[R-390] The R-390 Cookbook by A.J. Carmody: Beware!

[Roy Morgan]

R-390/URR Owners,

While finding the link to ST 32-152.pdf, "Visual Alignment..",  I came 
across another one called R390_cookbook.pdf, by A.J. Carmody from the Army 
MARS program.  It is intended for the R-390/URR ("Non-A") radio.

One of the mods he suggests replaces the two 6082 24 volt series regulator 
tubes with two of the more common and cheap 6080 6 volt ones. His method 
puts the two tube filaments in series with a diode to rectify the available 
filament supply to half wave.

THIS IS A BOGUS DESIGN.  DO NOT DO IT.

It turns out that a half wave rectified 24 volt supply produces a LOT more 
heater power than a full wave voltage of 12 volts.  The 6080 filaments will 
run way hotter  than they should.  They may overload the transformer 
filament winding, and likely both fail soon.  This is a classic dead horse 
that has been revisited and beaten a number of times on the R-390 list.

His other mods don't contain such egregious engineering errors, but some 
are undesirable in a time when we are less apt to permanently modify our 
radios.  (The parentheses below are mine.)

1) Power Supply Sub-Chassis Modification (the solid state diode mod for the 
B+ supply.  He does not use a dropping resistor to lower the un-regulated 
B+ to normal levels.)

2) IF Sub-Chassis Modification (Replaces the ballast tube with a 12BH7 or 
12BY7.)

3) AF Sub-Chassis Modification (The mod I complain about above - replaces 
the 6082's with 6080, and runs the new tube filaments way too hot.)

4) Low Impedance Speaker Connection (Uses external 600 to low impedance 
voice coil matching transformer.)

5) Antenna Input Conversion to 50 Ohms "To replace antenna input connectors 
with SO-239 Connectors that mate with standard ham equipment." (In this 
mod, he removes the "break-in relay" completely (I think he means the 
antenna relay actuator), solders jumpers to various parts of the antenna 
relay module and connectors, removes the two antenna connectors and 
replaces one of them with an SO-239 connector.  SHUDDER!!)

The last section of this document is a two page procedure to test the radio 
for signal plus noise to noise ratio and test each RF front end range band 
for internal noise level at a standardized gain setting.  This seems like a 
good procedure.  It uses an audio output level meter, the TS-585, but an AC 
VTVM with DB scale and a load resistor would work just as well.

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
roy.morgan at nist.gov --