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Old 04 June 2017, 13:44   #1
Alen
 
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Dead A1200 (+5V only 287Ohms against ground)

I'm trying to figure out if I can fix an A1200. Got a working power supply from an A500 and one from another A1200. When I connect them to this defect A1200, both power supplys make strange noises (one is ticking and the other gives a high frequency noise). Obviously something seems to be pulling too much power. I'm measuring on the +5V (Pin 1 of CN8) against ground only something like 287Ohms which seems to be too low. For testing purposes I just removed the C811 (1000uF) to make sure that this one isn't making a short circuit, but that's not the case. Well now its getting difficult - it could be anything beeing powered by the +5V. The Board is Rev 1.D. Schematic diagram can be found here: http://www.amigawiki.de/dnl/schematics/A1200_R1.pdf
Curious if anyone could problably point me in the right direction. Maybe something that often likes to cause problems and pulls the +5V down?


 
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Old 04 June 2017, 14:01   #2
bebek
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287ohm on 5V is only 17mA so it is very low current for PSU

Last edited by bebek; 04 June 2017 at 14:14.
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Old 04 June 2017, 15:09   #3
Alen
 
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Anyone with an open A1200 can confirm that its normal to have only around 287 Ohms on +5V line against ground?
 
Old 04 June 2017, 15:28   #4
Alen
 
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Yes - just having a conversation on the Commodore Amiga FB-group where Tristan Zondag confirmed that he also measures around 270Ohms against ground on the 5V line (and also somethingl like 2-3MOhms on the 12V line (which I measure too). So means nothing wrong with the +5V line as I was expecting it first...

So any other ideas/suggestions what might cause both power supplies not to work with this miggy?
 
Old 04 June 2017, 15:29   #5
bebek
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You need to measure voltage when it is powered up. Some caps can fail only when there is power.
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Old 04 June 2017, 20:18   #6
Alen
 
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I just did. +12V are ok, but on +5V line I have only 0.308V

I have a short circuit over R352, R362 and R309. In the schematic diagram they can easily be located with CTRL+F (Find). R352 and R362 are on Page 6 (Mouse/Joystick Ports). R309 is on Page 7 (Audio Power). Its all +5V against ground. Well something is definitively pulling the +5V down. Below R309 is for example the C307 (470uF) could be a candidate - but I doubt that one is making the short (if it would be a short, then I should still measure some Ohms from VCC above R309 against ground, because of the Ohms from the Resistor R309 itself...). What I definitively dont understand is: My meter is peeping (short circuit) when measuring over R301. That one can also be found on page 7 (Audio Decoupling). But here we have above it 12V. If 12V against ground is a shortcut, then it would be impossible to measure +12V on the connector... Really can't explain why my meter is peeping for short circuit over R301 but I can measure +12V properly...
 
Old 04 June 2017, 22:50   #7
bebek
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Those resistors ar low value below 5 ohm and you will measure them as a short - it is correct. They are connected through caps to ground and as you said resistance there is correct. You are in wrong place. Turn it on and feel all IC's with finger. One of them may be hot. Same applies to all ceramic capacitors - be careful as you may be burnt. There is also resistor 4.7 ohm that provides power to modulator. This one may go hot too if modulator is faulty. Already had two fails like this.

R301 is 10ohm and most of meters will beep (some even up to 50-60 ohm) - it is correct.
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