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11/84 will not power on, but peripherals will

NeXT

NeXT

Veteran Member
Joined
Oct 22, 2008
Messages
8,124
Location
Kamloops, BC, Canada
I have not run the system in a few months so I closed the circuit breaker and turned the key only to be greeted with absolutely no life from the front panel, but the Power Control Bus did respond and powered up the drives.
Cycled the key a few more times and I got life and weirdly no complaints about unstable voltage rails or any problems at all. Ran it for two hours and cycled the key to test something and it was stone dead again, save for the peripherals.
Took the lid off and there's no fans and no DC rails coming up at all according to the LED's. Left it over night in case this was some weird pseudo indication recap time was approaching and this morning it was still no better.
Where should I start troubleshooting the power supply? It obviously has standby power for the Power Control Bus.
 
Oh yeah. the 11/84-BA uses the H7202KA power supply with the H7211, H7200 and H7213 subassemblies. My initial suspicion is that none of the secondary supplies are being told to turn on.
 
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Okay looking back through my notes, the power control bus doesn't get powered from the host CPU, it's just closing contacts and doesn't mean anything, so the keyswitch works but the PSU isn't coming up at all.
Quickly took the lid off and poked into the supply with a thermal camera and I can see a few warm components, so part of the standby power system is working. I cannot so far find schematics for this PSU on bitsavers.
 
Well after scanning over the schematics and then pulling the PSU out (that isn't fun because it's a bit buried) I wasn't able to immediately pinpoint a fault and it was still exhibiting the same symptoms and left it alone for a month.
Then I closed the breaker this evening and again no life and then came back later and it responded to the key.

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It's sitting at ODT with the CPU running. All the DC rails came up and are stable. It's gotta be in the standby power circuit because the rest of the PSU is fine.
I'll leave it for a few days and see what it does.
 
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Which is what I thought as well since between months of no use the breaker is open and nothing is powered and then turning it on it might not work initially but after a while a cap somewhere has reformed enough to start working again.
BUT, on my first discovery I did that and left it to sit for both hours and days in the off position, then tried turning the key. No dice. It's unreliable with the system in "warm standby" or from a cold start.
 
NeXT said:
Which is what I thought as well since between months of no use the breaker is open and nothing is powered and then turning it on it might not work initially but after a while a cap somewhere has reformed enough to start working again.
BUT, on my first discovery I did that and left it to sit for both hours and days in the off position, then tried turning the key. No dice. It's unreliable with the system in "warm standby" or from a cold start.
Click to expand...
A real long shot, but back in the day some of those old power supplies used 47 ohm resistors for a discharge path. Not as likely that the resistor is bad as a bad solder job. It would be redundant to say something may be intermittent in the supply, but may be worth a good going over.
 
We had a lightning strike across the valley. Nothing in the house was damaged but it briefly interrupted the power. The /84 is again refusing to turn on.
It's been sitting here for the last hour until I could get home and physically turn the key to OFF but with the thermal camera I can see several resistors remaining warm, so standby power is present in this weird "will not turn on" fault condition.
 
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I hated troubles that just "went away" by themselves.

They had the annoying habit of coming back by themselves as well.
 
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