[R-390] Recap and other maintenance

[DJED1 at aol.com]

I finally launched into doing some serious maintenance to my Amelco  R-390A.  
Until last year, I believed that I had all Vitamin Q capacitors in  the 
radio, so no need for rework.  Then, I opened it up to replace C553,  just to be 
safe, and found a bunch of BBODs in the IF and Audio.  So I  resolved to recap 
the radio this year.  I plunged in and bought the 225  size orange drops.  I've 
recapped the audio and IF now, with one  problem.  I broke the very small 
standoffs supporting C 528 and C  529.  If anyone has a couple of spares, please 
contact me. I also replaced  a handful of resistors which had drifted high.  
Most of the 2.2Ks were OK,  but a mix of low value and high value resistors 
were up over 20%. I also checked  all the caps with a voltage source and a 
microammeter.  With 300V applied,  the BBODs leaked about 30 microamps, while the 
silver General Instrument caps  had undetectable leakage (less than 1 microamp). 
 This radio has had  any easy life, sitting in my place for the last 35 
years, so the BBODs do just  get old.  As soon as I find a standoff, I'll wrap up 
the IF and put  it back in the radio.  Sure looks nice with all those orange  
caps!  I took a reference set of measurements so I'll be able to see how  much 
of an improvement I get.
While making the initial measurements, I ran across an interesting  thing.  
I've been bothered by the inaccuracy of my meter, so I played  around with the 
resistors and gain adjustment.  I had shunted the 22 ohm  resistor with 5 
ohms, which greatly improves the zero set with the stock  potentiometer.  I found 
the 5 ohms had virtually no effect on the meter  readings, but if I increased 
the IF gain, I could get readings accurate to 3-4  dB except at the 100 dB 
level.  I then went back and checked the  sensitivity, because the IF gain had 
been set per C. Rippel's  recommendation.  I was surprised to find very little 
loss in sensitivity  with the higher gain setting (maybe 1.1 vs 1.0 microvolt). 
 I'll play  around with it more after I finish recapping and an alignment.  I 
think  shunting the 22 ohm resistor works well, but if I'm missing something 
let me  know.
One other question for the group- I'm running into the stop when I  calibrate 
using the zero set, so the radio is not set for operation with the  zero set 
in the middle of it's range.  To correct that, do I adjust the  PTO shaft vs 
the Oldham coupler, or do I offset the odometer as  described in correcting the 
overrun?  Couldn't find it in the  manual.  I'm also anxious to try out the 
PTO I picked up at a flea market  for $3.  It'll be a find if it works, and a 
source a spare parts if it  doesn't. 
Ed WB2LHI