VOGONS


PSU Oddity

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First post, by avxstudios

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Hi folks,
Wondering if anyone could chime in on this particular topic.

I'm tinkering with a 486 system that uses a standard AT psu, (P8/P9 connectors)
The system seems to get unstable the longer it has been on, which suggests the power supply is not working right, I've measured the pins and get the following:

P9/6 : +5Vdc
P9/5 : +5Vdc
P9/4 : +5Vdc
P9/3 : -5Vdc
P9/2 : GND
P9/1 : GND

P8/6 : GND
P8/5 : GND
P8/4 : -12.2Vdc
P8/3 : +12.2Vdc
P8/2 : +3.4Vdc
P8/1 : GND/Power good

P8/2 is the one that raises an eyebrow, as when the system boots the line shows 3.4V, and briefly as the CRT (which is part of the system) the line jumps to 4.9V then back down to 3.4V, and if left running the voltage eventually drops to 3.2V at which point the system becomes unstable and crashes / shows artifacting, I don't believe this to be a heat issue as the CPU is well cooled, to me it feels more like the prolonged heat is knocking already out of spec capacitors slowly further out of spec as they heat up, and allowing it to cool somewhat reverses this.

I'm just curious as to if the P8/2 line should be as close to +5Vdc as possible or not?

Thanks folks.

Reply 1 of 7, by TheMobRules

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P8/2 should be +5VDC, the same as P9/4, 5 and 6. Which is strange since these should all measure the same (unless for some reason your PSU has that line on a separate rail, which would be uncommon for an AT unit).

With the PSU off, please check continuity between P8/2 and the +5V wires of the other connector. It should be a straight short, if you encounter resistance there's probably some issue with the wire (maybe it has a cold joint or the cable broke at the connector).

Another possibility is that the voltage is dropping on all the lines and you haven't realized it, in that case yes, it could be a problem in the PSU or some short in some component on the motherboard that is bringing down the +5V line.

Reply 2 of 7, by avxstudios

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TheMobRules wrote on 2021-02-19, 20:40:

P8/2 should be +5VDC, the same as P9/4, 5 and 6. Which is strange since these should all measure the same (unless for some reason your PSU has that line on a separate rail, which would be uncommon for an AT unit).

With the PSU off, please check continuity between P8/2 and the +5V wires of the other connector. It should be a straight short, if you encounter resistance there's probably some issue with the wire (maybe it has a cold joint or the cable broke at the connector).

Another possibility is that the voltage is dropping on all the lines and you haven't realized it, in that case yes, it could be a problem in the PSU or some short in some component on the motherboard that is bringing down the +5V line.

Hmm,
I checked between P8/2 and each of the other +5Vdc lines and encountered resistance, to rule out a short on the motherboard I disconnected the P8 and P9 connectors from the motherboard and tested them again and there is still resistance.
The power supply isn't a standard unit, the system is an all-in-one CRT TV+PC, and the power board for the CRT also generates and feeds the power requirements for the PC side, so it's possible that P8/2 is fed from another rail and that this rail is defective.

I suppose an easy fix would be to bypass P8/2 and feed it the +5V from one of the other lines, but if there's an ongoing issue with the power supply I'd rather fix it properly.
Looks like I'll be pulling it out to check/replace a bunch of caps on my day off then!

Reply 3 of 7, by Sphere478

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out of curiosity what computer model is this?
because apparently some IBM computers didn't use that wire
https://ibm.retropc.se/5150_5160/misc/5150_51 … of_5170_psu.jpg

http://minuszerodegrees.net/5170/psu/5170_psu … f_P8_and_P9.jpg

A real AT (5170) power supply still supplies -5v. However, make sure that your P8, pin 2 is either NC or connected to the +5 rail. Courtesy of MZD:

https://encrypted-tbn0.gstatic.com/images?q=t … GdVJHA&usqp=CAU
this lists it as a power good signal though...

