Sep 7, 2013

Mascot 719

Got a "new" Mascot power supply to my workbench

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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!

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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:
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 Current reading seems to be OK.
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Edit: One of the readers asked about PCB pictures. Download here

5 comments:

  1. Fine method of telling, and enjoyable article to acquire factual statements. here

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  2. Hi, sorry my bad English.
    Recommend that you put capacitors on the outputs of the unit, is sensitive to HF.
    Volt increase when exposed to HF warning.
    https://en.wikipedia.org/wiki/Electromagnetic_compatibility

    Do you have more pictures so I can see the components present in (burnt some components)
    access to the schedule?

    Damaged components, burned down.
    Resistors R5
    Transistor T2

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    Replies
    1. Hey, I took some pictures and added them to the original blog post! R5 seems to be brown-black-brown = 100 ohm and T2 is BD136.

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    2. Sorry, there seems to be some problem with sharing .zip from google drive. It requires access, although I set "anyone with the link can view". I'll take a look about that tomorrow.

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    3. Ok, now it works! :) Changed from Google drive to Dropbox (no login required)

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