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Macintosh IIcx PSU Problems

CompuNurd

CompuNurd

68000
I recently got a Macintosh IIcx with some other stuff which is in need of repair. Today I went to power up the IIcx to see what the deal was...nothing.. not a single click, or so I thought. The noise the PSU was making was drowned out by the click of the power button. I held the power button, tick tock.. tick tock.. tick tock.. I noticed some movement in the PSU, so I took a look at it closer, thinking something was sparking inside. Tick tock.. tick tock.. tick tock, AH HA! I see a relay moving inside, and the PSU fan is running solid, no intermittent movement, hinting the caps may be good, maybe. What is this strange relay movement? Caps? Logic board caps? Shorted connection? What is going on here?!

 
Motherboard probably needs a recap. That was the issue with mine. Regardless, if the mobo caps are original they will need replacement by now anyway. So its a good place to start.

 
Not really. Those caps don't typically leak, they dry out. I ripped my prototype IIcx power supply all apart trying to get it to work! As I said, it turned out to be the mobo. I sent it to uniserver (member here) for a recap. Fired right up. I have about 4 IIcx type power supplies, they are all working.

Not saying they can NOT go bad. Just saying in your case a mobo re-cap will probably fix the issue and its needed anyway.

 
That trickle diode thing was from Apple long ago to known repair shops at the time (late 1980s) as they were getting back too many "defective" IIcx PSUs. They figured it out and then sent the info to "us" techs and we all went along like nothing happened.

I wonder that Apple did with the 4 PSUs from my IIcx? They died from massive power surges that were from electrical fires in my building, several floors below me during the late 80s / early 90s. The PSUs gave themselves up to save the rest of the system.

 
Ah.. ok. I was going to say. ;)

If you ask me though, that was a pretty cheesey design for a standby regulator though. IMHO. But I guess they were built down to a price point, so it is what it is, that or the technology just wasnt available then like it is now. Nowadays, you can use a simple 8 pin TNY regulator with a small SMPS transformer, and generate 5V or 12V standby voltage all day long. 

 
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One of the reasons why I left the (Mac repair) company was because the company owner took a bunch of new jack techs and had them replace the diodes on the PSUs and then sell the repaired PSU as a new one. At $275 for a $2 part and 1 hour of time with a fake "new" label, while claiming to the IIcx (and later ci) owners "It's going to take a week to order the part for you"... - he was making out like a fat rat! In some cases the original PSU was repaired and not replaced and he claimed that it was!

Eventually (many years later that is) his business practices would be caught, and though the company remained in business, he was arrested for unscrupulous and cheating business practices.

 
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So I desoldered the diode, and the PDF says to connect it in series to another diode of the same type. Why can't you just replace the diode?

It also says to put in a new capacitor at C34, but it doesn't sound like it is required to do so. Would I be fine by leaving C34 as it is?

 
Repairing and testing is always give and take. You replaced the Diode, OK. You want to test it, maybe it will work? Go ahead. If it works, replace C34? That's on you at that point. Do you have a spare C34 to throw in there? If you do I would say yes but you don't have too.

I have always thought that diode replacement was to remove the old one and replace with 2 (smaller signal) diodes in series with each other.

 
All your doing here is increasing the diode voltage drop. 

Heck, some Class AB power amplifiers used 3 diodes in series to get the correct bias point for the output transistors. 

 
The place I found the diodes requires you to buy a minimum of two, so I guess I will just trash the original and add the new ones in series.

 
My money is still on the board. No extra power supply to swap in? The IIcx supply is also found in the IIci, Q700 and not sure where else.

 
Elfen said:
One of the reasons why I left the (Mac repair) company was because the company owner took a bunch of new jack techs and had them replace the diodes on the PSUs and then sell the repaired PSU as a new one.
Click to expand...
Back when Mac discussions like this took place on Usenet (comp.sys.mac.****) and there was no Ebay (comp.sys.mac.wanted) there was an outfit offering to fix "dead" Q605s and LC/P 475/6 for $100 with exchange of the old logic board.

They were taking the old logic board, replacing the battery, and sending it back out again.  $100 to change your battery.  Quite a racket.

 
you may need a correct load? is it a 1 second tick-tick-tick? 

2 events trigger this. underload, and overload. Check ohms readings between your rails to ground. at the motherboard, and at the power supply. if the impedance is too low, or shorted, thats a problem. 

If the rail that contains the feedback network goes unloaded, or underloaded, it will cause an overshoot which the power supply will shut down and recycle from gross overvoltage condition. this trigger repeats until the correct load is produced. 

Same thing if its shorted, or overloaded. it trips the overload detect sense resistor on the source of the MOSFET. same trigger event occurs, the timing capacitor on the SMPS control IC sets this. 

 
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