Hello everyone,
I fear that I may have killed my apple IIe by plugging an IBM 5151 cga monitor into the game/joystick port. It no longer beeps, and the power led stays on. No video output. As to how this happened, I have an Apple IIe with a PC Transporter card from Applied Engineering. I plugged the monitor into the wrong port.
I've taken all of my expansion cards out, and measured resistance on all of the voltage rails to ground. The 12v rail had very low resistance (6.5 ohms), but raised to over 800 when I unpluged the keyboard.
I would really like to get this IIe working again, it has a few AE upgrades and is fairly unique. Any help on what could be wrong is appreciated!
Hopefully you didn't fry the IOU chip. Those are unobtanium. If you blew one of those out you'll probably need to get another motherboard and to do that you may need to buy another //e. If you're lucky maybe you only blew the NE558 timer or one of the 74LS251 multiplexors. Those would be the chips I'd try replacing first.
Evey pin on the 74ls251 is black, so I'm going to order a replacement for that! I'm keeping my eye out on a for parts IIe or a motherboard.
Thank you for the help!
The NE558 and the 74LS251 near the keypad connector are the two chips that are most directly connected to the game port so they are the most likely to blow if dead shorted or incorrect power is run through... Hopefully it will start working if you replace the 74LS251.
The very first thing to do is check if you still have +5V on pin 2 of J8, because by plugging a CGA monitor you have shorted this +5V to ground.
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That's a good point... maybe he popped something in the power supply. If no 5V there, then I'd check the other voltages at some spot like the power connector or slot connectors. Given his description of no LED, now that I think of it...
Also maybe if no 5V ... try removing the NE558 and 74LS251 and measure again. Possibly if one is shorting it could cause no power. Removed that shouldn't be if that was it.
If no change, first thing would be to open up the power supply and check the fuse. Possibly it blew. If not and still no power, then probably there might be a bigger issue.
I measured 3.98V at pin 2. I also just noticed the power LED on the board next to the power connector is rather dim. It get brighter right when I turn the power off.
3.98V is a little low on +5V obviously... If you pull the 74LS251 and re-test does it change?
Since you're getting something obviously your fuse in your power supply is OK, so no need to open it up to check that.
You can check the other voltages from the slots using this:
https://www.applefritter.com/appleii-box/APPLE2/D006b_resurectionIIe/AppleIISLOTpinout.jpg
I am pretty sure at this point the PSU has been damaged. If it's 4V on pin 2 of J8, it will be 4V everywhere on the 5V rail, unless of course the SN74LS251 is causing this through an internal short.
Can you unplug the PSU from the motherboard and measure its 5V without a load? Also remove the SN74LS251 and see if you get 5V without it.
If the power supply is indeed damaged, probably the best fix would be one of Re-Active Micro's new power supply boards. This system sounds like it is pretty loaded if it has power hungry cards like a PC Transporter in it. I'm assuming the power supply in there may be an AE one or some other 3rd party high output supply as the original //e supply would struggle under a heavy load of a lot of cards even when it was new. The Re-Active Micro board is supposed to fit into AE power supply chassis and it should give similar power to those.
So, this IIe already have a psu from Re-Active Micro. When I first got it, the RIFA caps not only blew, but it actually caught on fire and damaged most of the psu board (It did have an AE psu). All 4 rails test good when not plugged into anthing (12.16V and 4.97V for +5 and +12).
After removing the ne558 and the 74ls251, it's still 4.0V, but I am getting 4.96V at the test point on the front of the board.
How is that even possible?? Test point 1 in the bottom left of the motherboard, the 5V rail on the slots and pin 2 on J8 are all connected together. Maybe you have burned a trace?
In post #9, CVT wrote:
"Can you unplug it from the motherboard and measure its 5V without a load ?"
Uncle Bernie cautions:
Most switchmode power supplies need a minimum specified load on their main output voltage to work properly. Some even need minimum loads on all their output voltages. If you have no load, weird things can happen, such as the whole switchmode controller losing regulation, going in some emergency operating mode, or blowing up things. Alas, I can't tell how the Apple IIe power upply reacts to the "no load" condition, as the mine blew up earlier this year.
