[Hallicrafters] SR-2000, SR-400, SR-150 Chirp and audio distortion fix

[Jim Liles]

We've had several queries regarding a chirp during the transition from 
transmit
to receive and receive to transmit.  The chirp will be present on the second 
to last run of the SR-150, the last run of the SR-400, and any SR-2000 that 
installed the following service note that We wrote about two years ago.  We 
recommended the installation of a 5uf capacitor to ground on the line that 
runs from the VOX relay to VFO.  The problem that it corrected was a very 
slightly fuzzy sounding audio both receive and transmit at the lower audio 
frequencies.  This was caused by low level 60 cycle modulation/sidebands 
being present on the VFO output.  The origin of the problem is resistance 
build up in the line between the RIT/CAL pots and the varactor diode in the 
VFO.  The high impedance created allows 60 cycle crossover in the wiring 
harness.  Cleaning both the RIT switch and VOX relay will correct the 
problem.  That line carries no current and will develop resistance over time 
unless a conditioner like Deoxit is applied.  Once you have cleaned the path 
and checked for zero ohms, remove the 5uf capacitor which was added to the 
VFO line.  We recommend the cleaning of the switch and relay in all SR-150, 
SR-400 400A, and SR-2000's and the removal of the capacitor work around, if 
it is present.  You will be pleased with clearer audio.  Spectrum analyzer 
photo's of the VFO and audio distortion are posted on the SR-2000 reflector. 
Any questions call 847-571-8420  Kindest regards Jim K9AXN

NOTE:  If you have anything to add or change please do so in a pleasant 
way --- 73's