[R-390] Tech Query R-390 power supply

[Drew Papanek]

Roy Morgan wrote:

>Very likely this resistor is added as part of, or after, the solid state 
>mod for the rectifiers (That field change was done with a different EIB 
>number.).  This resistor is in the Audio Deck, and goes between P-119 pin 5 
>(the DC input from the power supply via the B+ fuse) and the input to choke 
>L602.  It is likely a 5 watt power resistor mounted with a bolt to the 
>chassis.

>If what I assume is right, then it would be quite possible to have an audio 
>deck withOUT the series resistor, and a power supply WITH the solid state 
>rectifiers.  This would be bad.

>I would get the audio module out and see if that series resistor is in 
>there. If it's a field change, I would add it to increase life of the 
>rectifier tubes. If your power supply uses the diodes, then it might be 
>optional depending on how rugged your diodes are.

The field change to add a series dropping resistor  was prompted, IIRC,  by 
6AK6 audio output tube failures.  The solid state rectifiers give increased 
B+ voltage.

The R-390A power supply's input choke already very nicely reduces the filter 
capacitor charging pulses.   In a unit equipped with tube rectifiers an 
added series resistor would do little to make life easier for the rectifier 
tubes.

The 12BW4 has lower internal resistance than the 26Z5W.  If replacing 26Z5W 
with 12BW4, an added series resistor might help prolong the life of the 
other tubes by keeping B+ down.

Drew

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