SB-300 Low sensitivity problem solved

Michael Waldrop w5rkl at YAHOO.COM
Wed May 17 16:10:19 EDT 2006

Hello everyone,
  After having troubleshot my SB-300 today I finally have the sensitivity back up to where it is suppose to be. What I found are as follows:
  1.  Both IF amplifier plate and screen resistors were out of tolerance
  2.  Bypass capacitors in both IF amplifiers were leaky
  3.  All 3 coax connections to the preselector variable capacitor had very poor
      solder connections.
  After replacing all IF plate and screen resistors (1k ohm 1/2 watt each), replacing
  the IF amplifier bypass capacitors (.01ufd each) and reheating all 3 preselector variable capacitor coax connections at the variable cap, the sensitivity has greatly improved. An example is as follows:
  All test conducted using the onboard calibrator at 100khz intervals. Both antenna and mixer coils were adjusted.
     80 meters prior to component replacement - barely 20 over
     80 meters after component replacement - 60 over 9
     40 meters prior to component replacement - barely S9
     40 meters after component replacement - 40 over 9
     20 meters prior to component replacement - barely S3
     20 meters after component replacement - 40 over 9
  15 and 10 meters also improved dramatically as well. On certain portion of the 80 meter band, the meter actually pegs. In addition, the audio output has also improved.
  Moral of the problem is:
  With Heathkit gear check every solder connection and reheat/solder if necessary, check for shorts on circuit boards or circuits, replace resistors if there is the slightest indication they are bad and realign the receiver upon completion. Capacitors can become leaky causing change in circuit operation. 
  One last thing. Check the screws that hold the feet to the case. I found the 1 of the rear foot screws was touching and, in fact, digging into the audio output transformer, not a good thing. It's obvious it's not Heath original, it's been cut in half. The other appeared to be rubbing on a wire from the antenna to a spare RCA jack. In short, check everything you can think of and then some. 
  I used a high intensity flash light on one side of the circuit boards and looked closely at the other. You would be surprised what you will find that normal visual inspection does not disclose. Remember, broken PC circuit board runs can possibly act intermittently or cause the piece of equipment to not work at all. The only way you'll find a very find crack or seperation is with a flash light shinning on one side while you look at the other. The other way is with an ohm meter checking each trace on the circuit board. Solder bridges, even the slightest skinny/thin wire, can cause problems especially in the input circuits where voltage is at a minimum such as IF amp input grids of less than 1 volt. Yes it's time consuming but the end result is a properly working receiver instead of just a nice cabinet with panel lights and a meter.

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