SG503/Repairs

Service Notes

• Out of Range At About 1 MHz - Suspect U390 - Plessey brand biggest problems.
• No Output and Flashing Display - Suspect Oscillator circuits.
• Display Does Not Vary With Output - Suspect Auto Ranging Circuits Q410 , CR410 , and 2420.
• No Display Verify that 1432C - 10 is lo , a Hi will disable display.
• No Display - Except In 50 kHz Position:
  • 1 . Check for Reference Clock signal at U460D- 11.
  • 2 . If signal is okay suspect Auto Clock Enable circuits.
• 50 kHz Oscillator Has No Output - Suspect C134 has leakage.
• Certain Bands Are Defective - Suspect Band Selection switch , selected coil , or selected capacitor.
• One Band Flashes at Lo or High End of Band - Adjust core in selected coil , especially 25-50 MHz and 10-25 MHz ranges.
• Oscillator OK But No Output From Buffer Amplifier:
  • 1 . Suspect LR190 is open , if open you'll have -20 Volts on 0190 Collector.
  • 2 . If Q190 Collector is okay check for signal on cathode of CR225A. If signal is present suspect the Pk - Pk Detector is open or the attenuators are bad.
• To Determine If Your Oscillator Circuits Or Leveling Circuits are bad - Remove Q300 and connect one end of a 2K12 potentiometer to Q300's Emitter circuit and the wiper arm of the potentiometer to Q300's collector circuit. If the oscillator breaks into oscillations where varying the potentiometer, your leveling circuits are bad.
• Levelling Circuit Defective:
  • 1 . Check for +.7 Volts at 1280-2 and for @ 6 Volts at U280-6.
  • 2 . If +.7 Volts is not present check divider string R255 through R265.
• +5 Volt Supply Bad:
  • 1. Lift F620 and see if +5 Volts now reads @ +11.2 Volts
    • A. If +11.2 Volts is present your problem is external to supply.
    • B. If +11.2 Volts is absent your supply is bad.
  • 2. If 0600 or 2620 is defective change both.
• Readout Miscounts At Higher Frequencies - Suspect U390 , 0350 , or 1400 devices or sockets.
• Levelling Problems - Suspect U225 and 2190.
• Sinewave Flattens At Higher Frequencies - Suspect Q190 , CR200 , CR202 , VR200 , and VR202.
• Levelling Or Loading Problem On One Range Only - Suspect selected coil on coil circuit board.
• No Output On A Number Of Ranges - Suspect plates of C100 are shorting together.
• Sinewave Distorted, has Spikes or Ringing - Suspect Q160 is open.
• Distorted Waveform At 50 MHz And Above - Suspect C204 , C208 , and C212.
• Clipped Waveforms (Positive, Negative , or Both ) - Suspect CR200 , CR202 , VR200 , and VR202.
• Peak to Peak Detector (U255 ) Checkout - Using a DM501 on the 200 ohm range you should have 50 Ω in both directions across the signal path pins. When checking across the control pins you should have 50 Ω in one direction and in the reverse position.
• Levelling Problems Across More Than One Range - Suspect U225
• Amplitude potentiometer Has No Effect:
  • 1. Suspect U225 is cracked , when using ohm meter it may check good.
  • 2. Suspect CR216 and / or CR218 is open.
  • 3. Suspect open cable between P230 and P265.
  • 4. Suspect U280 or 0300 is open.
• No Output:
  • 1. Suspect L200 is shorting to shield.
  • 2. Suspect 4.5 Volt supply is shorted to ground which will cause 1630 to overheat.
• 50 kHz to 100 MHz Lo Band Oscillator , All Bands Not Operating properly - Suspect LR135 is broken , not the coil by the resistor.
• Display Is Blank Except Left Hand Decimal Point - Suspect Clock Oscillator and associated Dividers plus Enabling Gates.
• Display Miscounts When Unit Is Hot:
  • 1 . Suspect Pre- scaler I.C.'s , U390 or 0400 , if over 1 MHz.
  • 2. Suspect 7490 I.C.'s if below 1 MHz.
• The Display Has One or More Digits Reading Zero With No Change In Frequency While Remaining Digits May Be Free-Running - Suspect 1430, 4432, 0435, 1436, or U437.
• No Display - Suspect Q290 , Q296 , or R475 is open.
• Display Is All Zeros With No Variations - Suspect L660 is open.
• Display Has One Or More Digits Blanked, But Not All, and Remaining Digits Normal - Suspect 1510, 1520, and 1530.
• Binding Range Switch - Coil circuit board or wafer switch on coil circuit board needs replacement.
• Display at @130 - 160 MHz, Goes To A11 Zeros And Stays There - Suspect 1390 or 1400, maybe heat sensitive.
• Unit Normal Except In Reference Positions Where Display Free Runs - Suspect defective cable between P280 and P290.
• Replacing L110 - If L110 requires replacement the old tuning slug should be used in the new coil. Otherwise, you may not be able to adjust the new coil to meet distortion and/ or levelling in the 25 to 50 MHz Band.
• Normal Tuning Slug Positions In Coils:
  • L116 Slug set to bottom resonance
  • L114 Slug set to top resonance
  • L112 Slug set to bottom
  • L110 Slug set to top
  • L100 Slug set to top