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List:       classiccmp
Subject:    Re: TRS-80 without floppy drive
From:       ard () p850ug1 ! demon ! co ! uk (Tony Duell)
Date:       2005-04-28 22:08:40
Message-ID: m1DRHBc-000IxtC () p850ug1
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> > One time when I powered up my Model 4, I got the unmistakable smell of
> > burning tantalum capacitor, and magic smoke came out. The darn thing
> > continued to boot and has worked fine ever since. I assume a decoupling
> > tant decided to go short-circuit, but I've not got round to pulling the
> > case to check.
> 
> Are you saying that it is OK to continue running a machine without even
> checking it out after such a malfunction?

Not unless you know what you are doing, and more importantly know you 
could put right any problems caused by said exploded capacitor. I do know 
the Model 3 and 4 pretty well (even if I say so myself), I've done 
component level repair on them, etc. I have all the schematics (including 
my only reverse-enginnered schematic of the PSU board, which is a lot 
more accurate than the one in the technical manual, which it seems was 
also reverse-engineered (based on the lack of identification of some 
components, lack of transformer winding details, etc)). In other words, 
if there are subsequent problems, I know what to do. 

A tantalum bead that exploded like this one did pretty much has to be a 
supply decoupling capacitor. In any other position, it would be able to 
pass enough current to blow up like that. Well, maybe if it was on the 
primary side of the SMPSU, but (a) if one of those shorted most likely 
that PSU would stop working and (b) from what I remmeber there are no 
tants on the PSU boards.

As the machine continued to boot, it was a fair bet that the capacitor 
was not shorting out any of the supply lines. It had burnt itself out. By 
the time I could have turned off the power it would most likely have done 
that. In which case there's little harm in running the machine with one 
capacitor effectively missing.

-tony
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