[DSP-10] Toko coil tuning

[Bob Larkin]

Hi All -  I ran across an interesting problem in tuning up a newly built 
DSP-10.  This may apply to some of the others, as it came, I think, from 
a TAPR kit. It affects the sensitivity of the receiver.

I found that the 2 RF input tuned circuits (L1 and L2) were at maximum 
inductance and not quite reaching the 146 MHz.  On the Toko coils, like 
these TK1044 0.1 uH, the maximum inductance is with the tuning slug even 
with the top of the metal case.  If yours is peaking at this point, it 
may not be tuned to frequency.  On mine, they tune with the top of the 
slug about an eighth of an inch down (3 mm).

Several solutions are possible.  The easiest is to add a 2.2 pF leaded 
capacitor across C1 and C2 and another across C4 and C5. That is, from 
the top side of C2 to the bottom side of C1, for instance. If it is 
easier, the C1 and C5 ends are ground, and the metal package of the coil 
can be used for the connection.

Cause: The value of C2 and C4 for the unit that did not quite tune 
measured about 11.5 pF instead of 12 pF.  The original ones that I used 
(I still have some in the 1996 bag!) were DigiKey PCC120CCT-ND Panasonic 
ECU-V1H120JCM (apparently discontinued), that measured right at 13 pF. 
One is low, the other is high.

You might check your DSP-10 and see if the slugs are below the top.

By the way, the minimum inductance for all of the Tokos is with the slug 
all the way inside, OR with the slug out the top.  But, never use the 
slug out of the coil package. It is mechanically not so stable when out.

73,  Bob  W7PUA