28 Nov
2007
28 Nov
'07
10:24 p.m.
I've finally acquired enough of the missing boards to try for a console
prompt. But I want to do this carefully. I don't want to fry any of these
modules if my power supplies are not on spec.
I'm still waiting for a power key that's in transit (graciously copied by
another classiccmp'er), but I figured I could just set the power controller to
local and crank it up.
So, after cleaning and reforming the removable capacitors[1], I figured
that I wanted to apply power to the system with all the loads removed, and
confirm proper voltages on the outputs.
I left the backplane completely empty of boards, and I disconnected the
power cable to the TU58 controller, just to be safe. I get a good AC
indication on the controller when I plug into the main AC supply circuit.
Switching the breaker on and the control switch from OFF to LOCAL gives
good air from the (loud!) blower, but...
What I see on the power controller indicators is the '+2.5 FAIL' and 'REG
FAIL' LEDs both light up, then several seconds elapse and the 'OVER
VOLTAGE' LED will flick on and the REG FAIL flicks off for about 1/2
second, then the whole sequence repeats. Each cycle takes about 8
seconds. A couple of photos of this can be found at:
http://www.rogerwilco.org/VAX11-750/psfault
I admit that I've never dealt with such a complex power system before,
and I've just spent the last hour or so reading through the 'VAX 11/750
H7104 Power System Technical Description' document that I found on
vt100.net. Trying to get my brain around what's going on, I came across
the fault isolation section and it seems that what I'm seeing on the Power
Controller LEDs maybe indicates a "Fault in the CPU backplane".
Of course, there is no CPU installed yet. Could this indication be normal
when no boards are installed?
I'm hesitant to install any of my precious boards until I'm convinced that
the power supplies are good, but maybe I have to install one (or more)
boards to actually close the loop on something.
Further reading tells me that there is a +2.5V-at-the-backplane monitoring
function in the +2.5V power supply. I haven't yet looked at this particular
issue up close, but maybe the monitor circuit is not complete without the
proper board in the backplane. Is this a sensible explanation? I just want
to be sure that putting one or more boards into the backplane is a safe
next step.
Thanks!
- Jared
[1] System hasn't been powered since 1994. I went through the motions,
but I don't think the caps needed any help; the readings during the
reforming process seemed to show them in good shape from the start.
29 Nov
29 Nov
2:09 a.m.
All,
On 29/11/2007, J Blaser <oldcpu2 at rogerwilco.org> wrote:
[Description of 2.5V rail power supply fault.]
Firstly, I have never used an 11/750 so I am offering some general
advice and assuming a linear PSU with overvoltage crowbar circuit.
Firstly, no smoke after power on, you have to be happy about that, and
at least some life!
If you have a reasonably accurate voltmeter of sorts, what are the
actual voltages the power supply is producing? So far it is not clear
if the fault is with the voltage generation or detection circuits in
the power supplies.
Thanks.
Simon
--
------------------------------------------------------------------------
"Well, an engineer is not concerned with the truth; that is left to
philosophers and theologians: the prime concern of an engineer is
the utility of the final product."
Lectures on the Electrical Properties of Materials, L.Solymar, D.Walsh
7 a.m.
From: fryers at gmail.com
All,
On 29/11/2007, J Blaser wrote:
[Description of 2.5V rail power supply fault.]
Firstly, I have never used an 11/750 so I am offering some general
advice and assuming a linear PSU with overvoltage crowbar circuit.
Firstly, no smoke after power on, you have to be happy about that, and
at least some life!
Hi
I am not familiar with these machine either.
Still, I suspect that he may be looking at a broble of feedback.
Many high power outputs use remote sensing to supply the correct
voltage at the load, not at the powersupply.
He states that he doesn't have anything connected. One would still
use common sense. Don't connect real components until one knows
things are working correctly. Put dummy loads on the supplies and
make sure that any remote sensing is properly connect
at the loads.
The loads don't have to be full current loads in many cases.
I've found that various auto lamps are enough to get a feel
for correct operation. For low current, dash lamps and for
high current, head lamps.
Don't expect these lamps to light with 2.5 volts on them, they
are just someplace to sink current. Lamps are almost constant
current devices, over small ranges of voltage.
Dwight
_________________________________________________________________
Connect and share in new ways with Windows Live.
