question about caps in apache vfo

[NR5P]

I'm replacing the caps in my Heathkit Apache vfo. I'm experiencing excessive drift even after hours of warmup time.  The caps in the tuning circuit I was going to replace with mica(not sure whats in there now).  there are 3 of them that are 4.7pf and in the schematic it says (N750).  What does the N750 mean?  Should I be installing temperature compensation capacitors in the tuning section? 

[KA0HCP]

Class 1  ceramic or Polypropylene (PP) film capacitors are typically used in frequency critical circuits.   The temperature compensating ratings for them are:  

Class 1 Ceramic Temp. Coefficient Codes (sample)
Ceramic
names    Temp Coeff   Tolerance    EIA letter code
P100     100     ±30    M7G
NP0            0     ±30    C0G  (ideal, zero drift)
N33           −33    ±30    H2G
N75           −75    ±30    L2G
N150    −150    ±60    P2H
N220    −220    ±60    R2H
N330    −330    ±60    S2H
N470    -470    ±60    T2H
N750    −750    ±120 U2J

*As you can see N750 is a lower grade, lower temperature stability, but lower cost.



Some commonly used class 2 ceramic capacitor types are listed below.  Class 2 used in less frequency critical circuits, may be slightly microphonic.

    X7R (−55/+125 °C, ΔC/C0 = ±15 %),
    Z5U (+10/+85 °C, ΔC/C0 = +22/−56 %)
    Y5V (−30/+85 °C, ΔC/C0 = +22/−82 %)
    X7S (−55/+125, ΔC/C0 = ±22 %) and
    X8R (−55/+150, ΔC/C0 = ±15 %).

[KA0HCP]

Note Silver Mica capacitors would also be a good choice for stability, though higher in cost.

[KA2DZT]

N750 caps have a greater change in capacitance per degree of temperature change.  Lower N numbers have less of a change.  Usually a decrease in capacitance in needed to offset the oscillator coil increasing in inductance as the coil heats up.  Does the VFO drift up or down?  Drifting down, you need less capacitance to offset.

The trick to the whole thing is to find what value of negative compensation you need to have a stable VFO.

There are many things that will make an oscillator drift.  Unregulated B+, filament voltage changing with different loads on the AC line.  The tubes heating up is another. 

You could try silver micas if you can find them.  I don't think they are made in neg temp type.

I spent many hours on the VFO in my HB xmtr.

Fred

[Pete, WA2CWA]

Early Apaches used a 6AU6 in the VFO. Heath recommended replacing it with a 6AH6 to reduce drift.

[NR5P]

I guess I'll explain in more detail my problem and maybe I won't waste a bunch of money.  When I press the spotting button the vfo is drifting up in frequency about 300hz the first 10 seconds or so and then slow down to a crawl.  Over time it will continue to drift and I constantly have to touch up the vfo during a qso on top of the normal 300hz drift I'm getting.  I don't know how far it is drifting over time I guess I need to check that also.  I thought a good start would be to replace all the old components with new including resistors.  But maybe I'm jumping the gun and should try to just compensate.  What would be "normal" for an apache anyways?  And is the drift I'm experiencing when the exciter stages are turned on with the spotting very excessive?  It seems to be the same on 40 and 80

[NR5P]

Oh and thanks for the tip on the 6ah6 in place of the 6au6.  I've never heard that before.  I'll look into that.  I have the 6au6 in mine

[Pete, WA2CWA]

Also check the NE2 and it's associated components in the spotting switch circuitry. I seem to remember having some drifting issues because the NE2 was not firing properly. I remember changing out the NE2 and maybe some the associated resistors. I remember changing out the original caps in the VFO (back in the 70's/80's??) with whatever silver micas (those little maroon colored ones) I had in my capacitor inventory. Have no clue if they were N750's. Wasn't a consideration. I was having drifting problems using the Apache on SSB with the SB-10.

[NR5P]

are all micas silver mica?  Probably a real stupid question but all I see on mouser are mica

[wa3dsp]

Is the 6AH6 a better tube to use in the Johnson (Ranger) VFO also in place of the 6AU6?

[K6IC]

The N-750 cap must have been there for temperature compensation,  not to save money,  in my opinion.

The dark brown dipped caps - DMs are not compensating caps,  neither are Silver Micas,  again in my opinion.

Not all Mica caps are Silver Micas.

It has become more difficult to find any compensaing cap,  as  digital circuits do not need them,  and there are few analog cirtuits being designed into products,  in general,  now.

Opinions ceased for now.  Good Luck,  Vic