[Elecraft] [K2 4922] BFO Range Problem Continues..

[Don Wilhelm]

Ron,

Yes, I see that your padding attempts were not successful, and the 
proper conclusion is that you have something other than the crystals 
that is incorrect.

The BFO is similar to any VXO - and the components involved are the 
crystals, the inductor and the capacitors.  In this case, the varactors 
D36 and D37 are the variable capacitors, so check to be certain they are 
the correct type - the body on the iSV149 is the smaller of the 2 
varactor types used in the K2.  If they are the correct type, check the 
voltage swing on the VBFO signal line (check at Control Board U10 pin 
14).  Enter CAL FCTR and tap BAND+ (the voltage should go to almost 8 
volts), then tap BAND- and the voltage should be near zero volts.  If 
those are correct, tap BAND+ again and check the voltage at RF Board RP6 
pin 1 - it should be the same as the voltage at CB U10 pin 14, if not 
check soldering on all VBFO points.  Also check the soldering at all 
pins of RF board RP6.

If that did not correct the problem, then I would arbitrarily replace 
D37, D38 and L33.

73,
Don W3FPR

Ron Hahn (EI2JP) wrote:

> 
> Don,
> 
> Updated data below.  No real progress with the padding.  :-(
> 
> 	A	B	10pf	20pf	47/122pf
> BFO Hi	4917.62	4917.63	4917.11	4916.8	4917.39
> BFO Lo	4913.59	4913.49	4913.3	4913.16	4913.02
> Range	4.03	4.14	3.81	3.64	4.37
> 
> A is the original set of BFO crystals
> B is the replacement set of crystals
> 
> Try 1: 10pf from pin 3 - 6 of U11
> Try 2: 20pf from pin 3 - 6 of U11
> Try 3: 47pf across C174, 122pf across C173
> 
> As you can see by the data padding the junction of the crystals only
> reduced the upper range.
> 
> So I then tried the recommended padding across C173 and C174, and, as
> you can see, it still fails to bring the lower range into spec.
> 
> Where to go from here?
>