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Model 4P - Bang, then Smoke


New Member
Aug 30, 2016
I am hoping that someone can help me here.

This is my first post. Whether wisely or not I have decided in my retirement to start playing around with electronics so hope you will forgive my lack of knowledge etc. I am also in the UK to some terms might differ.

My first problem is with my Model 4P. I fired it up after having laid idle for many years. All was going well for a few minutes until when there was a bang with the associated smoke an smell. I disconnected it and stripped it down and discovered a blown capacitor. There are two others of similar make and suppose they should also be replaced. I am enclosing some pics to make it clearer.

From what i have read this type of cap are not recommended so could someone please suggest what they should be replaced with.

Sorry about the dust - that will be cleaned when I find a suitable brush.



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Al Kossow

Documentation Wizard
Sep 1, 2006
Silicon Valley
From what i have read this type of cap are not recommended so could someone please suggest what they should be replaced with.


answer was:

As user EM Fields already suggested, I have already half-answered your question in the linked EE.SE post. Short answer is: get an X2 or Y2 capacitor (see below), these are almost always MKP (metallized polypropylene) film capacitors. Make sure you use a value close or equal to the capacitor it's replacing. Note that both paper and film resistors have very large tolerances (20% is fairly standard over operating conditions), so it doesn't matter if the value is exactly spot-on.

Your application is mains filtering, which is not a precision or high-linearity application. You do not need to worry about the exact type of capacitor. All mains-rated capacitors have similar performance, and performance is basically only dependent on their capacitance value.

What you do need to worry about is that the capacitor is safe to use. If the capacitor is connected between live and neutral, you need to use an X rated capacitor, I recommend X2 to be compliant with 99% of countries' electrical safety regulations. If the capacitor is connected between protective earth (PE) and live or neutral, use Y rated capacitors.

DO NOT use ceramic capacitors. They have very unfavourable failure modes that can cause fire or electric shock hazard.


Veteran Member
Sep 9, 2013
Riverside, CA, USA
My memory may be off, but I seem to recall noticing that Astec used X rated caps in their power supplies which we encounter in the TRS-80 machines, even in one spot where I would have expected to see a Y rated cap. I have been replacing the Rifa caps with some Panasonic metallized poly film X rated caps. I know I've posted the part numbers I used on VCF at least once, but I can't find my post(s) by searching. This exact question comes up frequently enough that I should write up a blog post to point folks to.


Experienced Member
Feb 6, 2014
South Wales, United Kingdom
Hello there

i reside in the UK, South Wales and offer full repair, refurb and modification of all TRS80 systems.
i also carry many expansion addons.

for more details you can find examples of my work on my Facebook page, BetaGamma Computing.