Voltage Testing - AC Low Logic Fault?
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by Bill Degnan - 04/19/2014 16:18 |
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regulators' output from front to back:
h744 - both +5v = 5.00 h744 - both +5v = 5.00 h744 - both +5v = 5.00 h745 - -15v = 15.06 (note this regulator has a +15 input = 16.1v ) h754 = -5v = -5.06 +20v = 20.09 Backplane readings -15v = -15.06 LTC = 2.29 +15v = 16.10 (input TO regulators) +5v = 4.76 * +5v = 4.76 * ACLO = 1.727v DCLO = 3.46v * Something in the backplane (a card I assume) is bringing down the +5v? What causes the ACLO to be 1.727v? I read that it should be closer to DCLO. The front panel BUS and PROC lights are on, front panel is unresponsive (still can't read/write to memory, halt/run, etc.) Thoughts 1. I read a web page that said another 11/40 owner pulled the connector from the power distribution board to the CPU backplane (assuming it doesn't have any power on that connector) and re-applied power. Doing so allowed him to interact (although incorrect results) with the front panel. 2. Do I have an open grant chain? The next three are from "Hints on Testing a Dead PDP 11 site http://www.psych.usyd.edu.au/pdp-11/hints.html 3. check backplane with oscilloscoe for a ripple, and make sure it's less than 200mV on the +5 supply. 4. "..If you find low voltages at the backplane, but normal at the regulators, check the connectors, especially the MateLock types used on the modular regulators .." 5. check the AC and DC low signals. These are bussed via the Unibus (and Q-Bus), and a failure in any system box can shutdown other parts of the system (depending on the supply design). Again, use an oscilloscope to check for pulses (it should normally be above 2.5 volts) since it may be putting the CPU into power fail mode. All the DC voltages may be OK, but a fault in the AC low logic can still stop the CPU from running. Fault in ACLO causing +5V to pull low? I need more documentation. Reply |
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