Mac II shuts off after 10 seconds?

[jonny_d]

Hey guys! I just got my first Mac II series computers, a II and a IIfx. They're welcome additions to my collection. However I'm having a problem with the II. When I press the power button it turns on and stays on for only about 10 seconds, and then shuts back off. During this time there's no video or startup chime. I'm getting 3.5V and 2.9V from the PRAM batteries, could that be what's causing the problem? Or could it be something else? I have a second II logic board with higher PRAM voltages (but still not the full 3.6), but the computer doesn't even turn on using that board. Can anyone help me get her working?

Thanks!

Jon

 

[Unknown_K]

Dead PRAMS keeps the machine from doing anything. If the machine shuts down in 10 seconds I would be looking for a short or something like that.

Remove all uneeded cards and see if it keeps doing that.

 

[jonny_d]

Removed all the cards and it still does the same. Can I use the IIfx power supply with a II board? They are different Apple part numbers, but are the voltages different? Are there any likely places to check for a short? Thanks!

 

[Kallikak]

A IIfx PSU is the same, but I'd be surprised if its the power supply from the symptoms you describe.

 

[equill]

One of the PRAM battery voltages that you quoted is under the requirement. A Mac II needs that both batteries be adequate. As a generality, 3.3V in circuit is the point at which to replace a lithium half-AA cell in any old Mac, and 2.8V is about the point at which a battery may become a hindrance rather than a help. The AIO Classics can (just) get away with no battery, but the more (than just time) that is stored in PRAM, the more critical is the lack of a properly functional battery. This can be fatally true if startup of video is affected by a faded/dying/kaput battery.

Are your batteries soldered-in, or on a little board of their own for easier replacement?

de

 

[jonny_d]

Good to know. I'll check the circuit voltage when I get home tonight. They're soldered in, and I don't have any replacements handy right now. Is there a good source (read: cheap ) for these PRAM batteries? I really should get a bunch for this and for some of the other Macs I have around.

 

[LCGuy]

Try Radio Shack or one of those sort of places. Also, anywhere that specialises in selling batteries should have 'em or be able to get them easily.

 

[Kallikak]

One of the batteries (the PRAM battery) is of no consequence for power on, but the second battery kickstarts the PSU and must be working. However there are easier workarounds for testing than soldering a new one in. I ran my mac II with two 1.5V AA batteries in series for a while in place of this battery without any problem (and a completely dead PRAM battery) so 3V is sufficient. You could try the same setup. Just connect ground to the chassis and momentarily apply the 3V to pin 15 of the power connector. Once started, voltage to this pin is maintained by the PSU itself. To shutdown, simply ground this pin. (You can find pin 15 since it is the last one on the side with the missing pin (pin 13).

However, if you are getting 10 seconds of power, the battery is doing a good enough job to get the power supply started and your problem is something trickier than simply getting the PSU started.

An old tech note talks about a thermal sensor that shuts down a Mac II if the temperature gets too high. Perhaps there is a fault in that circuit? Or maybe the PSU is dying and just cant handle any load?

 

[equill]

Half-AA 3.6-V lithium cells are available for USD$4 each in lots of four from the eBay seller ronaldo101.

de

 

[wally]

Perhaps http://home.earthlink.net/~gamba2/images/macIIsch.GIF will give you some ideas also. [] ]'>