CRT Repair Toshiba CN27D90 Tube Brightness Issue - Page 1
Forum section: electronics / repair

author: piepurveyor
author_rank: Contributor
timestamp: January 23, 2022, 03:35:47 pm
content: Hello,I'm trying to get some help with a repair I made recently to my CRT Toshiba CN27D90. I performed a full electrolytic cap replacement and upon reassembly and power up the TV now has issues with the picture tube being very dim and it seems unable to produce any white color. I suspect maybe a power issue but I'm not sure where to start checking, as actively diagnosing issues isn't my forte.Before the repair the TV did not display the issue described above. It had vertical geometry issues and some red color instability (as in after power on, the whole picture would lose its red channel and become just cyan/green, but this would usually stop after maybe 20 mins).I've attached some images below to help. I'm using 240p Test Suite for the test images. Colors display but it has issues with white.Any and all help is appreciated, thank you.

author: floobydust
author_rank: Super Contributor
timestamp: January 24, 2022, 01:12:39 am
content: Welcome to the forumIf you have new symptoms after recapping, then review your work you might have bad soldering, wrong capacitor value/polarity or new fractured soldering joints. Connectors as well, that they are not damaged or plugged in wrong during assembly.Whenever a gun is cutting in and out, I find it's usually bad solder joints on the CRT board/socket. Not a capacitor. While looking at the screen using a mirror, take a plastic poker and tap the main PCB, the crt board, lightly to flush out bad connections. With older sets it's common to use a magnifier and do a visual and go over the solder joints. It depends on the build quality, some sets have good wave-soldering and many do not. With CRT TVs and monitors, I'd always spend 15 minutes just staring at the board and re-touching up dodgy looking solder joints. Every set has them.Heavy through-hole parts can fracture solder joints from the tossing and flipping around doing the recap. Some people can be a bit too rough.You appear to have primary colours, green gun is a bit weak- but any mix of them is a trainwreck, especially to make white or yellow. This is odd, I'm suspecting a capacitor on a CRT (common) rail is no longer there and ripple current makes one gun affect the others. This is just speculation.Do you have the service manual, a scope etc. that kind of thing? While you can troubleshoot the problem, it takes a lot of time. If your only test equipment is your eyes, then it can be faster to review things with a magnifier, and bright bench light.

author: piepurveyor
author_rank: Contributor
timestamp: January 24, 2022, 02:13:50 am
content: Ty for taking the time to reply and for the insights so far.I have been inspecting the board with a magnifier looking at joints, but in order to be thorough, I'll need to fully take the board out again, which I'll do. In hindsight, I should have just worked on small sections of caps at a time, mainly where there was leakage (and there were leaky caps on the vertical geometry segment of the main board). I do not have any tools except my soldering equipment and a multimeter.I tested the voltage on the filament heater (I think that is the proper term) at the neckboard and coming off the flyback transformer because I did notice that the filaments in the tube are not glowing when the TV is powered on. At the transformer pin, it's outputting ~3.1 VAC. I traced it to a molex type connector which is running up to the neckboard. At the connector just adjacent to the flyback transformer it's measuring 3.1 VAC, at the neckboard connector pin it's 3.1 VAC, then it runs through a very low Oms (0.82) resistor which makes it 2.8 VAC at the neckboard pins just before the filaments.I have no idea though if these are good values b/c I do not have access to a service manual, as you mentioned.And as an aside, I took values of the R, G, and B pins at the neckboard: R=184.6 VAC, B=187.5 VAC and G=177.0 VAC

author: floobydust
author_rank: Super Contributor
timestamp: January 24, 2022, 04:46:48 am
content: I did find a schematichttps://elektrotanya.com/toshiba_cn27c90_tv_d.pdf/download.htmland thishttps://elektrotanya.com/toshiba_cn27c90_tv_d.djvu/download.htmlThe power supply outputs regulated +115VDC at C897 33uF 160V/F802, which I would check and a few rails the ones with new caps to see how they are doing i.e C889 3,300uF 35V/F803 for audio?, C892 1,000uF 25V for 18.5V and there's a handful of other voltage regulators.The flyback transformer has a dedicated winding for the CRT filaments, these are usually 6-9VAC but 16kHz AC squarewave which many multimeters do not accurately read. The X-ray protect circuit is also fed by this and somehow gets 20.1VDC from it so it must be higher than what you read.

