English Amiga Board - View Single Post - A1200 weak crackly sound

Photon · 14 October 2011, 00:42

Again, would appreciate if mods didn't merge, for readability.

Hehe, U15 is the opamp. It's just not marked on the mobo, found it in the parts list...

OK, I traced the -12V to the "VREF/-12V area" between Paula and the CIA by the floppy power connector. Found no bad components, diodes checked out, the small chips there didn't get warm. Traced it on all the way to the opamp.

It's definitely something between -12V and VREF that brings VREF down from +2.5 to negative values...
-~-
When -12V is not connected to the mobo, -12V at power connector and all the way to the opamp is 2.93V. VREF is then +2.4V

When -12V is connected to the mobo, -12V at power connector and all the way to the opamp is about -8.2 / -7.2V. VREF is then -6.3/-7.2V

-~-

"OK" ...

[[[[ NOTE: the rest of this is with NO -12V connected to the Amiga! ]]]]

I couldn't find any other component that is between -12V and VREF, than the opamp.

I already lifted the +/-12V pins on it. I lifted both VREF pins on it now, and STILL the -12V at power connector goes up to +3V...

The opamp is out of the game (unless someone protests). So this didn't give me hope :S(If I get the replacement tomorrow I'll remove the old one completely and test though)

Hm... something else that brings -12V up by ~ 5V, or <unconnected -12V> up to 3V....

OK! Found a clue!

When the Amiga has been turned off a long while, and I measure the unconnected -12V, it's 2.2V for half a minute, then rises in 1.5 minutes to 3.8V and stops there!

Where this strange voltage comes from I have no idea, but slowly rising (slowly shorting to +5V??) to me means a cap or a chip.

Incidentally, Paula gets finger warm in 30 seconds, then distinctly warmer the next 2 minutes. Coincidence or cause?

Paula doesn't seem to be connected to neither VREF nor -12V (only found DIL pinouts though), so this points back to the opamp. But it is disconnected from both VREF and -12V now. Even if the Paula pins are connected to it, that can't affect the -12V and VREF since the opamp is not connected to them. This points to the VREF generator being at fault, if no other chip uses VREF and -12V. I've found the schematic for the VREF generator.

Thoughts...

14 October 2011, 00:42	#11
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,615	Again, would appreciate if mods didn't merge, for readability. Hehe, U15 is the opamp. It's just not marked on the mobo, found it in the parts list... OK, I traced the -12V to the "VREF/-12V area" between Paula and the CIA by the floppy power connector. Found no bad components, diodes checked out, the small chips there didn't get warm. Traced it on all the way to the opamp. It's definitely something between -12V and VREF that brings VREF down from +2.5 to negative values... -~- When -12V is not connected to the mobo, -12V at power connector and all the way to the opamp is 2.93V. VREF is then +2.4V When -12V is connected to the mobo, -12V at power connector and all the way to the opamp is about -8.2 / -7.2V. VREF is then -6.3/-7.2V -~- "OK" ... *[[[[ NOTE: the rest of this is with NO -12V connected to the Amiga! ]]]]* I couldn't find any other component that is between -12V and VREF, than the opamp. I already lifted the +/-12V pins on it. I lifted both VREF pins on it now, and STILL the -12V at power connector goes up to +3V... The opamp is out of the game (unless someone protests). So this didn't give me hope :S(If I get the replacement tomorrow I'll remove the old one completely and test though) Hm... something else that brings -12V up by ~ 5V, or <unconnected -12V> up to 3V.... OK! Found a clue! When the Amiga has been turned off a long while, and I measure the unconnected -12V, it's 2.2V for half a minute, then rises in 1.5 minutes to 3.8V and stops there! Where this strange voltage comes from I have no idea, but slowly rising (slowly shorting to +5V??) to me means a cap or a chip. Incidentally, Paula gets finger warm in 30 seconds, then distinctly warmer the next 2 minutes. Coincidence or cause? Paula doesn't seem to be connected to neither VREF nor -12V (only found DIL pinouts though), so this points back to the opamp. But it is disconnected from both VREF and -12V now. Even if the Paula pins are connected to it, that can't affect the -12V and VREF since the opamp is not connected to them. This points to the VREF generator being at fault, if no other chip uses VREF and -12V. I've found the schematic for the VREF generator. Thoughts... Last edited by Photon; 14 October 2011 at 00:50.