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11:12 a.m.
dwight elvey wrote:
As I said, I hadn't looked at that closely yet. I think this is really
my next step. I'm having trouble finding a full set of PS Engineering
Drawings for this system, though, so it's means I have to sharpen my
mental pencil a bit more than otherwise, and get to work. I was being
lazy and hoping someone knew offhand if what I'm seeing is normal with
nothing in the backplane.
From: fryers
at gmail.com
All,
On 29/11/2007, J Blaser wrote:
[Description of 2.5V rail power supply fault.]
Firstly, I have never used an 11/750 so I am offering some general
advice and assuming a linear PSU with overvoltage crowbar circuit.
Firstly, no smoke after power on, you have to be happy about that, and
at least some life!
Hi
I am not familiar with these machine either.
Still, I suspect that he may be looking at a broble of feedback.
Many high power outputs use remote sensing to supply the correct
voltage at the load, not at the powersupply.
He states that he doesn't have anything
connected. One would still
use common sense. Don't connect real components until one knows
things are working correctly. Put dummy loads on the supplies and
make sure that any remote sensing is properly connect
at the loads.
Yes, this is a good plan, based on what I discover with the monitoring
circuit. Thanks for the nudge in the ribs, reminding me to slow down
and find out what's going on before committing real boards.
The loads don't have to be full current loads in
many cases.
I've found that various auto lamps are enough to get a feel
for correct operation. For low current, dash lamps and for
high current, head lamps.
I'd better prepare for high current! The supply leads to the backplane
are fatter than the battery cables in your car!...probably about 3/4"
diameter. (I have no idea on the wire gage...probably triple-0, or
something :) )
The hardware maintenance manual suggests that the basic set of CPU
boards will draw about 400-500 watts, some of which is supplied by the
5V supply, but still, at 2.5V, that's a pretty big current!
Don't expect these lamps to light with 2.5 volts
on them, they
are just someplace to sink current. Lamps are almost constant
current devices, over small ranges of voltage.
Ah, yes. Thanks for the pointers. Hope to have more to report later
tonight.
- Jared
11:02 a.m.
Simon Fryer wrote:
Firstly, no smoke after power on, you have to be happy
about that, and
at least some life!
Yes, I am quite pleased about this as well! Especially since I
completely disassembled both power supplies for cleanup. In fact, a
couple of the PS PCBs went through the dishwasher to clean up the
remains to 'rodent residue'. On the whole, it seems that I got them
back together properly! ;)
If you have a reasonably accurate voltmeter of sorts,
what are the
actual voltages the power supply is producing? So far it is not clear
if the fault is with the voltage generation or detection circuits in
the power supplies.
Valid points. What I see at the 2.5V output terminals on the PS itself
is a moving target. There's a cycle of sorts. First I see about 1.5V
that climbs up through 1.6, 1.7, on up to about 2.0V over a period of
seconds. The the voltage looks like it drops to 0V momentarily, then
back to 1.5V and the cycle repeats.
BTW, I'm using a little digital meter, so it's hard to tell what's
happening when with all the digits changing. On second thought, I do
have an old analog VOM I'll dig out and see if that confirms what I
think I'm seeing on the digital. I didn't determined if this is
synchronous with the LED pattern I see, but it probably is...I'll look
tonight.
- Jared
11:15 a.m.
J Blaser wrote:
Yes, I am quite pleased about this as well!
Especially since I
completely disassembled both power supplies for cleanup. In fact, a
couple of the PS PCBs went through the dishwasher to clean up the
remains to 'rodent residue'. On the whole, it seems that I got them
back together properly! ;)
You still want to clean dishes with that?
11:18 a.m.
woodelf wrote:
J Blaser wrote:
Don't worry, I ran the machine through another washing cycle ("Power
Wash") before putting dishes back into it! My wife wasn't sure about my
mental state at the time I washed the PCBs, though. She said the same
thing!
- Jared
Yes, I am quite pleased about this as well!
Especially since I
completely disassembled both power supplies for cleanup. In fact, a
couple of the PS PCBs went through the dishwasher to clean up the
remains to 'rodent residue'. On the whole, it seems that I got them
back together properly! ;)
You still want to clean dishes with that? 
2:38 p.m.
Hi Jared,
as far as I know, those power supplies are switchers.
I once read a story that the power supplies in the *11/780*
should not be run without a certain load.
Perhaps that has something to do with the voltage regulation ...
