LC Power Supply voltages

[Hornpipe2]

My LC won't boot at all. I have already ordered a new PRAM battery but while I wait for it to arrive I want to test my power supply and make sure it's still good too.

Can anyone help me out with the expected voltage on each pin of the power supply connector? Also, are there any dangers to the unit if I try to power it up without being plugged into the motherboard? I know some power supplies need a load connected or they can die when turned on...

 

[porter]

Have you tried powering up the whole machine, then quickly flipping off then on the power button?

 

[Hornpipe2]

I tried that, but sadly it still didn't boot. My symptoms:

* Power on LC on back - keyboard lights come on very dimly. No other noise, movement, video, nothing at all.

* Wait a minute. LC starts "clicking" faintly about three times a second. Keyboard lights blink in time with the clicking.

* Push keyboard power button at any time - nothing happens.

* Flick power switch - nothing happens, machine just goes back to clicking.

Anyway as to my original question, Vintage Macs mailing list archive has the answers. I am surprised I didn't find this before but here it is.

It is most likely that your LC power supply's leads are color-coded thesame as mine all are, so in order:

Black (2 of them) = GND

Orange = +5v

Yellow = +12v

Blue = -5v

In addition, on the "business end" of the power supply's output plug, the

number "1" is lightly stamped at the first Black wire's position, and the

number "7" is stamped at the Blue wire's position.

ETA: As these voltages match those on a standard PC ATX power supply I suppose I could run a very quick test by hacking up a known good PC PSU to see if I can get it working.

 

[Dennis Nedry]

Sounds like a bad power supply or bad logic board. Also it could be anything that's hooked up to the Mac. I have a Mouse that will cause a Mac LC to do this very thing, and when I unplug it, everything is fine. (It must have a short circuit.)

I would first try removing everything possible, i.e., remove any expansion card (ethernet / modem / video / IIe, etc) and also only hook up the power on the back, no keyboard, SCSI, etc. See if that makes a startup sound that way.

Even without a PRAM battery, it should make a startup sound.

 

[slomacuser]

I have a working LC board maxed out with 10 MB ram and 512k Video ram, willing for sale ... 35 EUR to USA and 25 EUR to Europe -shipping included.

 

[Gil]

I've had the clicking thing happen to a PPC mac. Pretty sure the logic board is toast.

 

[Hornpipe2]

Thanks for the tips. I will try out some of these when I get a chance (could be a couple of days or so). I'm not really invested in the machine so I doubt I will buy anything for it, but if something can be repaired then I may give it a shot.

Can you get an LC to power on without pushing the keyboard power key?

EDIT: Should I also try booting without any RAM installed?

 

[equill]

The LC does not have active power control ('soft power'), but an on/off mains switch at the back. There can be no expectation that it will start up from the keyboard, then, unless it is one of the LCnnn or AIO Performas equpped with pseudo-soft power, which my LCII and LCIII surely are not, even though I have no personal acquaintance with the LC. But even as almost-modern a Mac as a Q605 does not have pseudosoft or soft power.

Pseudosoft-power Macs (which include the CC) can display the rapid clicking from the PSU that you mentioned. The trick with reluctant CCs is to leave them with the AC switched on (behind the Mac) for 24-48hr, and then to use the power key on the keyboard for startup. Either a brand-new PRAM battery (or, in desperation) no battery at all rather than a weak (3.3V or less in your hand) battery is a prerequisite for this game, however. The flick-flick trick mentioned by porter can sometimes be of use if you have no battery at all, but the soaking on the AC supply is still a good thing in pseudosoft-power Macs.

The LC has 2MB of soldered-in RAM on the MLB, so you could start up from a System on floppy disk if you removed the paired (they are paired, aren't they? and no more than 4MB each?) 30-pin RAM cards. You could then also discount the possibly faulty nature of the HDD by disconnecting its logic and power cables.

de

 

[porter]

I would disconnect everything except the PSU and the speaker and listen for beeps of distress.

 

[Hornpipe2]

Update: Still no PRAM battery but I thought I would try a couple suggestions to see what happens. I opened the machine up and unplugged the hard disk, floppy drive, VRAM, and both RAM chips (leaving only PSU, fan, and speaker attached). Then I plugged it in to the wall and tried the power switch.

