[R-390] filtering on R-390 C101, C103

[jkharvie]

 Hi Alex - Good questionsI have attached two documents that will help you in understanding how these power supply filtering caps are electrically located in the power supply circuit.  I do hope you have acquired the knowledge to read a schematic, if not this can be acquired.  Depending upon the age and condition of the existing filter caps, I would treat them as suspect and likely needing to be replaced, unless evidence from performance validation shows otherwise.  While they might "operate", residual AC ripple is not to your receivers advantage.  You will need to try to understand how a power supply filter capacitor failure mode will likely result in regarding what else you might then need to repair.  
The Y2K manual contains a number of options exist regarding these filter capacitors.  If you power these up outside of the receiver, give them a good session with slowly increasing voltage and slowly increase current demand into a load such as a 25 watt stove oven light bulb initially.  Monitor output with a scope if you have one available.  The peak current is at first on cold inrush, once warmed up the current is about ~~1.3 or so amps if I remember correctly.  Try to operate the receiver at 115 volts rather than the standard 126 volts now on the US mains voltage using a variac and if available a 1:1 isolation transformer.  Good luck \VR\John

    On Monday, February 2, 2026 at 10:54:32 AM EST, Alex Hopper <ahopper0216 at gmail.com> wrote:   

 Hi Folks,

Later this week I am going to slowly start up an R-390/URR that I recently
purchased.  After careful visual inspection and removal of several of the
modules, I am confident enough to start it slowly on a variac.

My question is, what provides the typical 'filtering' of the rectified DC
voltage?  Is this C101 (the large rectangular metal box capacitor next to
the power supply)?

What does the other large, cylindrical capacitor C103 do?

Are these capacitors failure prone?  Should I expect the need to replace
them?

Thanks,
-Alex
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