Sep 7, 2013

Mascot 719

Got a "new" Mascot power supply to my workbench

Click to enlarge

I didn't expect very much from it but actually it's quite accurate and good power supply after one modification and calibration. At the beginning analogue voltage meter had about 0.5-0.7V offset. That may even break things especially in low voltages (for ex. 4.0V instead of 3.3V).

Datasheet says that it have <1.0mV load regulation @ 15V output, <1.0mV line regulation @ 2A load current and ripple voltage should be under 0.3Vrms. Sense wires are also quite nice addition.

After some tests I opened it up and  found a few calibration potentiometers. None of them appeared to affect volt meters offset until I found a place of potentiometer that had been replaced with ordinary 81k ohm resistor (series with meters coil in voltage mode). That was quite easy to replace with a 100k potentiometer and now it seems to be bang on!

Click to enlarge
Considered also beefier components to zener regulator that seems to get really hot.
..but then thought that if it have worked that many years without any problems, it won't break anymore :) 

Before and after the mod:
Images before and after the calibration:
Click to enlarge
 Current reading seems to be OK.
Click to enlarge

Edit: One of the readers asked about PCB pictures. Download here