English Amiga Board - View Single Post - Found my old A500 again, and it is nearly running

Shadowfire · 13 November 2018, 03:54

I tried to hunt down a copy of the Rev5 schematic to see if the circuit is any different, but no dice. Good news is, I have a working Rev 5 A500 here to measure and walk you through it.

http://amiga.resource.cx/photos/phot...res=hi&lang=en

shows a picture of the rev 5 PCB. RP401 is on the right hand side (further to the right, and slightly above, RP404). You need to take off the floppy drive to see it. In this picture it is a red component; for what it's worth, on my machine it is blue.

A little less than 10K is the correct measurement. This confirms that R401 (4.7K current limiting on the +5V pin of the joystick port) and RP401 (4.7K pullups to +5V) are good on joystick pins 2-4, however, 1 Meg means that you have at least one fault on the motherboard.

Once you have removed the drive and have RP401 exposed, perform the following checks:

Inspect the rear side of the joystick port, EMI421, RP401, and U15 for foreign bodies, solder bridges etc. which might be shorting pins together.

Measure from joystick pin 1 to EMI421 (side nearest the joystick port). You should get <1 ohm.
Measure across EMI421. You should get <1 ohm.
Measure from EMI421 to RP401 pin 6. (RP401 pin 1 is nearest to the right side of the board). You should get <1 ohm. (* this is the one I think is going to fail)
Measure from RP401 pin 1 to RP401 pin 6. You should get around 4.7K.
Measure from EMI421 to U15 pin 11. You should get <1 ohm. U15 is the 74LS157, not the 74LS38.

Get back with which measurement(s) failed.

13 November 2018, 03:54	#10
Shadowfire Registered User Join Date: Aug 2001 Location: Connecticut USA Posts: 617	I tried to hunt down a copy of the Rev5 schematic to see if the circuit is any different, but no dice. Good news is, I have a working Rev 5 A500 here to measure and walk you through it. http://amiga.resource.cx/photos/phot...res=hi&lang=en shows a picture of the rev 5 PCB. RP401 is on the right hand side (further to the right, and slightly above, RP404). You need to take off the floppy drive to see it. In this picture it is a red component; for what it's worth, on my machine it is blue. A little less than 10K is the correct measurement. This confirms that R401 (4.7K current limiting on the +5V pin of the joystick port) and RP401 (4.7K pullups to +5V) are good on joystick pins 2-4, however, 1 Meg means that you have at least one fault on the motherboard. Once you have removed the drive and have RP401 exposed, perform the following checks: Inspect the rear side of the joystick port, EMI421, RP401, and U15 for foreign bodies, solder bridges etc. which might be shorting pins together. Measure from joystick pin 1 to EMI421 (side nearest the joystick port). You should get <1 ohm. Measure across EMI421. You should get <1 ohm. Measure from EMI421 to RP401 pin 6. (RP401 pin 1 is nearest to the right side of the board). You should get <1 ohm. (* this is the one I think is going to fail) Measure from RP401 pin 1 to RP401 pin 6. You should get around 4.7K. Measure from EMI421 to U15 pin 11. You should get <1 ohm. U15 is the 74LS157, not the 74LS38. Get back with which measurement(s) failed. Last edited by Shadowfire; 13 November 2018 at 04:23.