What I'd do in this case is to pull all the irreplaceable "unobtainium" Apple custom chips and the DRAMs / ROM(s) out of the machine - if they are socketed - and then solder a red and a black stranded wire to the +5V and GND PCB pads on the power connector of the motherboard. Double check by measurements against VCC and GND pins of TTLs for the red wire going to their VCC (typically pin #14 on a DIL-14, #16 for DIL-16) and the black going to their GND (typically pin #7 on a DIL-14, #8 for DIL-16). Set a laboratory power supply to +5V with ~200 mA current limit (Apple II not plugged in yet). Plug the Apple II into the laboratory power supply. Expect the current limit to engage. Use voltmeter on a TTL to verify again polarity is right. Crank up the current limit in small steps (100 mA) or so and watch out for any ICs getting hot or making smoke signals. Immediately turn off the power if you spot one (you got one cuplrit). Some people use brief touches with spit wetted finders to probe the IC temperatures. This prevents blisters of a dying or dead IC get too hot. A thermal camera of course looks more professional but the older ones we had back in the day were bulky and needed a gulp of liquid nitrogen from the lab's storage tank to prepare them. Too much hassle. So even seasoned lab technicians did prefer the spit/finger method.
Once you found an IC that overheats, pull it and don't put a new one in yet. Proceed until nothing overheats anymore. Then you should get +5V at a current limit setting at or above the specified current draw of the motherboard.
This procedure allows you to find all the bad ICs on the motherboard which cause overcurrent and need to be replaced, without engaging the Apple II's switchmode power supply. Unless you have a proper mating connector and a load resistor in your lab, you can't really test it without risk of damaging either the contact fingers in its connector or the electronics inside.
Use these tips & tricks at your own risk. Be aware you still can damage things even if you follow the time tested procedures outlined above. I shall not be held liable for any incidental or consequential damages, loss of property, injury, and death, or any other effect, such as the end of the world as we know it, if you follow my well meant advice.
But the bottom line is, if you mess around with a damaged piece of electronic equipment, do it in a systematic and professional way. If you just start experimenting around and plugging things in and out, you may cause even more damage than there already is (been there, done that, made lots of smoke signals in my life).
- Uncle Bernie
This is not an issue, the PSU has internal load resistors for this purpose.
Oh, while I was writing my post #13, progress has been made, so please disregard that post.
In posts #11 and #12, attackracer and CVT wrote:
"After removing the ne558 and the 74ls251, it's still 4.0V, but I am getting 4.96V at the test point on the front of the board."
"How is that even possible?"
Uncle Bernie explains:
Voltage drop along PCB power supply and / or ground traces can do this. If there was a short circuit which lead to "frying" of such traces then they can get higher ohmic or open like a fuse. And then the supply voltage will seek another path ... which would be a weak power supply trace OR some IC or any other electronic component making a sneak path for this supply voltage.
You can find the approximate location of the damage by measuring the supply voltage beginning near the power connector. Measure supply voltage on the ICs, one per row is enough, and work away from the power connector step by step until you find a row where the voltage drops. The location of the fault is somewhere in between the last known good and the first much lower measurement (note that the actual traces can be routed in weird ways, but you then at least know where to start following the bad supply or ground trace). It's advised to first measure the +5V alone (this probe moves) while keeping the ground probe near the connector. If you don't find the drop in this case, the damage must be in the ground traces, so repeat the procedure keeping the +5V probe near the connector and moving the gnd probe across the PCB.
- Uncle Bernie
For the test points, from left to right I get -12v, 5v, then ~4v. the ~4v pad has no resistance between it and pin 2, but the 5v pad has about 74 ohms between it and pin 2
On my Apple IIe it's 0.39 ohms between test point 1 and pin 2 on J8. My multimeter leads are actually 0.33 ohms, so it's really 0.06 ohms when I substract the leads.
And just to make sure we are talking about the same test point:
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75.8 ohms between the two test pads on mine (The one with the arrow, and the pad to the left.)
All you have to care is the resistance between the pad with the arrow and pin 2 on J8. I don't understand what you mean by "no resistance". Do you mean they are connected? Also you should be getting -5V on one of the test pads, not a 5V and a 4V.
