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Old 17 June 2011, 18:59   #1
StLovell
 
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A500 Rev 3 help needed

I have an early revision 3 A500 that I'm trying to resurrect. Looking for any advice people can offer.

Initially all I got from it was a green screen and a 10/1 code blinking on the power light. There were no hot RAM chips and all the custom chips/CPU/Agnus have been tested in other boards.
I found that attaching a 501 in the trap door changed this to yellow but the 10/1 cycle continued.

I eventually got the board to boot by piggybacking another Ram chip over one which seemed to be bad. This however would only keep the machine stable for around 5 minutes before it crashes.
The capacitor for that chip seems to be dead but changing it for one of what I believe is the same value achieved nothing.

Any thoughts?
 
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Old 17 June 2011, 20:59   #2
Jope
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Replace the defective RAM chip that you found by piggybacking.
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Old 13 October 2013, 01:29   #3
jmvihtanen
 
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A fix to green screen / 10 + 1 blinks error

Ok, I will tell what I made to repair the A500 rev. 5 ROM ver 1.2.

I opened the cover. I powered the computer using my own power supply that I knew to be in condition and waited for 5 minutes. I measured the temperature of the 16 RAM chips on U16 - U31 using my fingertips. The chip on U24 was warmer than the other. I turned off the power. I desoldered a Texas Instruments 2x8 pin 100ns DRAM chip from a circuit board that was thrown away. I desoldered the original 150 ns Samsung chip on U24 from the Amiga motherboard. Note that the TI DRAM chip was faster or as fast as the original. I took the original chip away from the board using a screwdriver. On the same time I accidentally damaged the bus because the chip legs weren't completely apart from the bus and they pulled some bus wires from the motherboard. I repaired the bus using thin copper wires that I soldered in the place of the damaged bus. Other ends were soldered to the bus and other ends put to the holes. I checked the fix using a multimeter that beeped to indicate that the bus was in condition. This was done by putting the other electrode of the multimeter to a hole and the other electrode to the respective pin of a neighbouring chip. This rule had a couple of exceptions as the bus wasn't originally planned to conduct that way. Then I put the TI chip to U24 and twisted the copper wires around the chip legs from the back side of the motherboard. I soldered those pins that didn't conduct otherwise. In addition I had to solder the pin 16 (enumeration from upper left corner to lower right corner one row at a time with respect to the text on the chip). That was even though the multimeter beeped. However I noticed that the circuit conducted also through other chips and I that's why couldn't become convinced that the power pin 16 was connected to the bus. I pushed the hole 16 from a 45 degrees angle so that it touched both the bus and the leg. I powered the computer. The led started to light continuously after a second and the floppy drive started to check the presence of a disk. I turned off the power and soldered the pin 16. Then I checked the video output using a TV and it showed the hand holding a floppy -picture. I put the cover back.

Last edited by jmvihtanen; 24 October 2013 at 17:22. Reason: CORRECTION OF CHIP POSITION NUMBER
 
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