ebay power supply

[Bodge]

From all the great work done a lot earlier (Many thanks guys)... we could calculate the replacement for R34 this way :-

Given that the PSU spec says "If the PSU is out of regulation then Uout=42.8+/-0.5V DC."

Taking the potential divider made from R34 and R33 is 9K1 and 1k with a +/- 1% tolerance

Applying 42.8V to this divider gives us a Junction voltage of 4.238 +/- 1%

Calculated by (R1(1k)/(R1(1K)+R2(9.1K))* 42.8V = V divider = (4.238V)

So rough calculations for 49V +/- 0.5 we need a resistor R2 of 10K561 or 10.561K (which ever standard you prefer)

The nearest we I could get with standard values (1/R1)+(1/R2)+(1/R3)=(1/Rtotal) was (1/22K)+(1/22k)+(1/270K) = 10.569K or 10K569 (which ever standard you prefer)


Thus this should give an output voltage of 49.025 +/- 0.5V eg +/- 1%


To change R34 for the 3 parallel resistors this was my process.

Warning make sure the PSU has been off for a good time as the PSU capacitors hold High voltages for a considerable time 400v DC is a skin splitting voltage and quite a Deadly!! thing to touch.

Remove the 5 outer screws, remove the cover, Unscrew the top board another 4 screws, disconnect the connector connector on the end unscrew the two screws holing the grey i/o connector disconnect the mains link connector and the other 4way connector

Remove the two PCBS.

Un screw the two screws holding the Heat sink down unscrew the screw that you can reach on Diode de solder the other diode remove the heat sink.
De-solder R34 replace it with the new resistor made from 2 x 22k 1/4W 1% and the 270k 1/4W 1% resistors connected in parallel re-solder R34 Re-assemble in reverse order...
Now to test it.. the result of this change gave me an output voltage of 48.9V very slightly out from my calculated value but well within the +/- 1% tolerance on the resistors used.

excuse dyslexic spelling


pictures to follow.