IBM 5155 analogue display fault

[Tony Duell]

> Indeed there isn't.  Anybody who stares long enough at the 'No User
> Servicible Parts Inside' labeling on an IBM XT power supply, then makes
> the effort to find the right tool to open it up and finds the socketed
> 3AG fuse inside will know just a little bit more about IBM.

Well, I went further than that. I took the PSU apart, took out the PCBs, 
and traced out the schematics (along with schematics of the other 
'misisng' bits from the Techrefs, the PC/XT/AT supplies, the CGA monitor 
supply, the EGA monitor, etc). 

The Portable PSU is clearly a Zenith design, and is very similar to the 
130W XT supply. 

Now, as to that fuse. If it fails, there is a good reason for it. 
Probably a shorted semiconductor on the mains side of the supply. It is 
very unlikely that just replacing the fuse will do anything except blow 
another fuse. And the average user, alas, when he finds a machine that 
blows fuses will then put in the highest-rated fuse he can find, and 
probably do a lot more damage. There are good reasons, IMHO, to put SMPSU 
primary-side fuses away from normal users :-)

Of course if the user is me, then that label is total nonsense anyway. I 
will hapilly replace chopper transsitors, etc...

Or is it? When I'm working on the machine, it's broken, and therefore I 
can't be using it. So I am not a user :-)

-tony