Are these A500 PSU voltages ok?

Starglider 2 · 17 September 2018, 20:56

After a few failed CIA chips someone suggested checking the PSU. Are these voltages measured on the cable plug within tolerable levels do you know?

+12V line = +14.1V
-12V line = -13.2V
+5V line = +6.28V

Franchute13 · 17 September 2018, 21:04

Hello Starglider 2!
You must test the power supply with load, without load, the values do not work.
Test connecting an old hdd to 12v and 5v to see.
What you can not prove is the state of the capacitors, if they are in bad condition you will not see it in the voltage. Change the capacitors for doubts.
Bye

Starglider 2 · 17 September 2018, 21:34

Thanks. With the machine on, at the serial port test pins 9 & 10 I get -14.5 & -15.4V.

On chips the 5V pin measures about 6.1V.

Is any of that normal?

I don't have an old HDD to test.

Photon · 17 September 2018, 22:09

You don't need a heavy load like a harddisk to test the voltages. The +/- 12V can be 13V or so without issue, but the 5V must be within +/- 0.25V for the logic.

To me it looks like you have the same voltages with the machine connected and disconnected. Either the cable is faulty or the power connector is broken inside. I wouldn't say the PSU is broken yet, because it seems like you haven't put a load on the PSU yet.

Starglider 2 · 17 September 2018, 22:15

Quote:

Originally Posted by Photon

You don't need a heavy load like a harddisk to test the voltages. The +/- 12V can be 13V or so without issue, but the 5V must be within +/- 0.25V for the logic.

To me it looks like you have the same voltages with the machine connected and disconnected. Either the cable is faulty or the power connector is broken inside. I wouldn't say the PSU is broken yet, because it seems like you haven't put a load on the PSU yet.

To clarify, the machine is essentially working fine after replacing the CIAs. These voltages at the serial are with the machine on and showing workbench. Thoughts?

What would you do to load the PSU apart from having the machine on and at workbench?

Photon · 17 September 2018, 22:17

Nothing. I would rub my eyes, then replace the voltmeter post haste. But I wouldn't measure voltage on the serial port, I would measure it on the PSU plinth.

Starglider 2 · 17 September 2018, 22:56

Faulty volty huh? Ok, I think you're right. It's been acting strange actually. I've ordered a replacement. Thanks.

Daedalus · 17 September 2018, 22:58

Yeah, I'd get a second opinion on those voltages with another meter. It's possible for logic to still work around 6V, but I would expect the chips to get hot and cause lots of crashes, and eventually damage chips at that voltage.

17 September 2018, 20:56	#1
Starglider 2 Registered User Join Date: Jan 2014 Location: California Posts: 1,146	Are these A500 PSU voltages ok? After a few failed CIA chips someone suggested checking the PSU. Are these voltages measured on the cable plug within tolerable levels do you know? +12V line = +14.1V -12V line = -13.2V +5V line = +6.28V

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17 September 2018, 21:04	#2
Franchute13 Registered User Join Date: Feb 2013 Location: Argentina Posts: 281	Hello Starglider 2! You must test the power supply with load, without load, the values do not work. Test connecting an old hdd to 12v and 5v to see. What you can not prove is the state of the capacitors, if they are in bad condition you will not see it in the voltage. Change the capacitors for doubts. Bye

17 September 2018, 21:34	#3
Starglider 2 Registered User Join Date: Jan 2014 Location: California Posts: 1,146	Thanks. With the machine on, at the serial port test pins 9 & 10 I get -14.5 & -15.4V. On chips the 5V pin measures about 6.1V. Is any of that normal? I don't have an old HDD to test.

17 September 2018, 22:09	#4
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,602	You don't need a heavy load like a harddisk to test the voltages. The +/- 12V can be 13V or so without issue, but the 5V must be within +/- 0.25V for the logic. To me it looks like you have the same voltages with the machine connected and disconnected. Either the cable is faulty or the power connector is broken inside. I wouldn't say the PSU is broken yet, because it seems like you haven't put a load on the PSU yet.

17 September 2018, 22:17	#6
Photon Moderator Join Date: Nov 2004 Location: Eksjö / Sweden Posts: 5,602	Nothing. I would rub my eyes, then replace the voltmeter post haste. But I wouldn't measure voltage on the serial port, I would measure it on the PSU plinth.

17 September 2018, 22:56	#7
Starglider 2 Registered User Join Date: Jan 2014 Location: California Posts: 1,146	Faulty volty huh? Ok, I think you're right. It's been acting strange actually. I've ordered a replacement. Thanks.

17 September 2018, 22:58	#8
Daedalus Registered User Join Date: Jun 2009 Location: Dublin, then Glasgow Posts: 6,335	Yeah, I'd get a second opinion on those voltages with another meter. It's possible for logic to still work around 6V, but I would expect the chips to get hot and cause lots of crashes, and eventually damage chips at that voltage.

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