I'm new to forums, but found this forum while also seeking help fixing my Micronta 22-121. I'm an electronics novice, and hope you'll bare with my lack of knowledge of forum operation and electronics. I have downlaoded the service manual & circuit diagram, but the service manual doesn't mention my probem- the unit humps when I turn it on and then blows the fuse within a couple of seconds.

Also any guidance on where I can learn the proper netiquette for forums- like what are these icons I see below for and what do they mean if I use one?

thanks in advance

 

Hi there giantk, and welcome to the forums.

Unfortunately, looks like you used your E-mail address as a login. This isn't good, as it'll most likely result in tons of SPAM once the spambots find you.

I suggest that you delete your above post, delete your current account, and sign up with a new one that does not divulge your E-mail address.

What do you mean by "humps"?

Use of icons (smilies) in posts is optional; you're limited to four or five per post.

There's a toolbar above the text box area. I almost never use the text formatting options, as it makes quoting more difficult.

If by "humps" you mean that the voltage is going up and down rapidly before the fuse blows, that sounds like it's oscillating. Failed capacitor(s) would be a likely cause of that.

Unplug the unit from mains power, then open the case. Examine it carefully, see if anything looks burned, or if any part looks like fluid leaked out of it, or if the tops of capacitors look bulged or ruptured.

If you can take good photos of it and post them using the "Go Advanced" and "Manage Attachments" or upload them to Tinypic.com and resize them to 640x480, that would help us to help you.

Getting good lighting for taking pics of electronic circuit boards isn't real easy. Flash doesn't work at all; the resulting images are so harsh it's like black and white. Trying to take photos indoors using ambient lighting doesn't work well either, the photos are dark and muddy looking. The best lighting you can get is outside on a cloudy day; the lighting is very soft and even. Next best is outside on a bright day, with a white sheet draped over you and your subject.

A frequently-made mistake is trying to get too close to the subject, resulting in it being out of focus. Unless it's sharply focused, it's not much help.

 

I moved this to its own thread, as it may diverge from the original one - http://forum.allaboutcircuits.com/showthread.php?t=39287&page=2

I might read that as "hums", which suggests something like a bad filter capacitor, as the sergeant has mentioned.

 

"hums" - of course. I feel like a rock.

The most likely candidates are C1,C2, C4, C6, then C3, C5.
If you can easily remove Q1 and Q2, that would help in troubleshooting by eliminating much of the regulator circuit.

Schematic:

 

Why not temporarily lift one side of D2, D5 and D6 to determine firstly if its the "A" side or the "B" side that's bad?