OT: new info on fuse

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Sun Dec 28 20:24:34 2003

>
> Just got back from Radio Shack. I bought a fast blowing 250v 2 amp fuse
> and installed on in the laserdisc player. Unfortunetly, it blows just
> as fast as the .75amp fuse. So.... that means I have a dead short
> somewhere, right? How do I go about checking that?

Can you tell us a little about the PSU design? Is it a 'linear' PSU, with
an iron-cored mains-frequecy transformer? Or a switcher, with HV
capacitors (look for a couple rated at 200V each) and a chopper transistor?

If the former, can you disconnect the secondary windings of the
transformer from the PCB? If you can, then try a new fuse with the
windings disconnected. If that blows, then you have shorted turns in the
transformer, or a short in some component on the mains input side (RF
filter capacitor?).

If it's a switcher, then you have fun (for suitable values of 'fun'. You
need to check forwards from the mains input and at least find the mains
rectifier diodes and the chopper transistor. Check these for shorts. If
the transistor is OK, and one or more of the diodes is shorted, then
replace the diodes (use something rated at _at least_ 400V 2A) and try
again. If the chopper is shorted, well, you've probably got other
problems too, and _I_ wouldn't want to do much more without a schematic...

-tony
Received on Sun Dec 28 2003 - 20:24:34 GMT