bryce@hoser.berkeley.edu (Bryce Nesbitt) (11/05/87)
In article <2153@gryphon.CTS.COM> bilbo@pnet02.cts.com (Bill Daggett) writes: > >...Through Supra I learned that ALL the PALs have floating pin 10s >that should be grounded.... > >...in some way with details on how the user can get their machines fixed - >either take it to a dealer or "do-it-yourself"? Or have my eyes and ears >been closed up tight? GRIN... Check for ear wax and have your glasses cleaned. This is a re-post. If anyone out there has an A1000 that does not have this fix, it would be wise to save this message. It won't be repeated again and you *will* eventually need to have the modification made. Subject: PALs,jumpers and expansion on the Amiga 1000. Keywords: C Ltd.,TI,MMI,bunk,bull As many of you are aware, a company called C Ltd. produces memory and hard disk products. They acknowledge that some of their products (and products from some other manufactures) will act unreliably on some Amiga 1000's, and that if you chain two or more of those (*unbuffered*) devices you run a greater risk of failure. Symptoms range from total refusal to work, to intermittent crashes to intermittent loss of Kickstart (machine crashes all the way to Kickstart). They recommend as a solution replacing two PAL chips inside the machine. My repair person friend, Bruce Takahasi, does not subscribe to this belief. Replacing the PALs has not produced any fantastic improvement in any of the cases that he has tracked. He does have an alternate solution... First, though, here is what C Ltd. has to say:: > ...This problem stems from the speed of the PAL chips used by Commodore in > the manufacture of the Amiga. Early Amigas used PALs manufactured by MMI, > and these chips seemed to work well in almost every case, but at some point > Commodore started using PALs made by Texas Instruments. The TI PALs, > though having the same rating as the MMI parts, simply do not perform as > well causing the problems as stated above. Replacing the parts with the > much faster 16L8B part (any manufacturer's "B" versions are fast enough) > solves the problem. We have researched this problem to 'death' and are > confidant both from analysis of the system with the Texas Instrument PALs > and from experimental analysis of the system with the faster 16L8B PALs > that we know what the problem is and how to fix it... > > ...So in conclusion, the best solution to the problem is replace the PALs. > By the way, if there is blame in this matter it is probably on the > shoulders of Texas Instruments and their PAL chips. > > E.J. Lippert II C Ltd. Wichita, KS (316) 267-6321 Bruce's explorations lead to a different solution. The key clue was noise. A high speed 'scope revealed that the ground pins of two of the four PALs on the RAM/ROM daughterboard had a very high 90 milivolts of noise. This is at the very least a potential cause of problems... and one that would not be solved directly by swapping PALs. (Swapping PALs may fix the symptoms, but would not cure the cause). The RAM/ROM daughterboard has 4 layers with one each for ground and +5. One might assume that all four grounds would be securely tied together. This turns out not to be the case. Under a bright light it is revealed that the two PALs with high noise are isolated from the main internal ground layer. The classic reason for isolating grounds is to isolate noise. In this case it appears that these isolated PALs have an insufficient ground. The solution that has worked in several cases is to improve the grounds for these PALs. The ground pins of each of the four PALs are connected, and a clip extends from the last PAL (U6J) down to the motherboard. Here's a diagram: ^ clip | | J K L N ---X---O X---O X---O X---O O---O O---O O---O O---O .......................... The X's indicate pins to be connected. The clip connects to ground on the motherboard at a point just to the left of the power supply connector (There is a row of 8 capacitors with ground on the side nearer the connector). One other modification is made in certain cases. A ground is extended from the negative end of a capacitor at the "diving board" end of the daughterboard to a capacitor in the chip ram array. Newer Amiga 1000's already have this modification from the factory and thus don't need it. |\ /| . Ack! (NAK, SOH, EOT) {o O} . bryce@hoser.berkeley.EDU -or- ucbvax!hoser!bryce (") U "Fanitic: One who can't change his mind, and won't change the subject."