17 Feb
2012
17 Feb
'12
1:59 p.m.
Hey, does anyone have an electronic copy of the schematics for the
DEC 874-D power controller? I looked on bitsavers and didn't see
anything there, my paper originals are 1200 miles away, and I have an
874-D "buzzing" and dropping power periodically. Yuck!
-Dave
--
Dave McGuire, AK4HZ
New Kensington, PA
18 Feb
18 Feb
12:21 p.m.
Hey, does anyone have an electronic copy of the schematics for the
DEC 874-D power controller? I looked on bitsavers and didn't see
anything there, my paper originals are 1200 miles away, and I have an
874-D "buzzing" and dropping power periodically. Yuck!
My experience wih DEC power controllers (in general) is that thy;'re very
simple, and you can trace out the scheamtic of the control electronics in
half an hour maximum,. For the rest of it, it takes longer to draw the
components than to figure out what they do.
Much of the time a controlelr that buzzes/chatters has a dried-up
smoothing capacitr for the supply to the control electronics. I'd check
any sizeable electrolytics first.
-tony
10:23 p.m.
On 02/18/2012 03:21 PM, Tony Duell wrote:
That makes sense; they're pretty simple. I'll yank it and check
those capacitors, that's what I was figuring it was anyway, but I
generally like to have schematics (if possible) before tearing into
something.
-Dave
--
Dave McGuire, AK4HZ
New Kensington, PA
Hey, does
anyone have an electronic copy of the schematics for the
DEC 874-D power controller? I looked on bitsavers and didn't see
anything there, my paper originals are 1200 miles away, and I have an
874-D "buzzing" and dropping power periodically. Yuck!
My experience wih DEC power controllers (in general) is that thy;'re very
simple, and you can trace out the scheamtic of the control electronics in
half an hour maximum,. For the rest of it, it takes longer to draw the
components than to figure out what they do.
Much of the time a controlelr that buzzes/chatters has a dried-up
smoothing capacitr for the supply to the control electronics. I'd check
any sizeable electrolytics first. 
10:38 p.m.
[...], but I generally like to have schematics (if
possible) before
tearing into something.
Sure...but don't forget that any schematics you'd get would describe
what an 874-D was originally supposed to be. There are any number of
reasons why that might differ from what the particular unit you have
currently is.
Not that a supposed-to-be schematic is worthless, of course; just
pointing out that you likely would want to do at least some
verificatory tracing anyway, so creating your own schematic may not be
all that much extra pain....
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11:09 p.m.
On Feb 19, 2012, at 1:38 AM, Mouse <mouse at rodents-montreal.org> wrote:
Urr? I have about ten 874-Ds now, and in my life probably fifty more have passed
through my hands, and I've repaired maybe four of them. I have NEVER seen or heard of
a modified one. It's just that my schematics are still at my old house in Florida.
[...], but I
generally like to have schematics (if possible) before
tearing into something.
Sure...but don't forget that any schematics you'd get would describe
what an 874-D was originally supposed to be. There are any number of
reasons why that might differ from what the particular unit you have
currently is. Not that a supposed-to-be schematic is worthless, of
course; just
pointing out that you likely would want to do at least some
verificatory tracing anyway, so creating your own schematic may not be
all that much extra pain....
Well sure...but for anything more complex than a simple 874-D, if the goal is to get it
working, that can be far more work than is necessary. We've all run across erroneous
schematics, but those situations usually stick out like sore thumbs.
Just sayin'..
I'd be willing to just put another one in this rack, but my general philosophy is to
repair rather than to replace whenever possible. Plus, my place is a right mess at the
moment, so actually FINDING a loose one would probably be pretty tough! ;)
-Dave
--
Dave McGuire, AK4HZ
New Kensington, PA
19 Feb
19 Feb
1:13 a.m.
Dave McGuire wrote:
I have about ten 874-Ds now, and in my life probably
fifty more have
passed through my hands, and I've repaired maybe four of them. I have
NEVER seen or heard of a modified one.
Nor have I. However, I *have* seen a
different DEC power control that
was modified.
Specifically, it was a DEC Type 811 power control, and it was part of
the Type 30G Precision CRT Display attached to the PDP-1 at the Computer
History Museum. After we had the PDP-1 restored to operation, we
started working on restoring the Type 30. We identified a number of
things that were wrong with it, including the deflection power supply
(an EQR power supply made by NJE Corporation) being entirely missing.
After we repaired and tested all of the Type 30's power supplies, with
fire extinguisher at hand, we powered it up. We hit the power switch,
the fans spun up, then there was a very loud clack and the circuit
breaker tripped.
What we found was that the internally wiring of the 811 was completely
inexplicable. Documentation on the 811 was found in the museum's
archives, and the wiring was significantly different than indicated. As
wired, 120VAC was applied directly across the vane switch that closes
when it sense airflow across the deflection transistor assemblies. As
soon as it closed, it welded shut, and tripped the breaker.
