Power supply checks

Step-by-step instructions

Estimated execution time: 0,5 hours

Temporary connect a power lead to the AC terminal block:

The voltage over both 
electrolytic capacitors (e-caps) should be around 25V DC. While measuring this DC voltage also check the polarity of the capacitors. Immediately power off the device if there is a mispolarisation.

Assemble the backplane and power supply. Power on and check if the led lights up. Immediately power off the device if the led stays dark:

Before proceeding to one of the following checks, double check the different DC voltages on the corresponding pins on the right side of the PCB:

If you notice any discrepancy on the negative supplies, please read the below note.

Expansion pinout overview:

Well done

Congratulations, you have now finished the power supply checks section, click here to navigate back to the assembly & adjustment instructions.

Note about measuring negative supplies

The 79xx voltage regulators series made by National Semiconductor (Texas Instruments) and licensees have to be connected to a load consuming at least 5 mA, otherwise the output voltage can be higher then intended. Oscillations are not the cause of the overvoltage problem. 

This unexpected behaviour is typically for the 79xx series, the internal structure of the 78xx voltage regulators is different, such a fault does not accur here. 

To perform the negative supplies measurements correctly, temporary connect a resistor (any value between 220 Ohm and 560 Ohm will work) across both -5 VDC and - 15 VDC to the mass plane, so there is an adequate load on both regulators. You should have the correct readings now. 

Important warning

A limited number of components will be connected to the mains voltage and can be dangerous if used incorrectly. It cannot be stressed enough to disconnect equipment before and servicing. Depending on local regulations, use approved connectors, wiring and insulation for the mains voltage connection. If you are unsure do not continu and contact a professional for assistance.


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