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

[Jim Whartenby]

If you look at the final engineering report of the R-390/URR you will find that the radio receiver was designed to operate at a line voltage of up to 132 VAC.  Look on document page 14 of the above report; the variation in line voltage is specified as 115 VAC +/- 15%.  The idea that the AC line voltage has increased over time is just nonsense.   What has actually happened is that the lower line voltage limit was raised over time but the upper voltage limit has always been limited to 126 VAC, at least since 1927, at the very beginning of AC line powered radios.
Trying to control the charging current of a filter capacitor by monitoring and adjusting the applied voltage is a fools errand.  If you are just monitoring voltage and blindly adjusting a VARIAC, how is the filter capacitor charging current controlled?  The least expensive method to control filter capacitor reforming current would be to half wave rectify the AC line by using a silicon diode and placing a 7.5 watt incandescent light bulb in series with the filter capacitor.  This will automatically limit the charging current to a maximum of about 60mA.  Even if the filter capacitor short circuits during reforming, the maximum current available is low enough so that no internal heat builds up inside of the capacitor so no exploding filter capacitor.  This method will allow the filter capacitor to reform at up to about 170 volts peak which should be enough to prove that the filter capacitor is serviceable.
Of course there needs to be some way of discharging the filter capacitor once it has reformed so one needs a 27k 3W resistor across the capacitor so that the 170 volts will bleed off and not surprise you.  The cathode band on the silicon rectifier should be connected to the 7.5 watt incandescent bulb and the wide lug of the polarized line cord should be connected to the negative terminal of the filter capacitor that is to be reformed.  If safety is a concern, and it should be, use a GFCI protected outlet to power the above mentioned filter capacitor reformer circuit.Regards,Jim
Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Thursday, February 5, 2026 at 03:20:36 PM CST, jkharvie via R-390 <r-390 at mailman.qth.net> wrote:   

  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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