[R-390] Observation on the Cap Questions and Comments

[Charles P. Steinmetz]

Chuck wrote:

>Electrolytic's are generally used as filters in power supplies; you 
>may see the odd application as a cathode bypass in an audio 
>circuit.  The voltage rating of the replacement cap should be such 
>that the applied voltage, be it P-P or DC should not exceed 80% of 
>the rated voltage of the cap.
>
>In circuits in which audio or RF up to but not exceeding 455KC in 
>frequency, Sprague Orangedrops work wonderfully and Orangedrops with 
>Polypropylene media (the more modern 716P Series) in particular, 
>excell in audio circuits.
>
>Circuits which have RF whose frequency exceeds 455KC in benefit from 
>Ceramic Discs.  1KV rated caps, in my book, are considered a minimum.
>
>As an example, the average SP-600JXxx will need something on the 
>order of 38 .01 ceramics, 16 .022 Orangedrops a couple .5's and one 
>Multi Section Electrolytic filter cap.

A few minor quibbles/clarifications:

Power supply voltages pump up and down (e.g., due to turn-on surges, 
particularly but not exclusively in radios with SS rectifier diodes) 
and wander around (due to the power company, varying current draw 
(ovens cycling, etc.), resistive service neutrals, imbalance on the 
two 120v service phases, etc.).  Electrolytics should be chosen so 
the *maximum possible* voltage they see does not exceed 75-80% of 
rating.  If the radio has been recapped with NOS electrolytics or the 
electrolytics have been "re-formed," all bets are off.

Any modern film/foil cap will be better than the original paper caps 
were on their very best day, so in that sense using all ODs (for 
example) is an improvement over the radio as new.  On the other hand, 
disk ceramics are better bypass capacitors at all frequencies above 
audio and are just as good, value for value, at audio, so best 
results will always be obtained with all ceramic bypass caps (audio, 
IF, and RF).  This is particularly true for radios like the SP-600, 
which have nasty IF and IF harmonic leakage that gets back into the 
IF and RF stages all too easily.  I have done quite a few SP-600s, 
and have observed a 10-20 dB reduction in spurious responses from 
changing all of the bypass caps from ODs to disk ceramics.

Coupling caps at IF and audio and AGC time constant caps should be 
film/foil.  No ceramic, no mica.

Best regards,

Charles