AM on SB-300

[Michael Waldrop]

I've received a few request for schematic or exact wiring changes to allow an SB-300 to receive and produce audio on AM without an AM filter. 
   
  First, let me say again, the audio is bassy and this wiring change does not eliminate the need for an AM filter. This wiring change will allow you to hear AM audio if you do not have an AM filter, nothing more nothing less.
   
  DO NOT PEFORM THIS WIRING CHANGE WITH THE POWER CABLE PLUGGED INTO THE RECEIVER. UNPLUG ALL CABLES BEFORE ATTEMPTING THIS CHANGE
   
  Do not remove, change or otherwise modify any wiring to any wafer switch other than MS-1, the first mode switch wafer from the front panel. Each wafer has a "F" (front) and "R" (rear) section. MS-1 selects the desired filter, LSB/USB (same filter), CW or AM.  
   
  First, the circuit boards are as follows:
   
  Board in the middle of the receiver is the RF board. 
  Board on the left side of the receiver is the IF board
  The first IF amplifier is V6, a 6BA6, located closest to the front panel.
   
  Now, with the receiver out of its case, sitting on it's left side and the chassis facing you, locate the mode switches first wafer from the front panel. This is mode switch wafer "MS-1". Locate the pin closest to the chassis on the front of MS-1. This is pin 1. There is a bare wire soldered from pin 1 of MS-1 to a ground lug attached to the "Z" shaped shield.  Unsolder this wire at the ground lug. Reroute this unsoldered end to pin 12 on the rear of MS-1. Pin 12 is located on the rear of MS-1 closest to the "IF board". Pin 12 also has a wire going to the IF board at point "C". Tack solder the bare wire to pin 12. Do not, repeat DO NOT, remove the wire from pin 12 to point "C" on the IF board. 
   
  This completes the wiring change.
   
  Put the receiver back in its case and reconnect all cables. After warm up, tune to 7.435 or so and you should find a rather strong AM station. Turn the mode switch to "AM". Tuning in the station takes a bit of practice but you will be able to hear the audio from the AM station fairly well. 
   
  As I stated in the begining, this wiring change is not, in any way, a replacement for an proper AM filter. The reproduced audio is bassy and take practice to tune it in. With a proper AM filter, tuning is easy and the audio is much better, in fact allot better. 
   
  The SB-301 already has this wiring change done therefore, no wiring change is needed on the SB-301.
   
  You might ask yourself, I did at least, "Why have a ground on the filter output?". Honestly, I can't answer that. What I can say is, the signal in an unmodified SB-300 coming from the output side of the filter is capacitive coupled to the input of the first IF amplifier using the wafer switches. There is no direct wire connection between MS-1 front pin 1 and MS-1 rear pin 12. By rerouting this wire you are bypassing the capacitive coupling  through the switch. I have tested this with or without the wire from pin 1 to pin 12 of MS-1 with no difference in operation. However, the ground on pin 1 of MS-1 must be removed otherwise there is no audio output in AM mode without an AM filter. 
   
  I have had this wiring change in my SB-300 for awhile now without any problems. If you have both schmatics, SB-300 and SB-301, you will notice both receivers have the same wiring except for pin 1 of MS-1 is grounded in the SB-300 but not so in the SB-301.
   
   I hope to find an AM filter someday but until then, I'll use this wiring change. If you later install an AM filter, you do not have to remove the wire simply install the AM filter.
   
  73's,
   
  Mike
  W5RKL
   

		
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