.  (I locate 5 schematic
areas fed from U6 Pin 8).
7. I have checked the "Thermistor Board" to the extent I can without
removing it.  It looks to be OK visually.
8. I have not applied power and looked for heat or smoke but I have
been tempted.
9. I have checked for a reversed U8 regulator.

 

I would appreciate any advice that anyone might offer.  I would be truly
thankful.  Would it be helpful for me to know what resistance others show
between U6 Pin 8 and each of the 8 pins on the "Thermistor Board"?  I'm
thinking that might allow me to eliminate that board as a source of the
problem but not sure I am thinking correctly.  Thanks.

 

73, Wiley, KG4UPO

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[Geoffrey Mackenzie-Kennedy-3]

Wiley,

The resistance between U6 pin 8 and ground was 295 ohms in my K2 built in
2003, before the RF board was assembled with the front panel and Control
board.

Hope that this helps.

73,
Geoff
LX2AO


On July 14, 2012 at 10:25 PM, Wiley Kling wrote:


>I am building a recently purchased K2, serial # 7313.  In the past few
>years
> I have built 6 or 8 solid state kits, including a K1, without a hitch.
> However, I am now having a problem that is really making me feel like a
> "newbie".

 <snip>

> I am on page 62 of the manual and things have gone swimmingly until now.
> All of the resistance checks (RF Board) are fine except for U6 pin 8 which
> is supposed to read >100 ohms.  My reading is 38.3 ohms and I cannot
> locate
> the problem. I have searched the Elecraft Reflector and many folks have
> suffered the same problem.  Problem is, I can't find a reference as to how
> anyone resolved the issue.  I have done the following things:
> I would appreciate any advice that anyone might offer.  I would be truly
> thankful.  Would it be helpful for me to know what resistance others show
> between U6 Pin 8 and each of the 8 pins on the "Thermistor Board"?  I'm
> thinking that might allow me to eliminate that board as a source of the
> problem but not sure I am thinking correctly.  Thanks.
>
>
>
> 73, Wiley, KG4UPO

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[Don Wilhelm-4]

Wylie,

That is a voltage rail that you are checking, and you should measure
somewhere close to 250 or 300 ohms.  I assume you do not have the
Control Board plugged in yet - if you do remove it and remeasure.

Actually the resistance check at U11 pin 8 is not a very good check of
the 8A voltage rail because there is an 820 ohm resistor between U11 pin
8 and the 8A voltage rail.

Checking a voltage rail to find the fault is usually no fun at all. 8A
and 8B differ only because RFC16 is between them.  First step is to lift
one lead of RFC16 and measure the resistance to ground from both U6 pin
8 and the lowest reading end of R100 (on the bottom of the board).  You
are looking for a resistance greater than 100 ohms in both cases. You
are looking to see whether the 8A voltage rail or the 8B voltage rail is
the one with low resistance - that is the troublesome one.

Once you have determined whether it is 8A or 8B giving trouble, go to
the schematic - a searchable .pdf version is ideal.  You will search
through that schematic locating each and every place that the
troublesome voltage rail (8A or 8B) is shown on the schematic, and see
what is connected to it at each occurrence.  Locate that component on
the RF Board, and check it.  If it is a resistor or inductor, you may
want to lift one end to see if the fault lies beyond that component

My guess is that you have a solder bridge somewhere.  One quick place to
check is at connector J7 for a short between pins 6 and 8 or between
pins 5 and 7.

73,
Don W3FPR

On 7/14/2012 4:25 PM, Wiley Kling wrote:

> I am building a recently purchased K2, serial # 7313.  In the past few years
> I have built 6 or 8 solid state kits, including a K1, without a hitch.
> However, I am now having a problem that is really making me feel like a
> "newbie".
>
>  
>
> I am on page 62 of the manual and things have gone swimmingly until now.
> All of the resistance checks (RF Board) are fine except for U6 pin 8 which
> is supposed to read >100 ohms.  My reading is 38.3 ohms and I cannot locate
> the problem. I have searched the Elecraft Reflector and many folks have
> suffered the same problem.  Problem is, I can't find a reference as to how
> anyone resolved the issue.  I have done the following things:
>
>  
>
> 1. Reversed the leads on my DMM.
> 2. Performed the resistance checks with a second DMM.  Same results.
> 3. Checked polarities on all devices.
> 4. Spent 2 full days looking for cold or bridged solder joints.
> 5. I have not "lifted" and checked the resistance of all resistors as I
> am in the habit of testing them before installing.
> 6. I have checked the schematics and have given special attention to
> the soldering and polarity of all "8B" components.  (I locate 5 schematic
> areas fed from U6 Pin 8).
> 7. I have checked the "Thermistor Board" to the extent I can without
> removing it.  It looks to be OK visually.
> 8. I have not applied power and looked for heat or smoke but I have
> been tempted.
> 9. I have checked for a reversed U8 regulator.
>
>  
>
> I would appreciate any advice that anyone might offer.  I would be truly
> thankful.  Would it be helpful for me to know what resistance others show
> between U6 Pin 8 and each of the 8 pins on the "Thermistor Board"?  I'm
> thinking that might allow me to eliminate that board as a source of the
> problem but not sure I am thinking correctly.  Thanks.
>
>  
>
> 73, Wiley, KG4UPO
>
> ______________________________________________________________
> 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
>

______________________________________________________________
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Help: http://mailman.qth.net/mmfaq.htm
Post: mailto:[hidden email]

This list hosted by: http://www.qsl.net
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