I have a IIGS that does not startup. I am trying many things but have read that a bad power supply can be fatal to proper startup. I did some checking and get the above referenced voltage reading. First, does it matter that the voltage is almost double what it should be? Second, is it fixable? Finally, could this be the cause of my problem? When I power up, the 3.5 drive make a noise but nothing else happens. No screen display or sounds from the speaker.
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When you say the voltage is almost double? There are 4 voltages coming out of your power supply. +12, -12, +5 and -5. What is each one showing? Also to get an accurate reading of whats coming out of the PSU, it needs to be under some kind of load drawing from it. Such as connected to the motherboard. How are you testing the power supply to determine the voltage is almost double? Apple used 2 manufacturers for their power supplies. Astec and Dynacomp. You CAN get readings with a multimeter on an Astec with no load. They aren’t accurate but they will be close to what they should be. Definitely not double. The Dynacomp will need a load to be tested.
Look for the thread on here about replacing an Apple II power supply board with a Mean Well PT-65B board. That article is mostly about ][+ or //e style power supplies but the same basic methodology can be used for the IIgs power supply. Also if that is too DIY for you, Reactive Micro has a kit to replace power supply boards with theirs. They still offer a service to do the work for you as well I think, although I've done several of theirs and they weren't hard. I just recently did the MW PT-65B on an AE (vintage aftermarket) power supply too. If you can solder you can do it.
https://www.reactivemicro.com/product/universal-psu-kit/
I don't usually endorse shotgun capacitor replacement or "kits", but the DynaComp power supply may be an exception as it contains some electrolytics that fail rather dependably. This would also be an easy repair if that is the cause of your issue. These cap kits are available on eBay if searching out the capacitors yourself is too annoying.
The 1 µF, 50V replacement capacitors are to replace two 1 µF, 35V factory caps that were believed to be underspecified for the circuit.
When electrolytic capacitors are used over their rated voltage, some weird things can happen including behaving like a diode. I don't know whether your -5V problem is related, but I can't rule it out.
-5 V is only used for two things in a IIgs:
I suspect, but don't know for sure, that the machine will boot and run perfectly well without any -5 V, but with composite video output only.
If only the -5V is bad, I might suspect that the 7905 or one of the components associated with it (usually a couple of capacitors) are the culprit. I'm assuming it probably has a 7905, but I haven't seen the schematic for that so I don't know for sure they are using that -5V regulator, but it is super common.
You could do something similar to what people are doing to adapt a Mean Well PT-65B power supply, which is using a 7905 off the -12V wire to provide -5V. That might work for you if you can't find another way.
Or just spend the $22 (+ shippping) to buy a PT-65B, and a couple bucks for a 7905 and just swap your power supply board out. It would be very similar to the ones people have done for //e power supplies or the one I did for an AE power supply. I also use a piece of 1/8" plexi but that's not much cost either.
Are you sure you are measuring the right pin? That voltage sounds abuot right for the -12V line. If not maybe there's a short somewhere?
But yeah, that -11V could be rectified to -5V with a 7905.
I don't recommend relying on a malfunctioning PSU that gives out -11V on its -5V rail to get -5V through an aditional 7905.
It would be better to fix it. Here are the schematics:
http://www.appleiioz.com/AppleIIoz/Projects/Entries/2016/10/9_240V_Dyna_Comp_PSU_Circuit_Diagram.html
First check if you still get -11V on the -5V rail when you disconnect the motherboard, in order to be sure that it's not the motherboard causing it. If it's not the motherboard, check R36 and R37, as they are the only connection between the two rails inside the PSU. If they are fine, the problem is probably the 7905 (U2), it's diode 1N4001 or the electrolytic caps on each side of it. Actually I would replace all four electrolytic caps visible here (C19, C20, C24 and C25).
NegativeVoltages.png
To be clear, I agree with CVT that relying on a faulty output and trying to rectify it to be correct is probably a bad idea. I also think he's on the right track with his repair advice.
The reactivemicro PS is what I use in my 2 GS machines. It does not match the old schematics.
It seems likely that there could be a problem with the MB. The previous owner may have tried to fix with a new supply.
The Reactive Miceo power supply boards are usually pretty decent. If it gives bad voltages when running by itself (may need a dummy load, I don't know about that PS) then you can either buy another Reactive Micro board or you could go with a Mean Well PT-65B as was sugested before, One thought is maybe to try to contact Henry Corbis at Reactive Micro and see what his sugguestion might be.