No luck - fan didn't spin, speaker made no noise aside from that incessant clicking. I checked over the board for some evidence of damage or capacitor troubles, but didn't find anything obvious (it's really clean in there!) Also disconnected and reconnected the power supply to the board just to see if the pins were not making good contact... didn't help.

Powered on again, waited 5 minutes or so, then tried quickly cycling the power. Still no boot.

Next up: I will go ahead and check the pins on the power supply to see if they are putting out proper voltage. Also will install the PRAM battery when it arrives and then try booting. Maybe there is something else I am missing...

 

[Charlieman]

For the PSU test, plug in a known working hard disk. It doesn't matter whether it is SCSI, IDE or any flavour, as long as it auto starts. The drive will put some draw on the PSU so that you get some sensible meter numbers.

 

[Dennis Nedry]

But even as almost-modern a Mac as a Q605 does not have pseudosoft or soft power.

Neither does the Power Macintosh 6100!

 

[Hornpipe2]

A nice member of the Vintage Macs mailing list offered to mail me a working LC power supply at cost of shipping. I figured I would take him up on the offer - if it doesn't get my problems sorted, I can always re-gift it later. So now I am waiting on that plus the PRAM battery, and probably won't touch the LC until I get both those parts.

Also, just reaching here: I had a theory on how a bad PSU could have these symptoms. If it needs to hold a certain charge in order to start and sustain the boot, but it can't build up enough power, then the LC could go into a loop of constantly attempting to boot / draining a capacitor / cutting off all power to the board. This would explain the clicking (power cycling) and also why the clicking doesn't start until a couple minutes later (got to slowly build up that initial charge first). Plausible?

Anyway I opened up the PSU to see if there were any obviously bad caps or scorch marks inside, but there was an electrolytic that looked big enough to knock out a horse, so I put it back together without poking around anywhere. The only way I want to be killed by a computer is if that computer is part of SkyNET.

 

[Blessed Cheesemaker]

Out of curiousity, is there a way to fix a dead power supply?

I'm sure there is, the question is, is there a way to teach an electronics dullard like me how to fix a dead power supply...

 

[wally]

http://www.repairfaq.org/sam/smpsfaq.htm

 

[Blessed Cheesemaker]

http://www.repairfaq.org/sam/smpsfaq.htm

Thanks! Let me know when /smpsfaq.4dullards.htm is posted!

 

[tomlee59]

http://www.repairfaq.org/sam/smpsfaq.htm

Thanks! Let me know when /smpsfaq.4dullards.htm is posted!

The "official" troubleshooting methods work reliably because they make few assumptions, and so you should follow them in the general case. However, if you're looking for shortcuts that work more often than they have a right to, here you go: The most common failure in modern supplies (so-called "switching" supplies) is of one or more electrolytic capacitors. Quite frequently, a simple visual inspection suffices to detect failure -- there will be noticeable bulging on top (and/or perhaps on the bottom).

Also check for a blown fuse and perhaps a cooked resistor (again, you can detect these failures with ease); replace if necessary. Replace the defective capacitor(s), and you'll often be up and running.

Troubleshooting other cases involves more work and more sophisticated test gear, but the "method" outlined above will frequently suffice.

 

[Scott Baret]

After working on many LCs over the years I can tell you there's an 85% chance it's the power supply and a 15% chance it's the logic board.

It's not the PRAM battery if it's an original LC. I know a 475 won't boot without a working PRAM battery but a 475 will at least give you a startup chime before failing to display anything on the monitor.

 

[Blessed Cheesemaker]

http://www.repairfaq.org/sam/smpsfaq.htm

Thanks! Let me know when /smpsfaq.4dullards.htm is posted!

The "official" troubleshooting methods work reliably because they make few assumptions, and so you should follow them in the general case. .

Well, one thing I can do is I can follow directions. If it is an algorithm of sorts, then I'll just start at the top.

Of course, I won't neglect to make a visual inspection first...

Coincidentally, I finally unboxed an LC I got off ebay, and upon start up it sounds like I am tuning in a radio broadcast, so I may have the perfect rig to start working on. Thanks!