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REPAIRING THE SL1200 POWER SUPPLY
The stock power supply should deliver approx 20-21 volts. Aside from a dead turntable, the most obvious indicator of a bad power supply is the strobe lamp flashing each time you hit start/stop or when you apply pressure to the rotating platter. You can choose to service the existing supply, which is time consuming or you can simply upgrade and replace the entire supply with the KAB DC-1200 replacment 20V regulator board. Labor cost wise, it may be cheaper to upgrade the board. Servicing is not complicated. it is a simple basic 3 transistor regulator. It is a reliable circuit, but it is not short circuit proof. And usually that is how it gets damaged. Probing around or careless replacement of the target lamp are the usual causes of PS failure. The main power transistor rarely fails. and the usual cuprit is the to small transistor Q2. (2SD637-QRS).
You can read more about the DC-1200 here.