Weirdest RK05 problem I have *ever* seen. - test-drb@ccmp.vtda.org

11 Dec 1999

-----Original Message-----
From: Tony Duell &lt;ard(a)p850ug1.demon.co.uk&gt;
To: Discussion re-collecting of classic computers
&lt;classiccmp(a)u.washington.edu&gt;
Date: Saturday, December 11, 1999 6:57 PM
Subject: Re: Weirdest RK05 problem I have *ever* seen.

...
 >
> I have not even begun to look at the drive yet but I thought I would float
...
 > the problem here in case someone has actually had
something happen as weird
...
   as this.

 PROBLEM: Both drives don't work.. only "Write Protect". Is you tip either
 drive on it's left side they work fine, (blower starts up, lights,
 everything..). [not the right side] 
Are you saying that tipping one drive on its side makes _both_ of them
come up?

No.. even weirder.... Take any one drive (they are not connected
together)..and plug it in. The 5V supply works (Write Protect).. nothing
else happens. Flip EITHER drive on it's left side and it happily comes up!

I can't believe both drives have the same problem. Also, both drives have a
bad -15V when mounted.. flip it on its side and away it goes...

...
  From what I
rememberm the blower is controlled by a relay on the relay board on top of the PSU
chassis (rear left of the drive). The other relay
controls the spindle motor. The blower relay is controlled by either ACLO
or DCLO (I forget which), which, of course, is bussed between the drives.
I'd start there -- sounds like something is pulling that line active, and
turning off the relay.
 Yes.. the control logic won't let anything come on unless all three supplies
are functioning right.. -15V is the problem.

...

 Looks to be PSU problem but I have not popped the top yet. 
Possibly, although WrtProt working implies the PSU is doing something. 
Yes +5V is okay on both.

...

 In all the years I have worked on RK05s I have never seen this! 
Ditto...

I have 13 right now but 11 are 2 hours away.... so I have to get these
running quickly to test the 8A.. (it seems to be running).

...
 Do you have the prints? I'd certainly start by
looking at that relay
board (take care -- live mains!!!) and the signals that control it.

Yes.. but again far away  - storage (my home is filled with older PDP-8
stuff right now).. I went on highgate and had a look. It will be nice to
have a print set for everything next weekend (no more travelling).

john

PDP-8 and other rare mini computers

http://www.pdp8.com

...
 -tony