[R-390] AGC voltage issue

[Charles Steinmetz]

Put another way, there is simply no excuse to design for best-case 
conditions.  Competent design always focuses on worst-case conditions.

Replacing C551 with a capacitor rated at <450v, or with a 
metallized-film capacitor of any voltage rating, is designing for 
best-case conditions.

Charles


I wrote:

>Larry wrote:
>
>>I think the reason they are not rated higher is that the power 
>>supply is a 'choke input' and the voltage regulator is active 
>>immediately (putting enough load on the rectifier to minimize the 
>>initial 'power on' high voltage condition you see in many other 
>>power supplies).  Granted my 2 r390a's here have 26z5's
>
>The regulator circuit does not draw anywhere near enough current to 
>hold the initial voltage surge down (see previous posts in the 
>archives about choke-input filters).  If your B+ does not exceed 
>260v, it is because the 26Z5 cathodes warm up more slowly than the 
>other tubes in the radio (the current drawn by all of the other 
>tubes, even when they are only partially warmed up, is enough to 
>provide a sufficient load to hold the B+ down with the choke-input 
>filter).  And that is how it is supposed to work.  However, some 
>26Z5s warm up much quicker, and if they have been replaced with SS 
>diodes the rectifiers are active immediately, so the voltage rises 
>until all of the tubes have warmed up and are conducting.  (Also 
>note that if you switch the radio to Standby, the B+ rises because 
>the load is removed.)
>
>Depending on how you measure the B+, it is also possible that the 
>indication you see may lag and integrate the actual voltage (i.e., 
>your measurements could be under-reporting the actual peak).  The 
>time constant of a VTVM can miss this transient (due to both the R-C 
>noise filtering and the electromechanical ballistics of the 
>d'Arsonval movement).
>
>Best regards,
>
>Charles
>
>
>ps.  I don't understand why so many people seem to want to get by 
>with substandard parts, and why there is so much resistance to doing 
>things right (e.g., using a capacitor that is rated for less voltage 
>than it may see in operation, or using metallized film rather than 
>film-and-foil caps where the voltage could exceed 200v).  Jeez, 
>people, just buy the right part and use it.  IMLTTO, anyone who 
>whines about the cost of the right capacitor, or that they can't fit 
>it into the old can, doesn't deserve to own one of these fine 
>radios.  They should sell it to someone who will treat it right and 
>take up stamp collecting.