[Elecraft] Low output from 2nd crystal filter and SA602 product detector

[Don Wilhelm]

  Bob,

I certainly can say that any measurements made at the 2nd crystal stage 
and the product detector would have no effect at all on transmit current 
draw.

How much current is it drawing?  Many K2s will develop 16 or 17 watts of 
power, and if you are making your current draw judgment based on HiCur 
messages when the power is cranked up to near maximum, that may not be a 
problem at all, just a good performing K2 - turn CAL CUR up to 3.50.

Another thing that can cause higher than normal current draw is a 
voltage that is too low.  Check using a 13.8 volt power supply.  Use the 
internal voltage/current display to see how much the voltae drops during 
transmit.  If it drops more than a volt, check the tightness of the 
power supply leads at the supply, make certain the wire is of an 
adequate gauge (I recommend #16) and make certain the wire is well 
soldered to the plug.
The reason low voltage results in higher current (and not the other way 
around as in many other transceivers) is due to the way the K2 controls 
power - it tries to transmit at the power you set, and if the voltage is 
lower, the current must be higher to maintain that power.

Also be aware that the base K2 current draw and power output must be 
judged only when operating into a 50 ohm dummy load.  If you are using 
an antenna, all bets are off unless you can assure me that it is 50 ohms 
purely resistive at the frequency of operation.

To measure the Low Pass filters, you need to disconnect them from the PA 
and terminate the input side of the LPF with a 50 (47 to 51) ohm 
resistor.  If you have installed a header for the K160RX option,, it is 
easy - make sure the K160RX board (or unpcb) is removed and insert the 
ends of the 50 ohm resistor into the header between pins 5 and 8.  If 
you followed the manual explicitly, remove one end of jumper W1.

If you do not have the header installed, lift the right end of jumper W1 
and connect the resistor between the right hole for W1 and ground.  J14 
pin 5 is a ground.

With the input of the LPF now terminated, connect your miniVNA (or any 
other antenna analyzer type device) to the K2 antenna BNC.

Now turn the K2 on and select the 80 meter band (do not attempt 
transmit).  Scan from about 3 MHz to 6 MHz with your analyzer, and you 
should see a good SWR up through at least 4.5 MHz, and the SWR should 
increase substantially before you get to 6 MHz.  The point where the SWR 
begins to increase is the filter cutoff frequency, and that frequency 
must be outside the top end of the ham band but substantially lower than 
the 2nd harmonic of the lower band edge.

Switch to 40 meters and check that range - typical cutoff at 9.5 MHz 
(below 2 times th low end of the 60 meter band).

switch to 20 meters to check the 30/20m LPF.  Cutoff above 15 MHz but 
well below 20 MHz.
Switch to 15 meters and check the 15/17, filter - Cutoff above 22 MHz,
Switch to 10 meters and check the 10/12m filter - cutoff above 32 MHz.

If the cutoff frequency is above the ham bands, and the SWR is low 
within the ham bands related to the filter, then the LPF is OK.  If not 
examine the toroids and check the capacitor values.  Look on the right 
side of RF Board schematic sheet 3 to identify which components are in 
the LPF.

Other causes of high transmit current are problems with the PA bias and 
problems with T4 - measure the DC voltage at the base of Q7 or Q8, it 
should be typically near 0.6 volts, but no higher than 0.63 volts.

T4 should have the 2 turn green wire leads positioned toward Q7 and Q8, 
and the white 3 turn leads should be toward the LPF area.  The "link" 
wires should go straight through the hole in the side of the toroid core 
and solder to the pad on the same side - it does not cross over.  In 
other words there should be one wire from pad #5 to pad #6, and another 
from pad#7 to pad#8.

Good luck.


73,
Don W3FPR

On 2/1/2011 4:16 PM, n7zf wrote:
> I have a miniVNA analyzer.
>
> I followed the transmitter troubleshooting procedure and it warned me not to
> skip the receiver procedure, which is how I ended up at the 2nd crystal
> oscillator.  Since I have a good -130 dBm receive signal, can I assume that
> the low output from the 2nd crystal oscillator and product detector is not
> causing my high transmit current?
>
> Bob