Mac IIfx logic board with dead nubus slots

[nvdeynde]

My IIFx does power on, chimes, and even boots from a floppy or it’s hard drive. However I have no video output since all Nubus slots are dead.
I have checked the power pins of the nubus slots and all are ok.

The Apple Graphics card is known good, same for m1212 monitor, psu, Rom, ram since these parts work fine in my other IIFx.

IIFx Board is the old revision A, same one as in my working IIfx. Jumper settings are identical. All capacitors were tantalums, replaced them just in case with the axials too without result.
I don’t have a scope. Is there anyone with the same problem that can point me to the right direction to fix this ?

 

[IIfx]

The only things I can think of to check is the UF7 and UI7 chipsets - these are the NuBus bus interfaces. Also closely check the area around UK4, that is the NuBus clock gen chip.

Is it possible dirt in a NuBus slot is causing this issue?

 

[nvdeynde]

Yes I know UF7 and UI7 control the Nubus interface. The board is in pristine condition, it's an Apple Service part in original box that I bought more than 20 years ago.. It was always my spare board, benchtested the board once a year, however now suddenly the nubus slots don't work anymore.

It has zero dirt on it, always stored in good condition in it's original box, it looks New. Visually there's nothing to see around UI7/UF7/UK4.
Since I don't have a scope I'm afraid there's nothing further I can do. I think bad traces can be excluded since there was never electrolyte leakage on the board and haven't heard of Mac IIFX's with bad traces.

An option would be to replace UI7/UF7/UK4 but I don't have spares and these are probably impossible to get.

 

[joshc]

What sort of capacitors are on this board, electrolytics? The original I assume, not replaced?

You can have bad traces without it being visible. You can also have electrolytic fluid leakage without it being immediately visible, as it can pool underneath the caps. Have you checked continuity yet?

 

[mg.man]

FWIW, I've had a couple of 'new, old stock' Apple Service 'boards (a pair of IIsi, an LC) over the past year - all have needed a re-cap...

I was going to echo @joshc's query, but see you mentioned...

All capacitors were tantalums, replaced them just in case with the axials too without result.

Not sure what else to suggest.... sorry

 

[joshc]

Woops, missed that.

Yeah, doubt you'll find out what the cause of this is without a scope really.

 

[beachycove]

Are these the Nubus interface chips?

https://www.digikey.ca/en/products/detail/texas-instruments/SN74ALS240ADWR/556025

 

[trag]

Many (most?) of the IIfx boards didn't have any SM electrolytic caps on board. They were all tantalum. I know there are some out there with a few electrolytics...

@beachycove Those are generic logic chips, not the specialized NuBus transceivers. TI did make NuBus transceivers. I saw some datasheets for them some time in the last few years. Now, where (on which computer) did I save those datasheets....

One of these days I need to synchronize all my "Datasheets" folders.

 

[trag]

@nvdeynde Is it possible one of the jumpers on the logic board got moved/removed? I've never figured out what those jumpers do, but I know that if they're in the wrong configuration the machine doesn't work.

Unfortunately, my IIfx isn't accessible at the moment or I'd provide the needed configuration info.

 

[nvdeynde]

@nvdeynde Is it possible one of the jumpers on the logic board got moved/removed? I've never figured out what those jumpers do, but I know that if they're in the wrong configuration the machine doesn't work.

The jumpers are in the right position as I have 2 of these boards, revision A. As for the capacitors: there has never been any leakage on the board.
Other than the electrolyte axials, the SMD's are tantalum. The problems is not related to capacitors, I have tested these and the voltages match with my working board, same for power to the Nubus slots. There are no broken traces from the Nubus slots to UF7 and UI7 too.

I guess it's either the Nubus clock or one/both of the interface IC's. I think this can't be diagnosed without a scope. Then again if it's a faulty Nubus interface IC, the part is impossible to get.

 

[trag]

@beachycove For your reading pleasure:

I can't remember if htere's much difference between the two books and am too lazy to check at the moment.

 

[LaPorta]

Mine has some SM electrolytic.

 

[IIfx]

Mine had exactly 2 SM electrolytic. I replaced them with SM polymer. IIfx board is unique in that it has redundant pads for caps - 1 set for SM eletrolytic, and 1 set for sm tantalum. In the areas with axial caps there are sm pads underneath.