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Sharp CS-20A - Power Supply Faults


Large Ripple, Strange Frequency

The power supply was straightforward to reverse engineer and produce a schematic. Points to note:

Setup
  • Low voltage side is floating
    • This can be helpful in future
  • Negative supply regulator circuit partly in the main card cage
    • So plan to power up with supply connected to main cage

The electrolytics are sometimes easy to measure but in this case they are soldered to heavy tagstrip terminals.
Desoldering will be difficult and the entire supply circuit will tend to fall apart.
So start power up at 10 % of mains, watch carefully and increase by same amount every few minutes.
Surviving capacitors will get a chance to reform while other defects should declare themselves as early as possible.

Setup for power up:

  • Variac with meter on transformer primary
  • Voltmeter to monitor DC level of + and - rails
  • Scope AC coupled to assess ripple on + and -

First power up at 10%, no smoke but assymetric + and - rails.

Increase to 20% and find:

  • Positive rail 890mV ripple at 50Hz
    • large ripple and half of expected freuencey
  • Negative rail 113mV ripple at 50Hz with odd stepped waveform
    • 100Hz is discernable but one half cycle is reduced

 

 

Every five minutes or so, increase supply voltage until 50% mains.

  • Positive ripple 1.67V and frequency now 100Hz
  • Negative ripple 319mV, stepped waveform same

Interpretation

  • Bridge rectifier with unusual failure
    • positive side has one voltage-dependant diode
    • negative side has one partly failed diode
  • Filter caps low capacity/failed, likely worse on + rail

Plan

  • Replace bridge rectifier
  • Remove, test and replace caps as required