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Old 08 June 2018, 13:50   #1
RiP
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Exclamation Amiga 2000 Rev6 - Power LED doesn't dim

I have noticed that the power LED of my Amiga 2000 Rev6 doesn't dim at all and it's always stuck at 5V
I'm sure it used to dim last year when I get guru error or play game, not sure what happened

Last edited by RiP; 09 June 2018 at 12:46.
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Old 08 June 2018, 20:44   #2
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It's possible that the transistor used to brighten the LED has failed short, giving it a constant connection to near 5V. Bear in mind though that if you measure it without the LED attached, it will read close to 5V regardless of the dim/bright state.
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Old 08 June 2018, 21:15   #3
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Thanks for reply what transistor exactly?

I thought more and remembered that I fried one of the CIA chips when I was measuring the voltage of 1488 chip.
After that, I replaced 1488 and CIA then everything worked fine. I think the LED problem happened after that story
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Old 08 June 2018, 22:36   #4
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Transistor Q302 controls the dimming of the LED, and it's at the very front edge of the motherboard on the Revision 6.2 motherboard, so I suspect it should be there for yours too. If you check the voltage at the base of the transistor, it should change with the LED status, low voltage will mean bright LED, high voltage will mean dim LED. It might be easier to measure it at the resistor R902 which is right next to Q302. You can use an audio program like a tracker, or even a special shell command, for turning the filter on and off for testing.
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Old 08 June 2018, 22:50   #5
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Thanks again, what special shell command?
Another question, shouldn't I test U303 74F08 (pins 4,5,6) ?

Edit: are you sure about this part?
Quote:
Originally Posted by Daedalus View Post
low voltage will mean bright LED, high voltage will mean dim LED.

Last edited by RiP; 08 June 2018 at 23:59.
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Old 09 June 2018, 01:29   #6
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Quote:
Originally Posted by RiP View Post
Thanks again, what special shell command?
Quote:
Another question, shouldn't I test U303 74F08 (pins 4,5,6) ?
Yep, if the signal at the transistor/resistor isn't changing when the LED should be changing, the next thing to check would be the input to U303 (which is the output of the odd CIA (U300)).

Quote:
Edit: are you sure about this part?
Q302 essentially puts R307 in parallel with R308, causing a brighter LED when the transistor is on as the combined resistance between the two parallel resistors is now 50-something ohms, instead of 200 ohms of R308 by itself. Since the transistor is a PNP type, it's on when the base voltage is low. So low output from U303 means the LED is brighter, since the transistor switches on under that condition.
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Old 09 June 2018, 09:08   #7
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Ok, but how exactly should I measure with multimeter and what points exactly?
I don't know much from IC/transistors



Quote:
Originally Posted by Daedalus View Post
Yep, if the signal at the transistor/resistor isn't changing when the LED should be changing, the next thing to check would be the input to U303 (which is the output of the odd CIA (U300)).
Shouldn't I check the output of U303? maybe U303 has failed becasue I replaced the CIA and it should be fine.
I think that I was connected -12/+12V to the pin2 of U304 (1488) when I was measuring last year.

Edit: well, I measured the voltage at the base of the Q302, it's constant +4.2V even in dimmed mode when I reset.
The voltage of LED is always +5V. Does it help?

Edit2: I also measured the input voltage of U303 (pin5), it's +4.9V when i reset and 0V in normal situation.
Then it seems CIA chip is working fine.

Last edited by RiP; 09 June 2018 at 12:05.
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Old 09 June 2018, 12:51   #8
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Yep, they're the checks you needed to do. If the input to U303 is normal (and it sounds like it), and the output is a constant 4.2V, or possibly even 5V, then U303 has failed and should be replaced.
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Old 09 June 2018, 12:56   #9
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Quote:
Originally Posted by Daedalus View Post
Yep, they're the checks you needed to do. If the input to U303 is normal (and it sounds like it), and the output is a constant 4.2V, or possibly even 5V, then U303 has failed and should be replaced.
Big thanks, will let you know if it worked
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Old 10 June 2018, 15:54   #10
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Wow, replaced U303 and it solved the problem

Just a question, how to find out if the audio filter circuit is really working or not?
I need a change in the audio out to hear the difference =/
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Old 10 June 2018, 16:27   #11
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Easiest way would be to load up a MOD in Protracker and toggle the filter option on and off.
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Old 11 June 2018, 10:02   #12
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Yep, most MOD players and trackers will have a button for turning the filter on and off. Bear in mind though that U303 doesn't actually control the filter, which should still have been working fine and turning on and off as required, even if the LED wasn't changing.
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Old 18 June 2018, 11:59   #13
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Thanks all, finally I could find and test it with ProTracker. It was in the Setup => Filter: OFF (darker LED) / ON (brighter LED)

What a big difference, low-pass filter is terrible as hell
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Old 18 June 2018, 13:05   #14
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What a big difference, low-pass filter is terrible as hell
Sure is. I could never understand why most PC mod-players had the filter enabled by default. I can only assume the programmers were either stupid and/or deaf.
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