[Hallicrafters] SR-2000

[Jim Liles]

Rocco:
I would not worry about the ALC, it only responds to grid current flowing which is exactly what it attempts to dampen.  An SR-2000 that is operating properly and has a pair of good 8122's will deliver 1200/1000 watts to the antenna 80/10 meters.  What is important is whether you can drive the finals into the positive zone of the control grid when in tune mode.  This will be apparent as you increase the rf drive, the plate current will increase then decrease before reaching maximum drive.  If you can not get adequate drive in tune mode, one of two things must be done.  Fix the radio or alter the tune up process.  The SR-2000 was designed to drop the carrier frequency 100 Hz into the filter bandpass and force enough signal through to drive the finals in tune mode.  The insertion loss at this point is 30 to 40db and if the filter terminators are not aligned properly, adequate drive will be impossible.  So what is the consequence?  Here is what will happen to an SR-2000 that has inadequate drive in tune mode if you follow the tune up process from the manual.  Conditions: you have a pair of 8122's that are capable of 500 watts in tune mode but have only enough drive to provide 300 watts.  You tune it up for 10ma of screen current and switch to sideband.  The voice frequencies pass through the filter with significantly less than the 30 to 40db of attenuation --- more like 10db.  The added drive will send the screen current soaring and you may hear the insidious prink of death from an 8122.  A good test is to a short whistle in sideband mode while monitoring screen current.  If it is high, retune for a lower screen current.  It's better to chance a whistle rather than talk for an hour driving an 8 watt screen to it's demise. 
For your other issue, receiver sensitivity.  The most sensitive band in receive mode for an SR-2000 will almost always be 20 meters followed by 15 then 40, 80, and 10.  This is true with few exceptions and has a lot to do with the design of the first carrier oscillator and the gating through the 1N295.  If 20 meters is not the star performer, the 15 ohm resistor that is in series with the 20m carrier osc coil will be high, in most cases above +50%.  When I stated that the 15 m band could be affected by the adjacent 20m coil being resonant in the 15m band, I failed to mention that the other coils are not resonant in other bands.  Also, the 20m problem only appears if you happen to have the preselector knob slightly off when adjusting the 20m coils.  One other thought on output.  Neutralization problems can make an SR-2000 squirrelly and or cripple the output.  In the past I have destroyed a neutralizing tab or two with so many bends trying to get it right on.  Well, I took a ball point pen apart and commandeered the spring along with a longer screw and washer.  With these I spring loaded the tab so that I could test, turn off the hv, and with an insulated screwdriver move it a bit and re-test (Careful with the length of the mounting screw --- too long will hit the opposing ground mounting screw, tab has 280v). CAUTION, that tab has 280 v on it any time it is powered up.  Finally, disconnect all power, measure and restore the original mounting --- DO NOT LEAVE THE SPRING IN PLACE.  I was hesitant to put this last tip to pen and only a lunatic would do such a thing but for me, it works.  Lastly, if the peak output occurs on the higher capacity side of the plate current dip, you have too much neutralizing capacity and if it occurs on the lower capacity side, not enough.   
Kindest regards Jim K9AXN 

 
Original message
Jim,

My SR 2000 has a somewhat different gain issue.  10 meter segments are a bit less sensitive than the other bands.  On 80-20, 100 uV yields S9.  If anything, 15 is the hottest of them all; 50 uV yields S9 easily, which is what you describe as normal.  10 is down 1/2 S unit (100 uV = s 8.5).  TX drive is down commensurately.  I have spent a lot of time on the alignment, tubes, and component/voltage checks.  Tuned circuits seem to behave as expected on all bands.   IF alignment was as expected after replacing a few bad caps and resistors. 

Any idea how 15 could be hotter than the others?  Your thoughts on the inadvertent 20-m coupling are interesting, but I don't see how 10 would be affected.  And it's a real puzzler why 15 would have more drive.  Have not found that on any Hallicrafters (or anything else, for that matter).

This SR 2000 is on the bench after a lot of work over the last two months.  Seems to work pretty well, but also having a tough time getting over 350 W on CW (low voltage).  Seems to be too much AALC - 10 o'clock on the TX audio results in plenty of AALC meter action.  Max power out is constant for all bands with expected currents.  AALC tube voltages are now pretty close to the manual values.  Might be the finals - have not tried a known good set, yet.

73,

Rocco N6KN