My guess at this point is that you are getting 4V everywhere on the 5V rail and before when you said you were getting 5V on the test point, you meant -5V, which is a different rail. If this is the case, either a chip has shorted internally and is causing this, or what UncleBurnie suggested above with damaged power or ground trace.
I double checked, it's not -5v, it is positive 5v. the 4v test pad has less than 0.1 ohm between it and pin 2. I've repeatedly checked the psu and it's pins, and it is putting the right voltages out on the right pins. I'm not sure what could cause the -5v rail to become +5v, but something is quite wrong
Wow, that is bad. This means a short between the +5V and -5V rail somewhere on the motherboard or insde a chip. Since the +5V rail is rated for 2.5A and the -5V only for 0.25A, the +5V rail mostly wins this arm wrestling.
At this point I would inspect the back of the motherboard. You might be able to spot the problem visually.
I can't find anything visually wrong with it. I took out all of the socketed ICs (all of them, in this case), and now when powered up the power and fault LEDs flash rapidly, along with a very faint click from the psu, in time with each flash.
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Not sure why this is happening, but you should check the -5V rail capacitor on the motherboard (C15 or C17). Having its voltage reversed most likely has damaged it.
Are you still getting +5V on the -5V rail without the chips?
I'm also going to replace all the electrolytic caps on the board, as I'm convinced there is a short somewhere. It's still positive 5v after removing all the chips.
These I believe are tantalum caps, not electrolytic, which makes them a strong suspect. You can just unsolder one of their leads and check them with a multimeter.
I have a similar early rev-B //e board here that is about 6000 SN off from that one that appears to have black electrolytic caps instead of the silver ones. Maybe they're still tantalum but they look like ordinary electrolytic.
Anyway, if it is something affecting the voltages, the caps probably most likely to be bad I'd guess would be the ones nearest the power connector. But who knows.
The black STC caps are tantalum too. Mine has those. But they can be replaced with low ESR electrolytics without any issues. The Pravetz clone of the Apple IIe even uses regular electrolytic caps.
IMG_5533.JPG
Good to know. Those four caps are the ones I am suspicious of on the OP's motherboard given the weird voltages he is reading. It would seem like they'd be the most likely to cause odd power behavior if they went bad.
Weird, my board has 8 10uF 16v caps, but they are silver and are definitely electrolytic. Most small tantalum caps tend to fail suddenly and usually burn when they do. New electroclytics will take awhile to ship, but I'll post a reply if I get it working.
I think they are tantalum, at least they look like your typical axial tantalum capacitor: click!
If you can measure their ESR, you'll know right away. Tantalum caps are really low for their size. Also they tend to be really accurate when you measure their capacitance, which also doesn't increase with age.
Semantics disclaimer: tantalum capacitors are considered electrolytic too.
The board picture shows aluminum electrolytics from Tecate Group (T.I.), which were one of the few marques (or the only one?) made in Mexico.
I'm not sure they were damaged in the "incident", semiconductors are much more sensitive.
Yep, they are not tantalum. I can now see the black rubber on the plus side on C72.
Still regular alumin(i)um capacitors get damaged when you reverse their voltage. They are only safe under around 1.5 V reversed voltage.
I'd agree in general, but in this case, if I'm not mistaken the voltage faults are showing even with all the ICs removed. That being possible because this is an early Rev. B motherboard which is socketed. With most later boards 1/2 the chips are soldered in which makes that kind of testing a real pain to the point of being basically unfeasible.
Yes, with all the chips pulled out the short should be really easy to find, considering there is only one chip that needs the -5V rail, and that is the MM741 op-amp. Besides you don’t really need this chip unless you are going to load software through the audio port. It can be pulled out and the Apple IIe will work fine without it, at which point you don’t even need the -5V rail, unless you have a card that needs -5V.
In fact with MM741 pulled out, if you measure the capacitance between the -5V rail and ground you should be getting around 11uF, which is the added capacitance of the -5V rail main 10uF filter cap C17, the 7 slot-side 100nF bypass caps of the -5V rail and the 100nF bypass cap of the MM741 itself.