We never found out why the 811 was wired that way. We're pretty sure
that it wasn't supplied by the factory that way, and that someone did a
field modification, but we couldn't make any sense of how the modified
version could have been useful for anything. We rewired it to factory
spec, replaced the vane switch, and it has worked properly ever since.
Eric
12:12 p.m.
We never found out why the 811 was wired that way.
We're pretty sure
Err, no. Devices that short out the mains in normal operation are not
normally very useful.
that it wasn't supplied by the factory that way,
and that someone did a
field modification, but we couldn't make any sense of how the modified
version could have been useful for anything. We rewired it to factory
spec, replaced the vane switch, and it has worked properly ever since.
How'complex' was the modification, If it was 'just' connectiong some
wires to the wrong tags, particularly if they were screw terminals or
fastons, then it could have happened by accident. Particularly if some of
the wires are the same colour, or if the wires has been mirror-relfected
in some wat.
If it was a more major modification, how neat was it? Random bits of wire
hangl;ig loose, or neatly tied into the cableform?
I haev a Creed 444 teleprinte with the associated control unit. They are
linked by a piece of telephone cable (which I beleive to be original) of
the type that conssits of a number of twised pair. One wire of each pair
is whit,e the other is a differnet colour.
Anyway, some 'genius' at some point cut this cable midway along and
rejoind it, simply matching the colours. Which meant the white wires --
half of the total wires i nthe cable -- were scrambled. It did no
permanent damage (Creeds are stronger than that!), but it did casue some
inte3resting problems, like having one of the currnet-control lamps glow
all the time.
-tony
2:21 a.m.
On Sun, 19 Feb 2012, Dave McGuire wrote:
I should probably find a service manual for my Sony VPC-1272 projector;
it's power supply intermittently sings in the most annoying way. Like,
louder than the projector's fans.
Dave, I see you have your license again; interesting choice of a vanity
call.
--
Kris Kirby, KE4AHR
Disinformation Analyst
Much of the
time a controlelr that buzzes/chatters has a dried-up
smoothing capacitr for the supply to the control electronics. I'd
check any sizeable electrolytics first.
That makes sense; they're pretty simple. I'll yank it and check
those capacitors, that's what I was figuring it was anyway, but I
generally like to have schematics (if possible) before tearing into
something.
7:53 a.m.
On Feb 19, 2012, at 5:21 AM, Kris Kirby <kris at catonic.us> wrote:
On Sun, 19 Feb 2012, Dave McGuire wrote:
I should probably find a service manual for my Sony VPC-1272 projector;
it's power supply intermittently sings in the most annoying way. Like,
louder than the projector's fans.
Eeeeewww. That's probably a consumer-grade switcher, too. Sucky.
Much of
the time a controlelr that buzzes/chatters has a dried-up
smoothing capacitr for the supply to the control electronics. I'd
check any sizeable electrolytics first.
That makes sense; they're pretty simple. I'll yank it and check
those capacitors, that's what I was figuring it was anyway, but I
generally like to have schematics (if possible) before tearing into
something. Dave, I see you have your license again; interesting
choice of a vanity
call.
Oddly enough, that's my FCC-assigned call. I had a few possible vanity calls in
mind, but when this one came in, I thought it was pretty neat so I decided to keep it.
-Dave
--
Dave McGuire, AK4HZ
New Kensington, PA
12:16 p.m.
I should probably find a service manual for my Sony
VPC-1272 projector;
it's power supply intermittently sings in the most annoying way. Like,
louder than the projector's fans.
The serive manual for our Sony digital TV set doesn't incldue the SPU
board scheamtic :-(. On one page it claims this is because it was an
OEM'ed module, on another it's becuase field repairs to the PSU are
prohibited for safety reasons. Whatever...
You do get scheamtics of the receiver PCB, complete with BGA ICs and the
like.... But not of the bit you actually have a chance of fixing with
nornal tools...
-tony
12:03 p.m.
That makes sense; they're pretty simple.
I'll yank it and check
those capacitors, that's what I was figuring it was anyway, but I
generally like to have schematics (if possible) before tearing into
something.
Sure, I do know the value of schematics, that's probably why I have
several _thousahns_ of them for assorted devices, computer and
otherwise... It's also why I have been lnown to produce them for things
where the official diagrams are unobtainable.
However, it can be worth comparing the time/effort taken to get the
official sechmatic whith that needed to solve the problem without it.
Now, if you had a nasty logic fault in a PDP11 CPU, or in a multi-board
disk cotnroller, or something like that then I could agree it would be
worth spending condsderable time to get the printset. But when it's
somehing with a couple of dozen parts, many of them obvious, and no
clever tircks, then it may be quicker to just open it up and figure it out.
After all, a schematic won't tell you the fault. It should tell you how
the devie works, and it might indicate likely candidates that would cause
the observed vfault. But you might be able to spot those in the device
anyway.
-tony
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