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#1 2015-11-20 05:44:18

bbraun
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Registered: 2014-05-29
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External causes of CRT image instability?

After fixing up the Macintosh TV and G3 AIO, both have really nice stable images on the CRT on the bench.
I recently moved the into my computer/server room, which is small and packed with machines as densely as I practically can, including the rack with a bunch of always on equipment.  After placing the Macintosh TV and AIO in this room, the CRT image has a slight instability to it.  The image kind of wavers back and forth.  It's minor, subtle, but it's the kind of thing that bothers me and makes me go to an LCD instead.
The instability seems specific to this room/power/something, since if I move the machines to another part of the house (which also happens to be on a different power circuit), the image is again stable.

To my uneducated thinking this has to be interference from somewhere.  Whether it's RFI or power or what, somehow interference is entering the CRT circuitry.

We recently had a power outage, so I took the opportunity to keep everything in the room powered off, except these CRT's.  The image seemed stable.
I then started powering on some machines to see if and at what point the instability started.  I believe I have limited it to one of two machines: an xserve3,1 or my PC file server that has lots and lots of spinny disks.  I suspect the latter, and can narrow it down at the next good opportunity to have one or both of them offline.  I do actually use these machines regularly and don't have good replacements for them, so just getting rid of them isn't the first choice option.  Replacing power supplies or adding power line filters to them is definitely possible.

The only two ways I'm aware of that these CRT machines are connected to these apparent problem machines are: 1) physical proximity, with the possibility of some sort of RF emission or magnetic field causing the instability, and 2) power.
Ideally, I'd have a way to have these two machines in the same physical location, but on different power circuits as a way of telling which of those two things are the problem.  I don't have a great way to do that, but I can run a really long extension cord this weekend and give it a try.

Just for the sake of sanity, I removed all those crappy power strips from the mix.  I needed to anyway for several different reasons, and this was as good an excuse as any.

Ideally, I'd have an oscilloscope that I could connect to the power line to see if there's anything wonky going on.  I'm really wanting one of these, but probably not justified for this particular annoyance.  An RF spectrum analyzer would be cool too.  Sadly, I don't have either.

So, am I completely off my rocker and into conspiracy theory territory here?

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#2 2015-11-20 07:34:17

mcdermd
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From: Corvallis, OR
Registered: 2014-05-12
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Re: External causes of CRT image instability?

My money is on proximity. You have to keep arcade machines at least 8 inches away from each other because one monitor will interfere with the other. It'll wiggle like a club on Saturday. I'm sure there's something throwing off RF that's placed too close to your CRTs.


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#3 2015-11-20 15:47:09

Eudimorphodon
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Registered: 2014-09-02
Posts: 522

Re: External causes of CRT image instability?

Fluorescent light ballasts can be a problem, you don't happen to have any of those in the room?

(EDIT: Need to stop banging out glib answers without reading more carefully, apparently you have narrowed it down to emitting from one or two machines.)

How physically close to the possible culprits to the CRTs when you see this instability? I have seen some effects like that myself from floppy or hard drive motors placed near CRT screens (granted it's been a *long* time) but I sort of doubt you'd see it from more than a foot or so away so I'm *slightly* more in the "noisy power line" camp? Do you have a Kill-A-Watt lying around by any chance? If the whole room is on one circuit and you have an xserve *and* a PC with lots-o-disks pounding away I'm vaguely curious how many amps you're pulling.

It may also be worth noting, I guess, that an Xserve is only FCC class A approved, so it may well put out more RF interference than a consumer PC.

Last edited by Eudimorphodon (2015-11-20 19:07:53)


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#4 2015-11-20 19:47:01

bbraun
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Re: External causes of CRT image instability?

Thanks!  No fluorescents in this room.  The MacTV and the AIO are about 4' from the suspect machines.  The interaction of the two CRT's shouldn't be the ones causing the problem I'm looking for, since it seems to happen to each of them with the other one off.
I do have a kill-a-watt laying around somewhere.  The hassle with using that is needing to unplug everything to put it inline, and then unplugging everything again to remove it, but I can probably do that after today.  The PC has a Corsair TX-850 PSU in it and might be pushing its limits, and probably would be a good idea to upgrade that PSU anyway.
I'll also try the extension cord thing to attempt isolation of the proximity and power circuit.

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#5 2015-11-22 00:52:42

bbraun
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Re: External causes of CRT image instability?

The long extension cord running from my bench (where the CRT image is stable), to the current location in the other room where it is not stable, says it is not power related.  The machine in the same location exhibits the same wiggling instability regardless of where it seems to be plugged in.
I turned off the xserve3,1 and the image stabilized.  Or at least is substantially more stable than it was when the xserve was on.

So, AFAICT, it's the xserve.  I wonder what about the xserve is causing the issue.  PSU?  From a practical perspective, there's probably not much to be done about it.

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#6 2015-11-22 02:56:52

mcdermd
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From: Corvallis, OR
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Re: External causes of CRT image instability?

Fans.


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#7 2015-11-22 03:55:03

bbraun
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Re: External causes of CRT image instability?

They do have some pretty massive arrays of 40mm fans in them.  I'd imagine replacing them wouldn't really affect the interference problem I guess.

