help vfo-122 drifting bad

[kc2etm]

hi i have a vfo-122 on a johnson viking 2 with the radio keying it it continues to drifts postive over two kc in less then a minute and continues postive until i bring it back down manually same results with key in back of vfo
thanks
fred
kc2etm

[WA1GFZ]

Check the screen dropping resistor on the 6au6. The last one I rebuilt was rock stable with half the hardware missing after replacing the 18K that measured 40K.
Use a 20K 10 watter and it will never fail again.

[kc2etm]

Hi i have already changed that when got it
the old was cooked crispy lol
is it possible the resistor is to small and heating
up quickly i did put original value and
wattage just figured i would need to change it sooner
thanks
fred

[WA1GFZ]

as long as the 0A2 stays lit and the voltage is stable that is not your problem. Replace the tube and see what happens. I have two 122s and both are quite stable.

[kc2etm]

Well heres the thing i have two vfos and
two viking 2 radios ive mixes and matches
vfos to units and no matter the combination
it happens on both vfos and both radios
i have swapped oa2 around with spares
with no luck my tube tester doesnt test oa2
tubes
thanks
fred

[WQ9E]

Fred,

Very strange to have two of the VFO's doing the same thing.  I have 3 and although they aren't rock stable they behave nothing like yours.

Does this drift occur on both ranges?  160 and 80 operate the oscillator on 1.75 to 2.0 while it operates on 7.0 for 40 and above.  Most components that can impact frequency drift are specific to either range so this will help narrow down the problem.  Since it appears heat related use a soldering iron very close to the component body but not close enough to cause damage (or cooling spray if you have it) to determine which component is overly heat sensitive. 

[kc2etm]

well i followed your suggestion and when i got near c53 both units it changed looking in the manual c53 is rated as a 43 mmfd NPO zereo tempature coefficent  i replaced this part with a 43pf ceramic m and m type and it is much more stable now do you have any suggestions as to a proper replacement from what i read zereo coefficent means not affected by temp or stays same capacitance at different tempature ranges
thanks
fred

[WQ9E]

Fred,

NP0 (also known as C0G) ceramic caps are available from Mouser  (www.mouser.com)  There is no minimum order from Mouser and no handling fee but specify USPS instead of the default UPS for small orders to avoid the fairly high base UPS rate.  I am sure there are other places with NP0 caps but I really like Mouser pricing and service.

[kc2etm]

Hi just for anyone that may be following this thread i changed out c53 with a 43pf silver mica and now the vfo is solid for over an hour so i guess problem is solved
thanks for all the tips
Fred
kc2etm

[WQ9E]

Fred,

Glad you got it running properly!