IIfx keeps turning off/issue with KB/IIfx power switch?

[Byrd]

Hi,

I'm rebuilding my beloved IIfx at the moment, and have hit a snag with the machine turning off unless I hold the keyboard power switch down. Take finger off the keyboard power, it'll turn off. If I hold it down for long enough (eg. a minute or two), it will eventually "take" and be quite stable with no issues, but not every time. If I attempt to power up by the IIfx rear power switch, it will turn on and off very quickly - think on/off/on/off several times a second.

Both PRAM batteries are fresh and replaced, and it only has two NUBUS cards - a network card and Radius Videovision graphics card, 8MB/1GB HD, dual floppy drives, Astec power supply. I've also cleaned the IIfx power switch with contact cleaner spray.

Any tips? Thanks in advance.

JB

 

[Byrd]

Hi,

still having an issue with the IIfx power switch - very rarely it'll stay powered on when I release my finger from the keyboard power key, but most of the time it'll switch off immediately when this key is released.

I'm leaning towards the physical power switch at the back of the case being at fault; it doesn't lock down like some I've seen, could this be the problem?

Thanks

JB

 

[Concorde1993]

Sounds like your power supply is on its last legs. Astec was notorious for producing poor power supplies for Macs, as I am sure you are well aware of (I am surprised that an Astec supply would be in a IIfx; you would figure a Sony PS would have been used, considering the IIfx was the "flagship" Mac model for a short period of time).

There could also be potential problems with the motherboard. Look for bulging caps, and leakage.

100th post, btw.

 

[trag]

Sounds like your power supply is on its last legs. Astec was notorious for producing poor power supplies for Macs, as I am sure you are well aware of (I am surprised that an Astec supply would be in a IIfx; you would figure a Sony PS would have been used, considering the IIfx was the "flagship" Mac model for a short period of time).
There could also be potential problems with the motherboard. Look for bulging caps, and leakage.

100th post, btw.

It is unlikely to be IIfx logic board caps as the IIfx seems to have all tantalum caps and no electrolytic. Still, it seems that something is preventing the power-on circuitry from "latching". If you have the skill, I'd look at the power-on circuitry schematic (I think it's available at Gamba's site?) and try to determine what is essential to keeping the machine on, after the button has been pressed and released.

My off the cuff guess is something in the power supply unit, as there's very little to go wrong on the logic board in the IIfx.

 

[Concorde1993]

It is unlikely to be IIfx logic board caps as the IIfx seems to have all tantalum caps and no electrolytic.

Really, the IIfx used all tantalum caps? That's news to me. :beige:

That's also quite unfortunate, as those tantalum caps are notorious for exploding with age. Geez, the IIfx-series were slapped with pretty lousy components.

 

[Unknown_K]

At least Tantalums are easy to troubleshoot, they have either blown up or can be found with a dead short using a cheap meter.

 

[trag]

And tantalums don't leak corrosive crud all over the board when they die...

 

[Unknown_K]

Yea, but how many chips that cannot be easily replaced will they take out when they short causing a current spike?

 

[Byrd]

THanks all for the tips (and PM), I will take her apart one day and go over the PSU and motherboard. It does look pretty spotless though.

 

[Dennis Nedry]

There are a few electrolytics on the IIfx logic board contrary to popular belief.

Let's have a look at the schematic, courtesy of Gamba, and discuss the symptoms and possible causes.

From your observation, the button on the Mac and the button on the keyboard act differently. This is a good clue.

As you can see if you look for a bit, the top group of NAND gates are controlled by both power buttons, and it is responsible for turning the Mac ON using Q3. The bottom group of NANDs is NOT controlled by the keyboard. It is controlled only by the button on the Mac and a software shutdown and uses Q2. Something is triggering this section of the circuit too easily.

We must not forget our clue from before. What would make the Mac's power cycle around only from the power button on the back of the Mac?

Resistor R110 is intended to make sure that when both the "ON" nands and the "OFF" nands are going at the same time, so both Q2 and Q3 are turned on, that the "ON"s always win. Somehow the "ON"s are not winning.

I therefore suspect that transistor Q3 is to blame for the power fluttering. But for the power cutting out unless you hold the power button for a length of time, I suspect the bottom group of NANDs. These are possible symptoms of leakage from the small radial electrolytic cap that's in the area. Mine leaked.

-- Recommendations --

• Do not hold down the power keys, this drains your batteries
  
• Focusing especially on Q2, Q3, UI18, and UJ18, clean vigorously with soap and water with a toothbrush and rinse thoroughly. I give this a 75% chance of solving the problem if cleaned very thoroughly.
  
• This will not damage your Mac if you allow it to completely dry before powering it on.
  
• Replace the mentioned electrolytic cap if it looks bad to prevent future problems.