• Power Supply Schmatic and E-Cap Tests and Fixes

    Lauri Pirttiaho • 09/03/2026 at 18:46 • 0 comments

    To be able to verify and potentially repair a power supply sometimes requires figuring out the schematic in detail. For simple linear and switching regulator designs that follow the example circuits given in datasheets and application notes, that may not be necessary. This power supply appeared to contain some circuitry that I'm not familiar with, so the next step was to draw out the schema, with KiCad.

    Before plugging in the AC it is advisable to verify that each part of the circuit is in working order. Especially the electrolytic capacitors of this age (date code 1977 week 21 for C9, for example) are know to be often, though not always, severely degraded. In this case I used multiple adjustable lab power supplies to feed each of the circuits at the output side of the rectifiers.

    With some luck, all could have been just fine, but as often, some are not. The first step in the testing is to limit the current to something like 20-50 mA and slowly rise the voltage to the expected maximum, which can be determined from the specifications of the components, electrolytics and semiconductors. 

    In this case the first ramping of the capacitor C9 to 9V went fine, but on the next ramp it shortcircuited. Without current limiting much more damage would have occurred. Others seemed to be OK on that firs testing, but because of the one major failure I decided to desolder all e-caps and test them individually. 

    Each cap was first slowly ramped to the maximum working voltage without exceeding the charging current on 20 mA and held there until the leakage current went below the usual maximum leakage current CV/4000s (for 5300µF/25V that is 33 µA). The voltage was then mainatained for 10 min. Then the procedure was repeated.

    All remaining caps passed that test.

    After the leakage test each cap was measured with LCR meter (100Hz) for capacitance and Q. Requirements set was that capacitance was at least 80% of the nominal and Q above 10 (my arbitrary choice for this kind of a quality check). One capacitor, C7, had less than 40% left of the nominal capacitance. That had to be changed. C4, the other large cap, had 87% of the capacitance but Q was only 2. That indicates large ESR and that will need to be changed soon, too. Others were withing the requirements.

    Lacking an axial cap for replacing C9, two 4700µF/25V radial caps were soldered to a trip boar in parallel and the combination was soldered to the place of C9; attached with two-sided tape and a zip tie.

    After these fixes the PS appears to work correctly. The tracking section was tested by taking the tracking input from another channel of a power supply and the output voltage followed the input as expected. To understand the circuit correctly it was simulated with LTSpice (simulation files are in the attached zip file tracking-ps.zip).

    The next step is to begin studying the main board. At first by powering also it from external current-limited lab supplies.

  • Restoration of the power supply

    Lauri Pirttiaho • 08/31/2026 at 16:59 • 0 comments

    The restoration of a computer often begins with the power supply. It is critical not to turn on the potentially degraded and fragile device until the safety of the power supply and the other circuitry has been verified to not contain shorted components, especially the electrolytic and other capacitors. There is also a chance there are degraded solderings which may trigger a major destruction rare or unobtainable components.

    The first step in this project was to take out the power supply and verify that it is wired for the correct voltage (220V for Europe). 

    Though a bit unorthodox, also the PH-163 power cord receptacle was changed to a modern IEC C8 one for which cables are available. 

    The original socket will be saved for a chance that the fitting plug becomes available.

    The next step is to get familiar with the power supply. Though the CDP18S030 manual has the schematics and component layout of the logic board, the only thing about the power supply is the instructions for the voltage selector wiring.

    Therefore the next step is to draw the circuit diagram of the power supply.