So, as suggested, take a few headlight bulbs from a car and
connect that as a load for the power supply.
Do check for "sense" wiring!Henk.
Date: Thu, 29 Nov 2007 12:02:05 -0700> From:
oldcpu2 at rogerwilco.org> To: > Subject: Re: VAX 11/750 power supply fault> >
Simon Fryer wrote:> > Firstly, no smoke after power on, you have to be happy about
that, and> > at least some life!> > > > Yes, I am quite pleased about
this as well! Especially since I > completely disassembled both power supplies for
cleanup. In fact, a > couple of the PS PCBs went through the dishwasher to clean up the
> remains to 'rodent residue'. On the whole, it seems that I got them > back
together properly! ;)> > > If you have a reasonably accurate voltmeter of sorts,
what are the> > actual voltages the power supply is producing? So far it is not
clear> > if the fault is with the voltage generation or detection circuits in>
> the power supplies.> > > > Valid points. What I see at the 2.5V output
terminals on the PS itself > is a moving target. There's a cycle of sorts. First I
see about 1.5V > that climbs up through 1.6, 1.7, on up to about 2.0V over a period of
> seconds. The the voltage looks like it drops to 0V momentarily, then > back to
1.5V and the cycle repeats.> > BTW, I'm using a little digital meter, so
it's hard to tell what's > happening when with all the digits changing. On
second thought, I do > have an old analog VOM I'll dig out and see if that confirms
what I > think I'm seeing on the digital. I didn't determined if this is >
synchronous with the LED pattern I see, but it probably is...I'll look > tonight.
> > - Jared
4:28 p.m.
1) There is a minimum load required for the power supplies to regulate.
2) Check the backplane VERY carefully for bent pins. I spent weeks
debugging mine before finding the bent pin...
Clint
On Wed, 28 Nov 2007, J Blaser wrote:
I've finally acquired enough of the missing boards
to try for a console
prompt. But I want to do this carefully. I don't want to fry any of these
modules if my power supplies are not on spec.
I'm still waiting for a power key that's in transit (graciously copied by
another classiccmp'er), but I figured I could just set the power controller to
local and crank it up.
So, after cleaning and reforming the removable capacitors[1], I figured
that I wanted to apply power to the system with all the loads removed, and
confirm proper voltages on the outputs.
I left the backplane completely empty of boards, and I disconnected the
power cable to the TU58 controller, just to be safe. I get a good AC
indication on the controller when I plug into the main AC supply circuit.
Switching the breaker on and the control switch from OFF to LOCAL gives
good air from the (loud!) blower, but...
What I see on the power controller indicators is the '+2.5 FAIL' and 'REG
FAIL' LEDs both light up, then several seconds elapse and the 'OVER
VOLTAGE' LED will flick on and the REG FAIL flicks off for about 1/2
second, then the whole sequence repeats. Each cycle takes about 8
seconds. A couple of photos of this can be found at:
http://www.rogerwilco.org/VAX11-750/psfault
I admit that I've never dealt with such a complex power system before,
and I've just spent the last hour or so reading through the 'VAX 11/750
H7104 Power System Technical Description' document that I found on
vt100.net. Trying to get my brain around what's going on, I came across
the fault isolation section and it seems that what I'm seeing on the Power
Controller LEDs maybe indicates a "Fault in the CPU backplane".
Of course, there is no CPU installed yet. Could this indication be normal
when no boards are installed?
I'm hesitant to install any of my precious boards until I'm convinced that
the power supplies are good, but maybe I have to install one (or more)
boards to actually close the loop on something.
Further reading tells me that there is a +2.5V-at-the-backplane monitoring
function in the +2.5V power supply. I haven't yet looked at this particular
issue up close, but maybe the monitor circuit is not complete without the
proper board in the backplane. Is this a sensible explanation? I just want
to be sure that putting one or more boards into the backplane is a safe
next step.
Thanks!
- Jared
[1] System hasn't been powered since 1994. I went through the motions,
but I don't think the caps needed any help; the readings during the
reforming process seemed to show them in good shape from the start.
6183
days inactive
6184
days old
8 comments
6 participants
participants (6)
-
bfranchuk＠jetnet.ab.ca -
cctech＠daffy.vax-11.org -
dkelvey＠hotmail.com -
fryers＠gmail.com -
henk.gooijen＠hotmail.com -
oldcpu2＠rogerwilco.org