Repaired Metcal Mx-500 with parts laying around

[jonsig]

Good day.
Bought this metcal mx-500 older version from ebay, "not tested" . Ofc broken.. It was easy repair but i replaced the IRF530(Q4) with
IRF540 , increasing RDSon from 20mOhm to 60-80mOhm

100µF polyester 63V (C24) replaced with one of those monolithic ceramic capacitors from ebay (brown ones) "max" 50V

ZTX749 (Q1) => with 2SA966 lowering Ic 2A down to 1.5A
 (had to cross base - collector terminals on the to92)

And the mx-500 is back to life, but i´m not sure how long it will last. How bad is the monolithic cap as replacement ? i ´m hoping the to-92 wil have little headroom thou Ic capability is lower. IRF540 should generate more heat if i remember the datasheet correctly.

[1audio]

The cap may be stressed in that location. It has to pass some power and those monolythic caps change value with voltage, a lot. A good polyproplyene would be a better choice. The 2W 100 Ohm load suggests that there can be significant power so a cap rated for current may be required. ideally film and foil construction or an NPO/COG ceramic however that large and expensive for a .1U.

[jonsig]

Yes , replaced the c24 capacitor with film capacitor.  And replaced the old thermal goo with arctic mx-4 thermal paste. I have no idea if the semiconductors gain anything from that because of large contact area.