Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!unix.cis.pitt.edu!dsinc!bagate!cbmvax!daveh From: daveh@cbmvax.commodore.com (Dave Haynie) Newsgroups: comp.sys.amiga.hardware Subject: Re: Help! My 2000 motherboard is dead Keywords: dead motherboard megachip Message-ID: <21594@cbmvax.commodore.com> Date: 15 May 91 15:51:22 GMT References: <1991May13.031522.28822@mtu.edu> Reply-To: daveh@cbmvax.commodore.com (Dave Haynie) Distribution: usa Organization: Commodore, West Chester, PA Lines: 98 In article <1991May13.031522.28822@mtu.edu> cdh@mtu.edu (Chris Hooper) writes: >My Amiga is dead. >Problem/Symptoms: > A diode was burned out on the motherboard (actually burned clean > through). That's a real bad sign. It takes a lot of current to burn through a power diode. What the heck was connected to this machine? > It was replaced (1N4001) located near the power supply connector to the > motherboard. A technician at the company that sells the Megachip said What's a megachip? > diaode D400 is on the +12 power input. D400 is a "protection" diode between "+12V", the internal +12V supply, and "+12V_USER", which is the +12V supply that goes to your video and serial connectors. Damage to this diode indicates that you did something horrible to either the video or serial port. Often, plugging in a connector while the system is on can cause pins to short. There's no certainty that this diode was the only thing zapped, of course. > He then told me that the particular revision motherboard I have > (B2000 Rev 4.3) is missing a capacitor (C908) which is needed for the > Megachip board to work. He stated this capacitor is listed as 100pf, > but recommeded 70pf. I still have yet to put this capacitor in. On the B2000 motherboard, 900-series capacitors were additions made for FCC purposes. Sometimes they would be required for one revision of something, not for another. They should not have any effect on any proper expansion device (which I already gather this "Megachip" isn't). > White screen on power up (with *no* boards in machine, no floppy > in drive). Goes through gray colors, but when it should come up > with a work bench prompt, stays white instead. > Floppy drive clicks like it is waiting for a disk. > When disk is inserted, floppy stops clicking, however, access light > does not come on drive and drive does not spin. Pull the disk out > and the drive starts clicking again. Whatever zapped your +12_USER supply could very well have zapped something else. If you zapped it through the serial port, it's possible, though not real likely, that the 8520 CIA chip at U301 was also damaged. That would affect floppy and parallel port operation, along with some timing functions (this is the CIA timed off of HSYNC). If you zapped it through your video port, you could have damaged the Agnus or Denise chips. Agnus, of course, is the RGA/Chip bus master, and nothing works if there's a real problem with Agnus. It's really impossible to tell what actually is wrong over the net, you need a good repair technician. Also, since I gather this "Megachip" thing is plugging down into chip sockets, not a proper expansion bus slot, check out the damage that may have caused. Check any chips that you removed and make sure they are in correctly and don't have any pins bent. Check out the sockets, most "tower" type plug ins use Augat type sockets as their "feet". These sockets can bend the pins of the socket they go into to the point where the smaller pins of an IC no longer make good contact. > Most of my attempts at booting, however, ended up with the machine just > stopping completely before it finished booting. By stopping, I mean no > activity; CPU halted or waiting for something. From what you said, I expect that the CPU itself is OK. It's probably one of the custom chips, either Agnus, Denise, or that 8520, having a problem. My best guess would be Agnus. This is just a guess, though. To check out Agnus activity, you could probe around and look for RGA bus activity during non-CPU cycles, but you really need a logic analyzer to get real far with that, or at least a 'scope; a logic probe isn't going to help. > At this point, I pulled my machine all apart and removed the > Megachip. I replaced the original Agnes back into its socket. > When I reassembled the machine, the screen did not return! Either your pulling on the socket, or the presence of this Megachip thing, may have messed with the Agnus socket. These sockets are designed to hold chips, not tower assemblys, so as I mentioned previously, it could be that the tower bent back the socket's pins far enough to no longer allow the use of the socket as a normal chip carrier. Or perhaps you bent the pins in removing the original Agnus. In either case, that could explain some of your problems, but it doesn't have anything directly to do with D400 getting zapped, since Agnus doesn't use +12V, and in fact, all motherboard use of +12V would come straight from the +12V supply, not +12V_USER. > Sacrificing the neighbor's cat. (Just kidding, all you > animal lovers out there) Nope, cat sacrifices aggrevate Amiga problems, though they have been known to fix IBMs. You need to sacrifice a Marketroid to fix an Amiga, if you're looking for shortcuts. Actually, though, it's already too late, that only works within a week of the vernal equinox, as I recall... >- Chris Hooper Computing Technology Services Consultant -- Dave Haynie Commodore-Amiga (Amiga 3000) "The Crew That Never Rests" {uunet|pyramid|rutgers}!cbmvax!daveh PLINK: hazy BIX: hazy "That's me in the corner, that's me in the spotlight" -R.E.M.