[Elecraft] RE: K2 VCO Testing problem

[Don Wilhelm]

Marinos,

Marinos,

Based on your voltage readings at Q18, the VCO is not actually 
oscillating (indicated by a lack of negative voltage at the gate).  I 
don't know why you have any frequency reading at TP1, but I would tend 
to disregard it for the moment.

The first thing to check is the orientation of D13 and D8.  If those are 
OK, I would try replacing Q18.
All other components connected to Q18 are suspect, so check all of them 
for correct values.

Q17 could also be at fault - check it quickly by placing a short from 
the drain to the source and see if you get a negative voltage on the 
gate of Q18 - if so, Q17 should be replaced.

Also, check the soldering at relays K13, K14 and K15 and the value of 
C72.  Check the varactors D21 through D26 to be certain they are the 
correct type and oriented properly.

73,
Don W3FPR

73,
Don W3FPR

Marinos Markomanolakis, M.D. wrote:
> Dear Friends,
> I am in the process of building my serial# 6614 K2 (the 3rd one that I 
> have built since 2000), and run into a problem during the VCO testing: 
> First, the PLL reference Oscillator test checked within specs, but in 
> the next (VCO) test, while checking at TP1 with the frequency 
> counter,with the K2 at 4.000.10Khz, I am measuring a 7.59 to 7.60Mhz 
> with the last digits of the reading rapidly alternating.
> The DC voltage at the left end of R30 is about 5.3V with some 
> spontaneous fluctuation, and not responding to adjustment of the L30 
> as per the VCO alignment test in page 64 of the manual.
> Reheating the leads of T5, checking C72 and general check for 
> soldering issues did not reveal anything.
>  
> PLL reference oscillator output with the RF probe: Pin 1 of U4 1.08V 
> (ok), but VCO output signal (pin 3 of U3) 1.13V when only 0.3-0.4V are 
> acceptable and VCO buffer output (pin 6 of U6) 0.85V with the 
> acceptable range being 0.6-0.75V
> DC measurements :
> Q18
> G     0.0V (-1.0)
> S     4.3V (2-3)
> D     7.1V (6.3)
> U3 Pin#
> 1     1,1V (0.0)
> 2      3.9V (4.3)
> 3     3.8V (4.1)
>  
> U4 Pin#
> 3      3.5V (0.0)
> 7      5.0V (0.0)
> 8      0.06V (0.1)
>  
> The values in the parentheses are the acceptable per the manual, and 
> the rest of the IC pins are within specs.
>  
> Based on the above, I suspect that for some reason the U4 IC might be 
> the problem (eventhough I did observe antistatic and overheating 
> precautions), but since its replacement can be technically difficult 
> and would have to be posponed until the part arrives from Elecraft to 
> the SV1 land, I would like to have your opinion as to how I can 
> further troubleshoot this problem before starting changing parts etc.
>  
> Thanks, 73
> Marinos, sv9dru / ki4gin
>