From: Thomas Weeks To: Rich Howley Date: Mon, 13 Feb 1978 04:48:30 +0500 Subject: Re: Good Amiga gone bad Around 20-Feb-98, Rich Howley typed: >Hello Tom, >I thought you might be able to help me with an Amiga problem. No, I am >sure you can help. In fact, you could probably diagnose this with your >good arm tied behind your back. Well.. I wouldn't get OVERLY confident there buddy... I have made mistakes... ;v) >The computer is an A2000. >The trouble began when a Super Gen was unplugged and the plugged into >the video port with the computer on (I know, I know). I'm sure you are >familiar with Super Gen. Now there is no video (just diagonal lines) >and the power light pulses bright and dim when the Super Gen is plugged >into the video port and the monitor is plugged into the Super Gen. Ok.. go on... >If I plug the monitor into the video port directly then it works fine. I >have used another A2000 with the same Super Gen and it works. I have >used the same A2000 with a different Super Gen and it does not work. Good diagnosing. >So what got fried? Well.. NOTHING's 100% in this business.. but it SOUNDS like you may have well blown your RGB Port's Pico Fuse.... In short... some Video devices that run off the RGB port make use of it's 12V_USER line for power... To protect your power supply (and system all together) Commodore made the wise choice of adding a small motherboard mounted fuse called the pico fuse to their later revisions of the A2000 (early revs did not have this protection and purple smoke resulted). It should look like a small axial capacitor (blue, yellow, or green) but instead of having cap markings, you should see FUSE or F# silk screened next to it and see a current rating printed on the fuse its self (500-2000mA roughly, I would go with 1000mA since the 12V_user is routed through a 1N4001). I think most revs of the A2000 that have this fuse use F5 to identify it on the silk screening. I have seen many people blow these devices by plugging in "active" or incorrect devices to the RBG port. If you find that you have such a device (located at the rear of the mobo, under the PS, by the RGB connector) you can TEST it (with all power off and unplugged) by doing a simple continuity test on it. Shorted, it is ok, open, it is blown (check carefully...decoupling caps normally register as open to a continuity check!). You can get them at most big electronics parts stores... Digikey has them also (http://www.digikey.com)... or.. it looks like you can even pick one up at Rat Shack now too! (called a fast acting submini axial-lead fuse RS part# RSU11322864) although it is a 1.5A device... In a pinch, I HAVE substitued NORMAL micro fuse (5x20mm) like RS's #270-1049 with CAREFULLY soldered on leads. Looks like RS even has some with leads on them.. but don't use slow blow.. only fast acting. In the past, I got MINE through Digikey.. but they have a minimum order I believe. If the pico fuse is NOT your problem, next, check to make sure your 12V_USER diode is good (D400 on Rev 6 Mobo's).. if not that.. then start checking your decoupling caps around your RGB port (I have seen them go too...(they tend to break down and short if bad)). Hope all this helps! >- rich Tom D Tek -- __________________________________________________________________________ / Thomas W. Weeks |Team AMIGA Commodore Amiga 500/030 40MHz \ | A&M EET/Telecom Grad '95 | /// Hacked Into Black Tower Case, | | Authorized Amiga Tech (7yrs) | __ /// 9M RAM, SyQuest 44M, 700M-o-SCSI | | Contact Me @ | \\\/// IBM 286 Hardware Emulator, | | o------------o | \XX/ Home Brew Audio A/D, HP48GX | | Email: tweeks@texas.net |"Amiga, The Computer for the Creative Mind"TM| |Web:lonestar.texas.net/~tweeks|"Mac, The Computer for the Rest of Us"TM | \_____________________________|____________________________________________/