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Sir supak111 . . . . . . .
The specs tell us that the unit can work on down to 60 cycles with .1 ufd and 200k used as the RC time constant values.
You probably don't have the instrumentation capability of reading that C1 chip capacitor that is now present in the unit.
So initially, use a .1 cap shunted across it and start at the highest frequency . . least trim pot resistance . . . since that is going to be probably close to a frequency that the unmodified unit was capable of operating at before.
Then you increase the trim pot resistance, by slowly adjusting it . . . . . as that output frequency then drops, you can catch any "dodgy" action . . . . . or complete drop out . . . . . .as you work on down towards your desired 25 cycles.
Realizing that you have far greater range, using that units 2 meg pots capability, than the specified mere 200K ohms.
HOWEVER . . . . that oscillator circuit just might not like a high R and low C combination, and is better suited to using a low R and high C circuit.
If by chance you still can't tune on down to 25 HZ . . . . yet the unit is still reliably oscillating . . . .increase the added shunted capacitance to .2 ufd as you mentioned.
I think that it would not be needing more than that.
(As your cat presently has its Rubik's cube . . . . I can complete it in 67 moves.)
73's de Edd
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