[K2] KPA100 above 20m - not enough drive on USB; LSB fine

[dl0tuh]

An owner of a K2/100 (S/N < 2000) showed me the follwing problem: it looks as if the mic drive were not enough for driving 100 watts USB on 17m band. A whistle test could only reach abt 20 watts.

However, on LSB the same whistle test produces full power output. Playing with CAL FIL, I moved BFO frequency to the wrong side of bandbass, effectively making LSB instead of USB. It is definitely useless on the air, but full power output was reached in this test.

How do you think, where can this kind of difference between "right" (USB) and "wrong" (LSB) sideband reside? Up to now, I can only think about L34 on RF board, but isn't the Q there too low to make such a difference between sidebands? Builder Alert #22a is the closest thing I found on the web, but, again, that would influence both sidebands.

[Don Wilhelm-4]

L34 is not in the transmit path, so eliminate that from suspicion.
Put the BFOs for FL1 (must be OP1) back on the correct sidebands, then
try the following.  If you have questions about the proper passband
placement, review Part 3 of the K2 Dila Calibration article on my
website www.w3fpr.com.

I would re-check the carrier balance - do that on 40 meters or any band
below 15 MHz.  Connect paddles and close the dot paddle instead of
trying to connect a mike and using its PTT button - that way you will
not have any audio input while doing the carrier balance.
If you can observe the (low) RF output with an oscilloscope, you do not
have to be in the SSBC bal menu - adjust the carrier balance pot for
minimum RF, first in LSB and then switch to USB, compromise if necessary.

Any difference between LSB and USB must be on the KSB2 board.  Either in
the filter shape itself or in the BFO settings.  Check in receive for a
flat passband with very little ripple.  If there is a lot of ripple in
the passband, you may have a problem in the filter itself.

73,
Don W3FPR

dl0tuh wrote:

> An owner of a K2/100 (S/N < 2000) showed me the follwing problem: it looks as
> if the mic drive were not enough for driving 100 watts USB on 17m band. A
> whistle test could only reach abt 20 watts.
>
> However, on LSB the same whistle test produces full power output. Playing
> with CAL FIL, I moved BFO frequency to the wrong side of bandbass,
> effectively making LSB instead of USB. It is definitely useless on the air,
> but full power output was reached in this test.
>
> How do you think, where can this kind of difference between "right" (USB)
> and "wrong" (LSB) sideband reside? Up to now, I can only think about L34 on
> RF board, but isn't the Q there too low to make such a difference between
> sidebands? Builder Alert #22a is the closest thing I found on the web, but,
> again, that would influence both sidebands.
>  
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>
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>  

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