Repairing a video monitor in an IBM 3279 terminal - Page 1
Forum section: electronics / repair

author: jmaynard
author_rank: Contributor
timestamp: March 13, 2024, 12:48:04 pm
content: I'm trying to bring an IBM 3279 color CRT terminal back to life. Following their diagnostic procedures, I've isolated the problem to the video monitor, a unit they purchased from an OEM. There's only one logo on the monitor, and the only part numbers are IBM's, so I'm flying blind without a schematic. The power supply voltages are all good, and the rest of the electronics report good as well. The logo is the triangular symbol in the picture.Where do I start? (Aside from not getting across the 28 kV anode supply, that is...)

author: wasedadoc
author_rank: Super Contributor
timestamp: March 13, 2024, 12:53:59 pm
content: Is the CRT heater glowing?

author: sanleontexas
author_rank: Contributor
timestamp: March 13, 2024, 02:13:56 pm
content: There is no doc available on these products unless you can find what was used in R&D orin service planning. In the field we used what what doc you apparently have.Having said that and that it's been over 30 years since I worked on this product line, look fora missing white plastic washer about 3/8" diameter on the underside of the display unit. I remembergetting a call from another FE who had spent 2 or 3 days replacing FRU's multiple times when hecalled me for help. When I told him what to look for he called back a few minutes later to tell me ofhis relief and thanks. I had been there before. One of those problems that you always remember.That's the best I can do with my age and failing memory.

author: tunk
author_rank: Super Contributor
timestamp: March 13, 2024, 02:38:26 pm
content: I doubt IBM made the CRT and CRT electronics themselves.There could be a label on the CRT or PCB(s) that may identify who made it.And is this any help:https://terminals-wiki.org/wiki/index.php/IBM_3279

author: tom66
author_rank: Super Contributor
timestamp: March 13, 2024, 02:43:07 pm
content: That logo is Matsushita, the predecessor / original name of the modern Panasonic.https://deskthority.net/wiki/MatsushitaThe monitor likely works on something like CGA signals, is there some way to hook up an external monitor to this kit to see if those are correct?

author: inse
author_rank: Super Contributor
timestamp: March 13, 2024, 03:19:12 pm
content: What happens when you power up the monitor?Any signs of life, crackling sound of HV when turned off?Do you get a white screen when you crank up the brightness?

author: bostonman
author_rank: Super Contributor
timestamp: March 13, 2024, 03:24:54 pm
content: Those old monitors have a horizontal drive transistor usually near the flyback transformer that is a common failure.My old job was repairing CRT based monitors and we didn't have schematics, so we did a "shot gun" troubleshooting process.if you hear crackling when powering, chances are the transistor and flyback are good. If the CRT neck is accessible, try measuring the pins to look for (if I remember correctly) 6.3v DC (heater voltage).

author: jmaynard
author_rank: Contributor
timestamp: March 13, 2024, 06:44:38 pm
content: To the various questions...I haven't gone looking specifically to see if the heater is glowing, but it seems an obvious step. I do get signs of life like HV crackling at power-on and power-off.@sanleontexas (nice town!), I'll look for that washer. I'd have never guessed it would be a thing. Thanks!@tunk, those manuals are for an earlier version of the 3279 than the one I have (an S3G-1). I have the equivalent manuals for my terminal, and as @sanleontexas said, it's not much help here.@tom66, there's no connection for an external monitor, and while some versions of the 3279 could drive an external monitor, this version can't, at least not officially. If I can figure out what signals are what, I might try building up a cable.@bostonman, yes, I agree that the transistor and flyback are probably OK. I'll have to see if I can figure out what to probe for heater voltage and other signals without getting across some ridiculously high voltage.I'll try those things and see what else I can learn. Thanks, folks!

author: wasedadoc
author_rank: Super Contributor
timestamp: March 13, 2024, 06:57:54 pm
content: The ridiculously high voltage (10kV upwards) is not on the tube base. Easily recognisable thick cable going much closer to the front into a rubber cap about two inches diameter. That said, there can be several hundred volts on the tube base!