https://ibm.retropc.se/5150_5160/misc/5150_51 … of_5170_psu.jpg

seems the definitions for that pin are kinda unclear haha

🖥Craziest socket 7 build on a 430tx chipset
🖥Dual socket 7 build

Reply 4 of 7, by avxstudios

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Sphere478 wrote on 2021-02-20, 09:48:
out of curiosity what computer model is this? because apparently some IBM computers didn't use that wire https://ibm.retropc.se/ […]
Show full quote

out of curiosity what computer model is this?
because apparently some IBM computers didn't use that wire
https://ibm.retropc.se/5150_5160/misc/5150_51 … of_5170_psu.jpg

http://minuszerodegrees.net/5170/psu/5170_psu … f_P8_and_P9.jpg

A real AT (5170) power supply still supplies -5v. However, make sure that your P8, pin 2 is either NC or connected to the +5 rail. Courtesy of MZD:

https://encrypted-tbn0.gstatic.com/images?q=t … GdVJHA&usqp=CAU
this lists it as a power good signal though...

https://ibm.retropc.se/5150_5160/misc/5150_51 … of_5170_psu.jpg

seems the definitions for that pin are kinda unclear haha

Aiwa PC-MT601,
I grabbed an old broken PSU and made an adapter for it to pass the clean 5v to the pin that was reading 3.4v, and now the system is completely stable again.
If i don't connect that pin at all, the PC does not boot, so the lower voltage is definitely the issue with it.

Reply 6 of 7, by Sphere478

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avxstudios wrote on 2021-02-20, 10:02:
Aiwa PC-MT601, I grabbed an old broken PSU and made an adapter for it to pass the clean 5v to the pin that was reading 3.4v, and […]
Show full quote
Sphere478 wrote on 2021-02-20, 09:48:
out of curiosity what computer model is this? because apparently some IBM computers didn't use that wire https://ibm.retropc.se/ […]
Show full quote

out of curiosity what computer model is this?
because apparently some IBM computers didn't use that wire
https://ibm.retropc.se/5150_5160/misc/5150_51 … of_5170_psu.jpg

http://minuszerodegrees.net/5170/psu/5170_psu … f_P8_and_P9.jpg

A real AT (5170) power supply still supplies -5v. However, make sure that your P8, pin 2 is either NC or connected to the +5 rail. Courtesy of MZD:

https://encrypted-tbn0.gstatic.com/images?q=t … GdVJHA&usqp=CAU
this lists it as a power good signal though...

https://ibm.retropc.se/5150_5160/misc/5150_51 … of_5170_psu.jpg

seems the definitions for that pin are kinda unclear haha

Aiwa PC-MT601,
I grabbed an old broken PSU and made an adapter for it to pass the clean 5v to the pin that was reading 3.4v, and now the system is completely stable again.
If i don't connect that pin at all, the PC does not boot, so the lower voltage is definitely the issue with it.

Good sleuthing! 😀

Gonna try and fix the psu?

🖥Craziest socket 7 build on a 430tx chipset
🖥Dual socket 7 build

Reply 7 of 7, by avxstudios

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Sphere478 wrote on 2021-02-20, 19:22:
avxstudios wrote on 2021-02-20, 10:02:
Aiwa PC-MT601, I grabbed an old broken PSU and made an adapter for it to pass the clean 5v to the pin that was reading 3.4v, and […]
Show full quote
Sphere478 wrote on 2021-02-20, 09:48:
out of curiosity what computer model is this? because apparently some IBM computers didn't use that wire https://ibm.retropc.se/ […]
Show full quote

out of curiosity what computer model is this?
because apparently some IBM computers didn't use that wire
https://ibm.retropc.se/5150_5160/misc/5150_51 … of_5170_psu.jpg

http://minuszerodegrees.net/5170/psu/5170_psu … f_P8_and_P9.jpg

https://encrypted-tbn0.gstatic.com/images?q=t … GdVJHA&usqp=CAU
this lists it as a power good signal though...

https://ibm.retropc.se/5150_5160/misc/5150_51 … of_5170_psu.jpg

seems the definitions for that pin are kinda unclear haha

Aiwa PC-MT601,
I grabbed an old broken PSU and made an adapter for it to pass the clean 5v to the pin that was reading 3.4v, and now the system is completely stable again.
If i don't connect that pin at all, the PC does not boot, so the lower voltage is definitely the issue with it.

Good sleuthing! 😀

Gonna try and fix the psu?

Yup, definitely needs doing, using the other +5V to test is just a sanity check, if the capacitors or regulators are that far out of whack to make one of the +5v rails drop to 3.4v they likely all need addressing, I'm already due to replace a bunch of capacitors and a voltage regulator to resolve a brightness issue, so once those parts arrive I'll replace everything I have a matching part for, hopefully it brings back the +5v on that rail.