[R-390] AGC voltage issue

[Larry H]

Hi Ryan,  The reason I though a 400v rating would be ok for c551 is that the caps in the power supply and most in the audio deck are rated at 300v.  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, so I don't see instantaneous B+, but my B+ comes on quite fast.  It goes to 260v very quickly and then holds at 250.  I used my 11 meg ohm vtvm to measure the voltage at F103 so as not to load the voltage down.  Even if you have the SS recs, I don't see this changing much because of the design mentioned above.

Regards, Larry


On Tuesday, September 16, 2014 5:11 PM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
 


David wrote:

>Self-heal events (characteristic of metallized film) are harmless in 
>C551, the AGC Time Constant cap.

While the radio is operating normally, that may be so (although C551 
can end up with >200v across it with strong signals, as you noted in 
your AGC article, and my tests indicate that a metallized film cap of 
any voltage rating has self-healing events above 200v).  But during 
the startup condition I described in my message, C551 may have >400v 
across it for some seconds every time you turn on the radio, 
positively ensuring that there will be self-heal events.

If your point is that self-heal events at this location would not 
matter even if they occurred, I disagree.  Any self-healing current 
would flow through the grid of V506A.  Cumulative occurrences would 
damage V506's cathode.

Best regards,

Charles



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