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

[Roy Morgan]

At 08:29 AM 3/29/2007, pete williams wrote:


>  The  Power  supply in the  R-390 has 47 ohm current limiting  resistors 
> in the
>cathodes of the 26Z5W rectifiers.

Pete,

I trust you mean the R-390/URR ( "non-A" ) radio.

This radio has four more of those 47 ohm resistors under the 6082 series 
regulators. In that case their purpose is not current limiting but rather 
current equalization.  These 2-watt carbon resistors are often found 
drifted high, and more importantly, drifted to differing values.  SOOoo... 
replace them all!  With the 47 ohms in the cathodes, the effect of 
variation in gain among the four sections of tube are reduced.  Differences 
in the values of those resistors will cause unbalance in the cathode 
currents.  With 6082's more expensive than we'd like, running them with 
over-taxed sections is not a good idea.

I have used 47 ohm 7-watt square cross section ceramic cased wire wound 
resistors in my R-390/URR with good results.  The cost was very 
modest.  Teflon tubing on the leads may be a good idea.

While you have the audio module out, I suggest testing the capacitor in the 
regulator circuit that goes from the 180 volt output back to the regulator 
amplifier tube grid.  If this cap leaks, it will drive the output voltage 
down. It's purpose is to add gain to the regulator circuit for AC, 
particularly hum.

I also strongly recommend a FAN at the side of any of these radios with the 
6082 series rectifiers (the R-390/URR, the R-389 and the R-391/URR).  I 
have a photo of my fan plate that can be used to duplicate it for anyone 
interested.  Even a small sized fan blowing in on the regulators will keep 
them MUCH cooler than otherwise.  At the very least, find a 4-inch muffin 
fan and just put it next to the regulator tubes, no mounting really needed.


It *seems* to me that the original value for the 47 ohm resistors in the 
rectifier circuit would be fine, but only a rather careful analysis of the 
surge current pulses would tell you much with authority.  A first 
approximation might be to figure the difference in forward resistance value 
between the two tube types and add that value to the 47 ohms.  Maybe that's 
just what you did.  It could be that data sheets from the RCH HB-3 manual 
would indicate design values for those rectifiers.  There are sections of 
the HB-3 that deal with power supply design guidelines. Look that up if you 
can.  (If you can't let me know, I may be able to find/produce it in 
digital form.)


Roy



- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing
13033 Downey Mill Road, Lovettsville, VA 20180
Phone 540-822-5911   Cell 301-928-7794
Work: Voice: 301-975-3254,  Fax: 301-975-6097
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