author: bostonman
author_rank: Super Contributor
timestamp: March 13, 2024, 07:02:24 pm
content: Things to be very careful of:The CRT can build up a charge while unpowered, but you'd have to stick your finger under the boot on top; or the boot is removed and you get close to that port.If you attempt to take measurements while powered, be careful. I once got hit with the flyback voltage because it snuck under the boot. I was replacing a CRT and turning the yoke while looking at the screen, the voltage hit me between my index finger and thumb - I never stuck my hand inside a powered monitor again.Touching the deflection coil terminals while powered can lead to a good shock/burn.Be careful capacitors don't keep a charge after removing power. In the past I dealt with cracked PCBs and the bleeder resistors were disconnected from the capacitors as a result. The company I worked for retro fitted touch screens onto existing monitors not built by them. As a result, it pushed the CRT back, and, when the monitor would take a slight hit on the base by an end user (or maybe during shipping), the tall components would hit the bottom of the CRT, flex the board, and crack.

author: jmaynard
author_rank: Contributor
timestamp: March 13, 2024, 09:56:52 pm
content: Several hundred volts is ridiculously high voltage to get across with one's hand... but no, I do not plan to touch anything on the tube base with anything but a Fluke Scopemeter probe.

author: inse
author_rank: Super Contributor
timestamp: March 14, 2024, 08:34:56 pm
content: No progress on the monitor so far?

author: jmaynard
author_rank: Contributor
timestamp: March 16, 2024, 05:25:18 pm
content: The heater is glowing, so that's good.Nothing on the screen at any point, including both full brightness and the monitor test (turning the brightness all the way down to activate a momentary switch). One of the lugs on that switch is broken off, however, so I'm not confident of that result.I do get a short green flicker of horizontal lines on the screen when I turn the unit off. Not entirely sure what that tells me, though.Going to go back and probe the power supply voltages to make sure everything is good. I'm beginning to think the monitor might not have a problem at all.Edit: Power supply voltages are all good. I'm seeing sync at the monitor input. Trying to get video signal probed, not entirely trivial.Edit 2: Adjusting the G1 Bias control got the separator line to display! And when turning the unit off, the raster was visible for a moment as it shrank on the screen. I'm going to say the monitor is good and my problem (if any) lies elsewhere.Edit 3: I got it into offline test mode. The green is extremely dim even with the Gi Bias control all the way at one end of its travel. Red and blue are brighter. All three need G1 Bias and the brightness control to be all the way at the end of travel to be visible. There aren't any crntrols labeled as drive, though. Wonder if the CRT is just tired and worn out, or...?Edit 4: Well, that's a setbck. I had it in test mode showing a red screen...and then activated the display override switch. It went "fsssh" and quit, and now the power supply is acting fried: +5 comes up but intermittently drops out, and the ready light never comes on. Time to pull the power supply and see what fried.

author: inse
author_rank: Super Contributor
timestamp: March 16, 2024, 10:00:38 pm
content: What a bummer!What is the override switch for?

author: jmaynard
author_rank: Contributor
timestamp: March 16, 2024, 10:23:57 pm
content: It's a test: when activated, it's supposed to display a complete, unblanked raster.Pulled the power supply out, found two power electrolytics that appear to have let the magic gunk out, ordered replacements for all of the power electrolytics. Should arrive Tuesday. A mere $22.

author: wilfred
author_rank: Super Contributor
timestamp: March 17, 2024, 12:31:29 am
content: Are you one and the same jmaynard associated with Hercules?If so consider this the Internet equivalent of being recognised in the street.

author: inse
author_rank: Super Contributor
timestamp: March 17, 2024, 06:12:36 am
content: Electrolytics for $11 a piece- what do they look like?

author: jmaynard
author_rank: Contributor
timestamp: March 17, 2024, 12:05:00 pm
content: I am! And my goal here is to use the terminal to talk to Hercules with.

author: jmaynard
author_rank: Contributor
timestamp: March 17, 2024, 12:05:59 pm
content: I'm replacing 7 caps, so it's more reasonable than that. The most expensive is a 330 uF/200V, two of them at $3.70 or so. That $22 includes shipping. I'm in Minnesota, so getting stuff from Digikey is fast and cheap.

