So, by interpreting the above info, you can thereby see that the filtering /decoupling action by shunting the unwanted signal to ground is much more evident by the LOW capacitive reactance impedance exhibited by that capacitor when being used at the higher frequencies. Aside;
Sir . . . . . Brian
Your posts hic-cupped, and multiplied !
Now, considering that a capacitors effectiveness in filtering or decoupling RF signal or "trash" fron the power supply or its sub busses has deteriorated downwards . . . . . that effect that it is having at 88-108 Mhz or its I.F.'s associative, typical 10.7 IF frequency, will certainly be more effective at those higher frequencies.
Less so, at the lower frequencies, thus opening up the possibilities of interstage coupling of potential RF feed thru between stages via the power supply.
And with the low time constants involved , the likely possibility is the effect being a low frequency oscillation or most commonly described as "botor-moating".
Example:
Take a minimal .01 ufd value of capacitance being present for the bypassing of these potential frequencies:
455 Khz . . . .. .(AM IF) --------------> 35 ohms
540 Khz . . . . . BCB-------------------> 29 ohms
1740 Khz . . . . BCB-------------------> 9 ohms
6 Mhz . . . . . . .SW--------------------> 2.6 ohms
12 Mhz . . . . .. SW--------------------> 1.3 ohms
10.7Mhz . . . . (FM IF)---------------> 1.5 ohms
88 Mhz . . . . .FM--------------------> .18 ohms
108 Mhz . . . . .FM--------------------> .147ohms
It effectively shunts them right on off to ground in a hurry .
You did not specify the "small" as per being of tube construction or possibly . . . That "pocket" almost clinches it as being a small transist-a-mo-rized type of design . . . right ?
If the latter case . . . I would be expecting the time onset, simple "dried out" electrolytics syndrome on the power supply buss . . . e.g lowering their effective capacitance value, but more importantly . . . their skyrocketing onset of higher ESR.
>
73's de Edd
:I have a small Grundig pocket radio that motorboats and squeals badly on the AM and SW bands, but is not as pronounced on FM band. Usually this is due to leaky capacitors, shielding, but would there be such a difference between AM and FM?
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