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  #1  
Old 07-07-2007, 03:12 AM
phe phe is offline
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Default debugging the protect circuit

I'm trying to figure out why the flip flop in IC L on the memory board is not changing state. I've checked the voltage on lines 20 and 70 of the bus when I flip the switch to protect and unprotect. Both lines go high when I flip the switch in either direction (protect or unprotect). With the system powered off, pins 20 and 70 are open (infinate resistance). So there is no short between them.

I've checked resistance across the three pins from the protect/unprotect switch. With the system off, the top and bottom pins of this switch show zero resistance, so something is shorted across those two pins. I would not expect this to be correct.

Anyone have any ideas of further places I could check to find out where the problem might be. I can't image it is a bad switch - that would be nearly impossible wouldn't it? But shouldn't these pins show infinite resistance between them? I'd like to find other signs that point to a bad switch before I desolder and remove it.

Paul
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  #2  
Old 07-07-2007, 08:22 AM
sje sje is offline
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Default

If the two throws of an SPDT switch have zero resistance between them, then there's either a broken switch, bad wiring, or a bad circuit design. I'd say that the switch is by far the most likely culprit.

Get an extra switch or two before starting the desoldering. It's always good to have a spare.
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  #3  
Old 07-07-2007, 05:00 PM
marty marty is offline
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Talking protect circuit

After removing the switch, check the circuit for a short, and if you have not destroyed the switch , check it. If the switch is the problem, them most likely you have dicsovered the problem. If it is still there , then take out the IC, If you don't have a socket on the IC, I would suggest putting one in. If the short is still there, them most likely either you have a short or a pcb short. Sometimes a PCB short can be taken care of with a voltage on one side and ground on the other. I would start with 5V before going to 12V ,, BUT This is a VERY Last resort, I would try every other method I could think of before doing this. I have had to short a High pin to ground to get an internal short fixed, but this was on some Very old Imsai front Panel connections, but the front panel now Works!!
Thanks Marty
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  #4  
Old 07-09-2007, 01:02 AM
phe phe is offline
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Default

Thanks for the helpful reply, Marty. I pulled the switch out. The switch is OK. The problem must be in the circuit. I've been visually inspecting for solder bridges and have not seen any. I pulled one of the ICs (I think it was IC W) and there was still a short. I'll be away from home for a couple of days so I cannot try anything new until I return on Tuesday. Anyone have any idea if there is a different IC that I should pull?

Paul
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  #5  
Old 07-09-2007, 03:01 AM
marty marty is offline
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Talking protect circuit

Hi Paul;
I wrote a long letter to you and somehow it got lost, so if you see 2 posts from me this is why.
First, try this, the two wires that go to the switch find the pins on the IC and Carefully bend them up, and attach another switch to them and connect the middle connector ground or to +5V depending on what it needs, I don't have a schematic in front of me. And see if the protect unprotect circuit works.
Also look around the traces that go from the switch to the IC and see if there is another trace shorted to it, unrelated to the protect circuit.
If there is no change them pull the wires to pins 20 and 70 and check them out using either a scope or a logic probe or a volt meter. And see if you can get them to show the proper values. If they work then either you have a problem in the Mother board or the memory board.
Thanks Marty
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  #6  
Old 07-10-2007, 03:01 PM
marty marty is offline
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Talking protect circuit

Hi Paul;
I have some of the circuit in front of me. So connect your other switch the middle pin to ground. there are two pull-up resisters R13, R14, check that they are BOTH 1K Resistors. they then go to a pair of inveters. the protect goes to pin 3 of the '04, and the unprotect goes to pin 1 of the '04, these should invert the signal. So with them being pulled up they should the LOW on their out-put side. on the protect side pin 4, on the unprotect side pin 2. If they are HIGH then either the inputs are not being pulled up, or shorted to a LOW signal or to ground or the IC is not an '04 or not working correctly, (BAD!!) The 8T97 (IC X) should pass the signal through and not alter it at all, HIgh in should be a High out, and a LOW in should be a Low out. Pin 10 and 9 on the protect side and pins 4 and 5 on the unprotect side. Check ALL Traces surrounding any IC or Trace Coming or Going to the 04 or the 8T97 or the solder pins 20 and 70.
Thanks Marty
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