author: jmaynard
author_rank: Contributor
timestamp: March 21, 2024, 01:05:22 am
content: It runs! Got the caps in and installed, and gave it a try...Now, need to figure out why the screen - the green in particular - is so dim. That picture was taken with the room lights off. The text that looks pink is supposed to be white. That's also with the G1 bias turned all the way down and the brightness turned all the way up. Are there drive adjustments or something on the monitor somewhere?Edit: Is this something that says I need a CRT rejuvenator? How do those things work with different picture tubes, anyway? Or was there one standard basing for them?

author: bostonman
author_rank: Super Contributor
timestamp: March 21, 2024, 03:13:27 am
content: I think most monitors have individual color (RBG) adjustments; usually on the neck board (which you need to be careful so you don't crack the CRT neck).The CRT could be aged and the color guns are weak, which is a good possibility. I don't remember if the color circuits have capacitors that would go bad, but maybe those too are bad.In any case, the color was adjusted by using a video source (or a piece of software) that displayed a white screen. Then, by eye, tweak the pots until the screen is white. Thought I still had software (which was on a 3.5") that displayed a grid pattern (for aligning new tubes), white screen, and individual colors, but I can't locate it.

author: inse
author_rank: Super Contributor
timestamp: March 21, 2024, 05:48:19 am
content: Great to see it running again:-)If the terminal has been used like this, mainly displaying green text, it’s not unlikely that the green cathode is spent.The CRT rejuvenators try to remove a passivated emitter layer by overheating the cathode (afair).The saying is that the fix is only temporarily, but are you planning to put 100‘s of operating hours on the set?If you happen to know someone who knows someone that has access to a rejuvenator (and the right adaptor), give it a try…

author: m k
author_rank: Super Contributor
timestamp: March 21, 2024, 07:50:07 am
content: Quote from: jmaynard on March 13, 2024, 12:48:04 pmUsually the monitor part is a some sort of a standard thing.Maybe some old multisync monitor can be connected without a problem, if needed.Some have sync polarity selectors.Generally would be a mechanical mess though, sort of.

author: wasedadoc
author_rank: Super Contributor
timestamp: March 21, 2024, 08:53:30 am
content: The "whiteness" should be adjusted when displaying both "just above black" and peak white. In the professional TV world one would use either a line sawtooth or a line staircase. These let you see what is happening throughout the range as you adjust the controls. "gain" or "drive" for peak white and "background" (or whatever name) for down near black. Usually there is interaction between them so a bit of iteration is required.

author: bostonman
author_rank: Super Contributor
timestamp: March 21, 2024, 02:00:25 pm
content: QuoteOh yes, I forgot about the staircase and/or gray scale going from white to dark (?) gray.

author: inse
author_rank: Super Contributor
timestamp: March 21, 2024, 05:02:31 pm
content: Ok, but after all it’s just a ASCII terminal, no TV or hires monitor for image processing.As long as text is readable, it should be fine, I would say …

author: audiotubes
author_rank: Regular Contributor
timestamp: March 21, 2024, 09:51:36 pm
content: Quote from: inse on March 21, 2024, 05:02:31 pmNot ASCII! EBCDIC!

author: jmaynard
author_rank: Contributor
timestamp: March 22, 2024, 03:12:52 am
content: Did some measuring with my handy dandy Scopemeter...and got some weird readings: G2 voltage is way low, and the waveform on the green cathode is 14 volts P-P while the other two are about 20. That would explain why the green is dim. May not be a weak tube after all. Then again...

author: jmaynard
author_rank: Contributor
timestamp: March 23, 2024, 02:31:20 am
content: Pulled the plastic shields off the monitor. Found the video amplifier hiding int he bottom...and the drive controls. Saw that the green drive was turned down relative to the red and blue. Turned it back up, and got the green visible. Yay! The voltages are still funny, though. I'm not doing to dink with them till I get a much better understanding of what the controls do.

author: jmaynard
author_rank: Contributor
timestamp: May 27, 2024, 01:01:59 am
content: Just to wrap this up: Turns out that the only thing it needed once I got it to the poin of being able to see all three colors was to adjust the grid 1 and screen controls. Once I did that, it brightened up just fine.I've now got another with the same monitor in it (or nearly so), with what seems to be a cracked voltage tripler in it, but that's a problem for another time.Thank you all for the help!