Originally Posted by PeteJ (Post 974669)

thanks, I should perhaps test the resistors in the video circuit next with the multermeter to see if one or more has failed. I will have a look at the 600's circuit diagram and try to understand what I'm looking at :)

No I've not replaced the NE555, I have replaced the failed capacitor there now that was causing the reset loop. When I googled the NE555 I read that it was a timer so I thought with the reset loop fault being caused by the bad cap there that this was used just for the systems startup / reset timing?

Originally Posted by FOL (Post 974675)

I would put it back on. Your trying to find an issue but you have parts removed.

Originally Posted by PeteJ (Post 974678)

Hi, I've not removed the NE555, it was just the bad cap there that I removed originally but that's now been replaced.

Originally Posted by FOL (Post 974703)

Spose its a good thing its effecting all outputs. Gives you a place to focus on. You could check U31, as your missing green.
Maybe swap U31 and U32 around see if issue switches to another colour.

Originally Posted by PeteJ (Post 974876)

I can't seem to find any information about the (PC)74HCT244T or a datasheet. The 74HCT244D is readily available to buy from various sources. I don't know what the difference of the two are though and whether the 74HCT244D will work in the Amiga. I'll see if I can find a hi res photo online of other A600 motherboards and see what is installed on them.

Originally Posted by Jope (Post 975064)

The crucial part here is 74HCT244 .. if that matches and the chip is in the same package, then it will work.

Originally Posted by jimbob (Post 975180)

I would be suspecting the circuit of Q212 before U31. Definitely check it out before attempting to remove SMD chips.

I'm guessing you don't have a scope but with just a multimeter you can make a good stab at diagnosing a faulty transistor. With the board powered off measure the resistance and continuity/diode reading of each transistor terminal to ground and to the other terminals.

Compare these results with the same measurements of the equivalant circuit for red and blue ie. Q211 and Q213. If any measurement is a long way off from its red/blue partner you probably have localised the fault to this transistor circuit. In particular, the diode measurement between terminals should be in the ballpark of the known working circuits, pay attention to meter lead polarity when making this measurement. It could still be the transistor itself or one the passive components around it.

Then you have to find which one, you could for example check that R212 is not open or short circuit, or you could bridge C212 with a leaded capacitor, (any value ~0.1uF-1 uF will do), to see if the green flashes up, that will tell you that C212 is open circuit, etc.

(Sorry if this doesn't make sense to you, bit tricky to write detailed instructions as I don't have any of my amiga or electronics stuff with me just now)

You can also compare the voltage on each terminal of Q212 with Q211/Q213 when the board is powered on. Do this with a steady white or black screen if possible.

In case you don't have them, best available A600 schematics can be found here -

http://www.amigawiki.org/doku.php?id...ice:schematics

Originally Posted by jimbob (Post 975180)

I would be suspecting the circuit of Q212 before U31. Definitely check it out before attempting to remove SMD chips.

I'm guessing you don't have a scope but with just a multimeter you can make a good stab at diagnosing a faulty transistor. With the board powered off measure the resistance and continuity/diode reading of each transistor terminal to ground and to the other terminals.

Compare these results with the same measurements of the equivalant circuit for red and blue ie. Q211 and Q213. If any measurement is a long way off from its red/blue partner you probably have localised the fault to this transistor circuit. In particular, the diode measurement between terminals should be in the ballpark of the known working circuits, pay attention to meter lead polarity when making this measurement. It could still be the transistor itself or one the passive components around it.

Then you have to find which one, you could for example check that R212 is not open or short circuit, or you could bridge C212 with a leaded capacitor, (any value ~0.1uF-1 uF will do), to see if the green flashes up, that will tell you that C212 is open circuit, etc.

Originally Posted by PeteJ (Post 975531)

After giving myself a quick crash course on how to use a multimeter I've run through the tests that you've suggested and you were on the money with a fault at Q212 it seems. I tested the red and blue Q211/Q213 and they both gave the same readings : touching either the positive or negative probe on the 3rd pin, they display with the other probe 0.32 at left pin and 0.18 at the right pin. Q212 (green) however displays a reading off the scale for the left pin and 0.50 for the right pin. I tested the resistor at R212 which shows a correct reading of exactly 151. I did also test the other resistors in the green circuit which gave correct readings R231B (1K) displays 977 and R234B (75) displays 75.

Now that I've already recapped the board and tested the resistors in the circuit, I guess that narrows it down to the transistor at Q212 so I should replace it. Can somebody please tell me which smd transistor to buy? There are no markings on it and the circuit diagram as far as I can see doesn't detail that. I'm unsure of the rating or package type to order.

Thanks.

Originally Posted by FOL (Post 975534)

Look with good light,they have numbers on them.

Originally Posted by PeteJ (Post 975784)

Does anyone know if the transistor I'm looking for is an SOT-23 package? My board is rev 1.5 but I've looked at some other A600 motherboard photos and the rev 2b board has resistors in those positions with what looks like just 1A written on them. Does that mean DC Collector Current is 1A? Maybe the Fairchild Semiconductor BCW66G is suitable?