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#8 2015-11-22 05:08:12

mcdermd
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From: Corvallis, OR
Registered: 2014-05-12
Posts: 931
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Re: External causes of CRT image instability?

Custom chicken wire cage, lol


Daily Drivers: 27" iMac 2.8 GHz Quad-Core i7 (Late 2009), 21.5" iMac 2.7GHz Quad-Core i5 (Late 2013), 11" Macbook Air 1.6 GHz i5 (Mid-2011)
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#9 2015-11-22 22:14:57

Eudimorphodon
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Registered: 2014-09-02
Posts: 522

Re: External causes of CRT image instability?

You should drag an AM radio in there and see if the Xserve can make it sing through RF interference. That was all the rage back in the Altair/TRS-80 Model I days.


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#10 2015-11-22 22:42:22

TheWhiteFalcon
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Registered: 2015-04-27
Posts: 504

Re: External causes of CRT image instability?

You may need some sort of homemade faraday cage for the Xserve.

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#11 2015-11-22 22:49:55

bbraun
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Registered: 2014-05-29
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Re: External causes of CRT image instability?

Yeah, I actually have already.  I was reading a bunch of ham radio operator pages about how to help your neighbor not mess with you, and a couple horror stories about detecting home wiring problems.  Places like the back of the rack, particularly around networking gear makes it go nuts.  It's pretty hard to get much of a directional signal off the AM radio, so I'd try unplugging stuff around that as well.

I doubt I'll be doing any sort of faraday cage setup for the xserve, although it would be kind of cool.  Maybe the whole rack needs a big chicken wire box around it!

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#12 2015-11-23 17:56:56

Eudimorphodon
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Registered: 2014-09-02
Posts: 522

Re: External causes of CRT image instability?

bbraun wrote:

Yeah, I actually have already.  I was reading a bunch of ham radio operator pages about how to help your neighbor not mess with you, and a couple horror stories about detecting home wiring problems.  Places like the back of the rack, particularly around networking gear makes it go nuts.  It's pretty hard to get much of a directional signal off the AM radio, so I'd try unplugging stuff around that as well.

Next time I'm at the colo I'll have to bring an AM radio with me to dowse around the racks with for larfs; it never really occurred to me to think about it but now I'm dying to see how much noise torrents off the front of a mostly-populated 13 slot Catalyst...

There are several issues of 80' Microcomputing magazine (available on archive.org) that detail HAM enthusiast's attempts (there was a lot of overlap between HAM and the early personal computer hobbyist demographics) to tone down the noise from their Trash-80's so they could use them inside their HAM shacks. Personally I used to get a kick out of leaving a radio on a low volume next to the Model I's keyboard so I could hear it "thinking"; when running BASIC programs the TRS-80's maximum operations-per-second was right inside the audio Hz range so repetitive loops generated very distinctive tones... good times. smile

Of course, I don't know what inside an Xserve might be buzzing at audio frequencies; maybe the PWM signals for the fans?


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#13 2015-11-23 18:04:28

bbraun
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Registered: 2014-05-29
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Re: External causes of CRT image instability?

With mcdermd's comments about the fans, I'm guessing it's the magnetic field generated by the electric motors of a couple dozen 40x40x28mm fans crammed in a 1U enclosure, since they're all just big electromagnets anyway.  IIRC the xserve's fans are rated up to 9000 RPM.

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#14 2015-11-23 18:43:25

Eudimorphodon
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Registered: 2014-09-02
Posts: 522

Re: External causes of CRT image instability?

Possibly, but... I dunno. The Model I used to hash the screen of any CRT within 20 feet of it despite having no moving parts whatsoever, but the comparison is sort of complicated by the fact that the Model I was generating NTSC-frequency video output and therefore probably having a direct effect on the TV on that level. Does what you're seeing look more like there's a magnetic field in the room so strong it's directly screwing with the electron beam? (Or, by extension, the yoke or the related high-voltage circuitry?) That sounds... pretty nuts to me. I've seen what I'm pretty sure is that from (again) fluorescent ballasts and big electric motors, but that's only from a couple feet away, not across a room.

I wonder if there's some rational way you could fit either the Xserve or the Macs with a variety of different grounded tinfoil hats and use the observational data to wheedle out what frequency spectrum is causing the "wavering". wink


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#15 2015-11-23 22:35:16

bbraun
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Registered: 2014-05-29
Posts: 1,064
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Re: External causes of CRT image instability?

Yeah, not really sure.  Unfortunately there's no good way to describe it other than a "wiggle".  The focus doesn't seem to change, nor does the coloration.  I tried to grab a video of it, but it doesn't seem to come out very well.

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#16 2015-11-24 00:06:34

volvo242gt
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From: Duvall, WA
Registered: 2014-05-22
Posts: 402
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Re: External causes of CRT image instability?

At our old house, my original M1212 used to jiggle like jello if I had the computer table against one wall.  Seemed to be worst when it was closer to the closet.  Only thing I can think of was that we had our old 1972 vintage Zenith TV on a dresser in the next room over and it was somehow effecting the M1212, even when it was shut off.  I don't remember the TV jiggling, though.

-J


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#17 2015-11-24 04:03:54

uniserver
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From: Sf, Mi
Registered: 2014-05-15
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Re: External causes of CRT image instability?

tinfoil-hat-and-tinfoil-cat.jpg


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