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U3 On Quadra / Centris 610

Phipli

Well-known member
I have a 610 board on the way, and have been checking over the photos for damage. I've spotted something strange comparing to other boards online. On some boards U3 is populated and on others it isn't. I can't quite read the chip label in photos.

Anyone know what U3 is and why it is only fitted to some boards? Its near the video port.

 

joshc

Well-known member
It's probably not anything crucial like an Ethernet controller I don't think. It's not big enough and it looks like an off the shelf part? I don't have a 610 board anymore otherwise I would see what the part number is.
 

MrFahrenheit

Well-known member
I don't think it is ethernet - boards without it still have AAUI.

When I said Ethernet I meant the AAUI port. There are 610 models which shipped without AAUI (just a blank area where the port is). They also lack some chips on the logic board that control the Ethernet.

There was a 610 board recently on eBay. I can’t see which one is U3
9C9018E5-4EDB-4F5E-958C-5F9A691DF8D3.jpeg
 

MrFahrenheit

Well-known member
Here’s a photo of a 610 board lacking Ethernet. Looks like it lacks the Sonic chip, which I don’t think is what you were referring to. Apologies.
D77C9537-4B7C-49A1-8C34-78B5A9421F54.jpeg
 

Fizzbinn

Well-known member
U3 is an 8 leg surface mount chip below the video connector.

U3 is unpopulated on my Quadra 610 logic board (which has onboard Ethernet). I actually have 2 non-working Centris 610 boards, one with onboard Ethernet, one without and they both have U3 populated.

Perhaps U3 is something needed for video timing with a base 10Mhz clock chip (C610) but not with the 12.5 Mhz clock chip (Q610)?

I recall the Q610 with Ethernet has an extra clock chip as compared to the C610 which apparently just makes use of that base 10Mhz clock chip.
 

MrFahrenheit

Well-known member
U3 is an 8 leg surface mount chip below the video connector.

U3 is unpopulated on my Quadra 610 logic board (which has onboard Ethernet). I actually have 2 non-working Centris 610 boards, one with onboard Ethernet, one without and they both have U3 populated.

Perhaps U3 is something needed for video timing with a base 10Mhz clock chip (C610) but not with the 12.5 Mhz clock chip (Q610)?

I recall the Q610 with Ethernet has an extra clock chip as compared to the C610 which apparently just makes use of that base 10Mhz clock chip.

That would make a lot of sense. The 20mhz system speed of the Centris compared to the 25mhz on the Quadra would likely have more than just an oscillator change.
 

Phipli

Well-known member
Bolle is on to something - the RAMDAC (U10) supplier is different between the boards I've checked. Working hypothesis : AT&T boards don't have U3, Apple © ones do?
 

Phipli

Well-known member
Nope, back to @MrFahrenheit 's theory - I found a 20MHz board with the AT&T part and U3 :)
 

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  • apple-macintosh-820-0377-quadra-610_1_a05d0c6b7f00247f15cfecf37ea6085b.jpg
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Daniël

Well-known member
Might it be the LM385-1.2 voltage reference IC? I'm far from knowledgeable in this field, but this chip does come in that package, and voltage references are apparently used in ADC/DAC circuits:


EDIT: Fairly sure it is. Apparently the Amiga 1200 uses it too, and various images show that the 1.2V Voltage Reference chip next to the RAMDAC (ADV101 or VP101) was also sometimes populated, sometimes not. In the images linked you can see this (look under the cap, under the yellow RCA jack):

With:
a3ecd4b2-5ae0-464b-aec6-3ecefd369a08.jpg


Without:
a1200mb_rev1d4_lightgreen.jpg


EDIT 2: On the 1200, the difference seems to stem from which RAMDAC was used. The ADV101 requires an external 1.2V voltage reference, the VP101 has an internal 1.2V volt reference. This is likely the same for the Quadras, it depends whether the fitted RAMDAC has it or not, which might differ even between AT&T chips.
 
Last edited:

Phipli

Well-known member
Might it be the LM385-1.2 voltage reference IC? I'm far from knowledgeable in this field, but this chip does come in that package, and voltage references are apparently used in ADC/DAC circuits:


EDIT: Fairly sure it is. Apparently the Amiga 1200 uses it too, and various images show that the 1.2V Voltage Reference chip next to the RAMDAC (ADV101 or VP101) was also sometimes populated, sometimes not. In the images linked you can see this (look under the cap, under the yellow RCA jack):

With:
a3ecd4b2-5ae0-464b-aec6-3ecefd369a08.jpg


Without:
a1200mb_rev1d4_lightgreen.jpg


EDIT 2: On the 1200, the difference seems to stem from which RAMDAC was used. The ADV101 requires an external 1.2V voltage reference, the VP101 has an internal 1.2V volt reference. This is likely the same for the Quadras, it depends whether the fitted RAMDAC has it or not, which might differ even between AT&T chips.
Bolle was saying voltage ref as well. I hadn't considered there might be two AT&T RAMDAC variants.
 
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