PDP 11 gear finally moved

John Robertson jrr at flippers.com
Sat Jul 18 11:42:54 CDT 2015

Oh, sorry, didn't realize they used switchers for the PDP-11s.

However I was talking with a friend of mine last night about my error, 
and he told me that the switching supplies for the PDP-11s were very 
unreliable back in the day. He often had to troubleshoot the machines 
back then. A common failure was caused by static electric shock to the 
machine would blow the supply. NO carpets allowed!!

John :-#)#

On 07/17/2015 2:19 PM, Jay Jaeger wrote:
> Ummmm - his PDP-11/34 most certainly does use switching power
> regulators.  ;)
> On 7/17/2015 4:06 PM, John Robertson wrote:
>> On 07/17/2015 11:53 AM, Mouse wrote:
>>>> I do find this witch-hunt against capacitors to be curious, given how
>>>> few I've found to have failed.  I suspect a lot of it comes from
>>>> audiophools who think this is the way to fix anything...
>>> Perhaps.  But not all of it, certainly.  I'm currently four for four
>>> fixing dead flatscreens by re-capping their power supplies; I imagine
>>> others have similar experiences.  It's not a huge stretch to imagine
>>> that other power supplies may have similar issues; even if it turns out
>>> to not be the case, there is probably at least a little "can't hurt
>>> anything, right?" running around.
>>> /~\ The ASCII                  Mouse
>>> \ / Ribbon Campaign
>>>    X  Against HTML        mouse at rodents-montreal.org
>>> / \ Email!         7D C8 61 52 5D E7 2D 39  4E F1 31 3E E8 B3 27 4B
>> This is not surprising given the vintages of the machines. Modern
>> machines using switching power supplies (15kHz+) must have capacitors
>> with low ESR and high capacity to run properly.
>> Older linear power supplies ran at 50/60hz and as such the capacitors
>> had much less ripple current (and low frequency to boot) to deal with
>> and the engineers typically over designed the values of capacitors to
>> allow for some degradation. The machines you are playing with cost
>> fortunes back in the day - they HAD to be reliable as possible.
>> Modern caps run at or near their rated temperature (105C) last around
>> 1,000 to 5,000 hours. The old linear supplies rarely heated the caps
>> much over 40C and thus the caps would last decades...I put fans on our
>> LCD monitors in our games and they last just fine.
>> No fan? Expect a year or two at most before failure.
>> John :-#)#

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