author: piepurveyor
author_rank: Contributor
timestamp: January 24, 2022, 01:58:15 pm
content: Ok, great. I'll start looking at those as soon as possible and I will see how they are functioning. Thank you.

author: jh15
author_rank: Frequent Contributor
timestamp: January 24, 2022, 04:52:44 pm
content: Back in monitor repair days, many units had bad capacitors on the crt board. Check carefully or just replace them, usually 3 or 6 cheap low value caps.

author: piepurveyor
author_rank: Contributor
timestamp: January 25, 2022, 07:19:30 am
content: I just want to preface again by saying that diagnosing problems isn't a strength of mine, but I've taken some measurements and added some photo refs in case they may help. I'm quite the amateur in this department so I pre-apologize.I probed the board at the suggested capacitors, and I was testing by just measuring from the positive terminal and taking the voltages.My formatting is: -Capacitor position, capacitance and voltage rating, measured DC voltage at positive terminal.-C889 3300uF 35V @ 22.2 VDC-C897 33uF 160V @ 114.5 VDC-C892 1000uF 25V @ 16.5 VDCDiagramNumbered Capacitor ListI took several more measurements from a few of the capacitors and I've attached a layout diagram I made for myself just to help me keep track of everything. The measured DC voltages are outlined in Red. All the capacitors numbered in the list correspond with the numbers on the layout diagram. The voltages of the fuses are there on the post-it too, F801 and F802. C810 was curious but I don't know what it should be reading, I couldn't locate it on the schematic.I've added 2 more references as well. I don't know if they will add anything, but I figured they might help with something. There are a pair of ICs in the area around C889, C897 and C892 that I took a picture of. One looks normal, the other is maybe corroded? It's got some green tinted build up on it (can been seen in bottom left of the image of the main board). Also, the board between the 2 ICs has some black speckling there, I'm not sure what that is but it's the only place on any of the PCB that looks that way.PCB discolorationMainboard Image

author: Renate
author_rank: Super Contributor
timestamp: January 25, 2022, 01:40:02 pm
content: Silly question: Are you sure that the service switch on the CRT board is in the correct position? It looks like you don't have any luminance signal.

author: floobydust
author_rank: Super Contributor
timestamp: January 25, 2022, 07:48:16 pm
content: The recap looks reasonable, but awkward to convert the reference designators to your cap #'s and need more pics of the underside on the pcb. In just the one pic, the solder joint on C892 (-) is broken, needs a redo. I'm sure many more need touch ups, I can see the factory work has a few puddle patterns which means they can fracture easy. What solder are you using, it seems to have no flux?The green slime is in a few places on the board, it seems to be moisture related? I would just use a q-tip and IPA to clean it off and let it dry as long time.What did you do on the two caps on the CRT neck board?C490 10uF 250V CRT neck board video driversC530 470uF 16V CRT neck board video drivers cascodeThe service switch seems to be on the neck board as well, it's got two positions, one disables the drives but I don't think we're there yet, to doing any CRT calibration.Others caps I would check:C201 220uF 16V luminance pwr Q202 buffer 13.9V typC403 470uF 16V jungle IC VCC. 9.5V typC268 2.2uF 50V jungle ACCC267 100uF 16V jungle ACC +12V pwrC265 220uF 16V jungle ACC +12V pwrC510 2.2uF 50V jungle IC apc

author: Renate
author_rank: Super Contributor
timestamp: January 25, 2022, 07:53:18 pm
content: The service switch only affects luminance.

author: floobydust
author_rank: Super Contributor
timestamp: January 25, 2022, 08:05:51 pm
content: No, here the Service Switch either disables vertical deflection or the CRT drives, so you can set up the screens and cathode bias.

