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I have a iic that won’t boot. Need help

Had one that kept crashing and doing weird things... Reset wouldn't reset the system. Replaced the CPU and it ran fine.

Had a IIe that had strange memory issues at random. The ground pin on the MMU had been replaced and the pin was corroded/discolored. Replaced that replacement pin with a nice one from a donor chip and that took care of it.
 
The reset line should is active low so it should initially be low and then, after a short period of time, go inactive (+5 volts through one of the resistors in in RP1, as mentioned already by retrogear). If the signal remains low then something is keeping it low. The signal is generated by the IOU IC (UE14) and, aside from the processor itself, connected to the following other ICs:

IWM - UD2
SER1 - UD11
GLU - UD6

As well as the keyboard connector (J9) on pin 29. My recommendation would be to disconnect the keyboard and pull the IOU (if socketed) and see if pin 40 of the processor is at +5 volts. If not then pull the other ICs (if socketed). Any one of them could be pulling the reset line to ground.
 
oldpcguy said:
The reset line should is active low so it should initially be low and then, after a short period of time, go inactive (+5 volts through one of the resistors in in RP1, as mentioned already by retrogear). If the signal remains low then something is keeping it low. The signal is generated by the IOU IC (UE14) and, aside from the processor itself, connected to the following other ICs:

IWM - UD2
SER1 - UD11
GLU - UD6

As well as the keyboard connector (J9) on pin 29. My recommendation would be to disconnect the keyboard and pull the IOU (if socketed) and see if pin 40 of the processor is at +5 volts. If not then pull the other ICs (if socketed). Any one of them could be pulling the reset line to ground.
Click to expand...
Thanks, The IOU is the only IC on your list that’s socketed but I’ll start there. I’ll let you know if I get +5v at pin 40 on the CPU with it removed. If not, I’ll go from there.
 
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Mochatea396 said:
Thanks, The IOU is the only IC on your list that’s socketed but I’ll start there. I’ll let you know if I get +5v at pin 40 on the CPU with it removed. If not, I’ll go from there.
Click to expand...
Ok, I removed the IOU and powered the board and I get 5V on pin 40 of the CPU. With the IOU in, pin 40 shows only 1.86v. And now it’s back to no logic reading at all on pin 40 with the IOU in. No logic at all was what I was getting initially, then I started getting low logic, now it’s back to none, remove the IOU and the logic stays high. I just checked the IOU again and it’s still good. That’s the one socket I haven’t replaced yet.
 
Does pin 28 of the IOU have 5V supply ? Look at the reset pin 15 with a scope on DC mode. Is it also a constant 1.86V or is there pulse or noise on it ? I can't imagine how it would end up with 1.86V.
It's supposed to be 5V high or 0V low.
 
retrogear said:
Does pin 28 of the IOU have 5V supply ? Look at the reset pin 15 with a scope on DC mode. Is it also a constant 1.86V or is there pulse or noise on it ? I can't imagine how it would end up with 1.86V.
It's supposed to be 5V high or 0V low.
Click to expand...
IMO the most likely reason is a problem with the IOU. Either it is bad or something connected to it is causing it to misbehave.
 
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I replaced the IOU socket just to rule out that as a possibility. You never mentioned if I should have the IOU in or not so I tested those pins with it out. I have 5v on both pin 15 and pin 28 of the iOU socket. When the IOU is in, I still get 5v on pin 28 but pin 15 shows about .15v. I even checked the IOU once again in a working machine and it boots fine and the working boards IOU does the same thing on this board as the other IOU. I’ll have to agree with oldpcguy, since it doesn’t look like the IOU is the villain but something connected to it must be causing it to misbehave. But what? Also, now I have low logic back at pin 40 on the CPU. For some reason I lost all logic at pin 40 at one point but now it appears to be back. When the IOU is in the socket. Still staying low still.
 
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Mochatea396 said:
I replaced the IOU socket just to rule out that as a possibility. You never mentioned if I should have the IOU in or not so I tested those pins with it out. I have 5v on both pin 15 and pin 28 of the iOU socket. When the IOU is in, I still get 5v on pin 28 but pin 15 shows about .15v. I even checked the IOU once again in a working machine and it boots fine and the working boards IOU does the same thing on this board as the other IOU. I’ll have to agree with oldpcguy, since it doesn’t look like the IOU is the villain but something connected to it must be causing it to misbehave. But what? Also, now I have low logic back at pin 40 on the CPU. For some reason I lost all logic at pin 40 at one point but now it appears to be back. When the IOU is in the socket. Still staying low still.
Click to expand...
Any chance you'd be willing to install the IOU with pin 15 bent out of the socket? Then measure the signal coming out of that pin.
 
I didn’t feel comfortable bending a pin out on the IOU so I took a 40 pin socket and removed pin 15. I’ve done this before and it works well. Push the IOU onto the socket and push the assembly onto the socket on the board. Pin 15 isn’t touching anything. I am however getting no voltage to pin 15. I still get 5v to pin 28. I know this sounds like the IOU is bad but it’s not.
I did try the original ROM back when this problem first came about. I had hoped my 4X was bad, but same with the original
 
Mochatea396 said:
I didn’t feel comfortable bending a pin out on the IOU so I took a 40 pin socket and removed pin 15. I’ve done this before and it works well. Push the IOU onto the socket and push the assembly onto the socket on the board. Pin 15 isn’t touching anything. I am however getting no voltage to pin 15. I still get 5v to pin 28. I know this sounds like the IOU is bad but it’s not.
I did try the original ROM back when this problem first came about. I had hoped my 4X was bad, but same with the original
Click to expand...
Using that configuration what is the voltage measurement on CPU pin 40?
 
Mochatea396 said:
With pin 15 of the IOU not making contact, pin 40 on the CPU is now 5v.
Click to expand...
I am going to have to give this one some thought as it appears to be the same issue I am having with an Apple IIc Plus system.

Just to confirm you're getting ~ .15 volts on IOU pin 15 when you have it in the socket which disconnects pin 15 from the system board? What are you using to measure with?
 
reading Jim Sather's book, the IOU provides a 33ms delay at power up before releasing pin 15. The disk controller will mask that with a 100ms delay.
Does the drive light turn on when you power on ? The IOU also controls the $C0xx addressing which includes the disk controller addresses. A problem with
the disk controller could potentially hold pin 15 low
 
That’s a valuable book I understand and an actual original copy gets huge $$. Ok, we’re looking at the IWM possibly being faulty not sending the proper signal to the IOU? Do you think it’s worth desoldering it from the board and testing it? The best I can test it with is the Back Bit Chip Tester. It will test the IWM. It’s been pretty accurate testing MMUs and IOUs. That’s the best I can do really, I’m not desoldering another one off a working machine and swapping them. And no drive light on the keyboard. Just the power led. I don’t have the keyboard or drive connected now but I clearly remember only the power led when they were connected.
 
it might be worth a try especially since you didn't see a drive on indication. Sather's book is actually about the //e but the principle of holding the IOU
until the drive is ready would be the same I think.
 
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