[R-390] [R390] Field Change No. 7, R390A

[K2CBY@aol.com]

In the process of overhauling and re-capping my R-390A I decided to change 
R702 from 56k to 220k per the Field Change 7 instructions. Following the 
overhaul, the sensitivity and S+N/N went west. To troubleshoot the problem, I 
consulted Chuck Rippel's notes of 12/24/2001 and measured the oscillator 
injection voltages at the cathodes of the respective 6C4 mixer stages. All 
was well until I got to the 3rd Mixer (pin 7 of V204) where the injection 
voltage as measured on my scope was only 1.5 v p-p rather than the 6 v p-p 
that Chuck called for in his note. I restored the 56k screen resistor and 
Presto! the injection voltage was back up to where it should be. The 
sensitivity went up and the noise dropped. Further research showed that with 
R702 = 220k the VFO screen voltage drops to about 30 volts while with 56k in 
the circuit it is nearly 70. A 6BA6 -- even one with robust gm -- a has 
difficulty oscillating under these conditions because the grid coupling 
capacitor is only 15 pf. By the way, the output waveform from the VFO was 
clean as a whistle viewed on a 75 MHz scope, so I doubt very much that the 
Field Change 7 mod was intended to eliminate harmonics.
 
Miles, K2CBY



--- StripMime Report -- processed MIME parts ---
multipart/alternative
  text/plain (text body -- kept)
  text/html
The reason this message is shown is because the post was in HTML
or had an attachment. Attachments are not allowed.
Please post in Plain-Text only.---