author: Renate
author_rank: Super Contributor
timestamp: January 25, 2022, 11:47:56 pm
content: Correct, but if it's in the wrong position/flakey only the color difference signals hit the base of the cascode.

author: piepurveyor
author_rank: Contributor
timestamp: January 26, 2022, 04:19:39 am
content: I've been noticing that b/c my model is a D90 and the schematic is for a C90, some things are a little different, but most are in the right place. On the neck board the 2 capacitors areC490 10uf 250V @208 VDCC930 470uf 16V @ 13.5 VDCCaps near the jungle chipC201 220uf 16V @ 13.5 VDCC403 470uf 16V @ 9.4 VDCC265 220uf 16V @ 11.4 VDCC510 2.2uf 50V @ 11.4 VDCC268 on my board is ceramic and I just can't locate 267, at least there isn't an electrolytic cap with that position.I did, though, to my chagrin, notice this.That's the neck board service switch. On the left is before I did any work on it, the right is as of today. It got knocked off during the recap somewhere.I don't know what exactly it would cause, but I'd guess either it doesn't matter too much or that it's bad.Also, if viewing the mainboard and deflection boards post recap from the underside would help, I can post some detailed photos. I'll have time tomorrow to do so.*Edit* My solder does have a flux core, I believe, but I also use a no-clean tack flux syringe when soldering.

author: floobydust
author_rank: Super Contributor
timestamp: January 26, 2022, 10:31:06 am
content: The switch (wiper) is needed for normal operation, Y-ch signal is gone otherwise. You don't want to bump or bang anything at the CRT neck! I'd be looking around for the switch parts.

author: Renate
author_rank: Super Contributor
timestamp: January 26, 2022, 10:39:43 am
content: Quote from: piepurveyor on January 26, 2022, 04:19:39 amSay what? "Oh, I accidentally destroyed this part on my TV but it probably doesn't do anything."I don't believe that you're going to be able to fix that switch in a reliable way. It's better to remove the switch and replace it. If you have a center-off toggle switch that will do nicely. If not, just use a piece of wire (connecting the path shown by the thick line in the schematic).

author: piepurveyor
author_rank: Contributor
timestamp: January 27, 2022, 04:48:29 am
content: Ok, I took your and Renate's advice and took care of the switch (I made a temporary fix and soldered a jumper between the proper 2 pins) and that brought the TV back.Live and learn.The link to the album below has some of the test images I took afterwards.https://imgur.com/a/6Hj96AMJust let me preface and say I've never adjusted anything in/on the TV. Nothing has been done to the yoke. None of the rings have been disturbed. The image has some issues but I'm hoping most, or all can be solved through menu/service menu adjustments. I just don't want to make things worse. The image has a few geometry issues like you can see on the linearity test in the upper right and the small grid pattern, it bends slightly toward center on top and bottom. It also has convergence issues, I think it's convergence, in the corners as you move off center. The corners are also a little fuzzy.

author: Renate
author_rank: Super Contributor
timestamp: January 27, 2022, 10:20:37 am
content: That looks pretty good for a TV. Sure, it could always be better, but try it on some subject matter besides crosshatch.

author: BrokenYugo
author_rank: Super Contributor
timestamp: January 27, 2022, 06:44:41 pm
content: I'm not sure these basic consumer TVs ever had perfect geometry and whatnot, or if they even can be aligned that well, consider they were mostly used to watch over scanned broadcast TV where such things would rarely be noticed.Focus is fairly straightforward to peak up, it can drift over time. Just be warned it's a touchy control that actually sets a comprise between a focused center and focused edges.

author: Renate
author_rank: Super Contributor
timestamp: January 27, 2022, 06:57:27 pm
content: The purity (on red) looks good. That's often the problem when things get magnetized over time and the built-in degausser doesn't do well enough. Heck, sometimes the silly PTC burns out and you get no degaussing.

author: piepurveyor
author_rank: Contributor
timestamp: January 28, 2022, 05:03:14 am
content: Yea I may make a small adjustment to the yoke, as the whole screen is slightly tilting right, but tbh it looks OK in use.