TDS744a ("a" is for Adventure) - Page 1
Forum section: electronics / repair

author: k1mgy
author_rank: Regular Contributor
timestamp: August 10, 2016, 10:25:38 pm
content: E-bay purchase of TDS744a. Waited for one that would not power on.On arrival disassembled first looking for signs of charring, soot, loose bits. Did not uncover PSU.Attached to mains isolated variac and brought up slowly. Fan started to spin up briefly.. then a click.. then no fan. This repeated cyclically. Shut it down.Uncovered PSU.No problems obvious. Removed and inspected board. Tested a few caps. All seemed in fine order.Hunch: removed connector to video display board. Unit powered up. Attached external VGA monitor. Unit works fine, including probe compensation.Shut it down. Shined a light upon video display board. Lots of dust and crud. Hey.. what's that black stuff in the corner?? Why, a Molex connector attaching the horizontal deflection coil to the driver board is black! The connector was quite literally melted in place. Had to cut wires to remove board.Removed board. The molex connector used has three pins, but only the outer two are used. The inner pin had literally dropped down from the PC board onto the chassis. I found a pile of black crud when the board was removed. (Photo 2)Theorized that perhaps the connector had become loose over time, a resistance occurred at the connector, and eventually this caused the heating and a short. However, once the horizontal deflection leads were cut, there was no short measured at the connector. Yes, I removed the charring and cleaned the area nicely. The board was not damaged. Adjacent components checked out fine.The deflection coil measures around 400 milliohms. The vertical coil about 5 ohms. From what I read, it's possible that the H coil can come in under an ohm.To test my theory, and after thoroughly cleaning and inspecting the board and CRT (plus the deflection yoke which looked pristine), I put everything back together. I replaced the connector to the H coil with a pair of power poles. I then disconnected these and re-assembled everything.Same issue. It seems the +24V rail is screwed to ground within the display PCB.So it's not a shorted deflection coil.The adventure will continue shortly.

author: stj
author_rank: Super Contributor
timestamp: August 11, 2016, 01:21:30 am
content: you can check the deflection coils with an inductor / LCR meter or a "ringer"

author: k1mgy
author_rank: Regular Contributor
timestamp: August 11, 2016, 02:59:12 am
content: Great suggestion!My DER-EE DE-5000 (I love this meter!!) tells me the deflection coil that was attached to the melted connector is 60uH. The other coil is 7.0mH.I don't know what these should be...Meanwhile I found a shorted NPN switching transistor. Removed, but this did not relieve the almost 8 Ohm short at the +24V buss.

author: Jwalling
author_rank: Unknown
timestamp: August 11, 2016, 10:04:15 am
content: Quote from: k1mgy on August 11, 2016, 02:59:12 amHi Mark,You contacted me via Ebay. Do you have the schematics?http://www.hakanh.com/dl/docs/hardtofind/TDS544A_A30-32(Disp).pdfThere's some pretty obvious suspects for a short on page 3.You wrote "However... if I need a CRT, do you have one? Can you test CRT? "I only have one CRT on my shelf, and it's dim. Saving it for a shutter transplant.I could test one, though.Jay

author: k1mgy
author_rank: Regular Contributor
timestamp: August 11, 2016, 12:11:02 pm
content: Jim,Thank you for reaching out in this small world.I may still need that driver board. Let's see what I can unearth now that I have a schematic. (I don't quite have Dave Jones' intuitive touch just yet).-m

author: Jwalling
author_rank: Unknown
timestamp: August 11, 2016, 12:50:44 pm
content: Quote from: k1mgy on August 11, 2016, 12:11:02 pmI'm not Jim,but check Q245 and CR156 for starters. Which transistor did you find shorted?Jay

author: k1mgy
author_rank: Regular Contributor
timestamp: August 11, 2016, 01:29:46 pm
content: Sorry Jay. I'm reading-challengedI found Q205 shorted. Well.. not fully. It's definitely GOP, tho.MJE13009NPN PowerResistance measurements:base-emitter 69 ohnsbase-collector 242 ohmscollector-emitter 173 ohmsHowever the schematic is for a different (probably earlier) version of this board. Mine has a HV section that is mounted differently.Photo:Shorted device circled in green...

author: Jwalling
author_rank: Unknown
timestamp: August 11, 2016, 02:03:36 pm
content: Quote from: k1mgy on August 11, 2016, 01:29:46 pmMight have to trace out the +25V... If it's anything like the older schematic, it doesn't go to too many places, as there's an LM317 which regulates down to +21 which goes everywhere. The LM317 near the LCD shutter connector is OK?Jay

author: poot36
author_rank: Frequent Contributor
timestamp: August 14, 2016, 06:12:49 am
content: If you have a current limited power supply set it to 24V output and slowly wind up the current until you can feel any of the components getting warm. What gets warm is most likely the bad component.

author: SeanB
author_rank: Super Contributor
timestamp: August 14, 2016, 07:54:36 pm
content: My bets are the TDA1170 and all the electrolytic caps attached to it, especially the bootstrap capacitor C274. Replaced many of those in old monitors. Might even still have a board or two around with them still on it.The horizontal coils cooking the connector was fairly common on CRT units, as the current peak in those coils is pretty high to deflect the beam to the extreme of the CRT. Any oxide on the pins increased resistance, and the 10A current soon cooked it.Dead LOPT transistor change the base resistors and speed up capacitors as well, R319, R201 and C221, and check C323 and the driver Q330. Check the pins of T325 do not have dry joints, preferably by unsoldering it and cleaning the pins then resolder it.

author: k1mgy
author_rank: Regular Contributor
timestamp: August 15, 2016, 12:15:06 pm
content: Thanks to both suggestions. I was almost there (applying a current-limited 24volts)...Yes, replaced all the electrolytics. "R319, R201 and C221, and check C323 and the driver Q330. Check the pins of T325 " I will do that promptly. Thank you!!Weekend tinkering:Without a schematic specific to this board (the one that's available doesn't tag up with specific components and layout) I continued to go 'round in circles on the theme of a short in the 24V rail. In fact, once I removed the defective NPN power switching transistor, I had removed the short, but didn't know it. I continued to chase down an 8 ohm (or so) "short", and at the wrong location. So this had me remove a few other things (I replaced the LM317 just in case, and managed to damage the TDA1175 vertical deflection IC in the process. Checking the rail, it's fine, but to be certain I connected the board back to the scope to confirm.So, now waiting on delivery of a replacement transistor, and a TDA1175.Whilst tinkering, I disemboweled the CRT and its driver board from a Tektronix 11401, and managed to find what I believe is a similar power transistor (however, its markings do not google anything, so it's meant to be proprietary!). I also measured the resistance of the vertical and horizontal coils of the CRT itself. These confirmed that what I thought was a short in the horizontal deflection coil, isn't The 11401 CRT also measures around 500 milliohms, with the vertical about 24 ohms. I therefore believe that the TDS744a CRT's coils are likely in fine shape.More soon..

author: k1mgy
author_rank: Regular Contributor
timestamp: August 18, 2016, 12:59:39 pm
content: With thanks for all the great suggestions and ideas...The CRT works, but has issues.Replaced the defective Q205 with a direct equivalent: Fairchild FJP13009TUReplaced as a precaution the TIP31 driver, and the vertical deflection IC TDA1175. Unfortunately the replacement did not come with an integrated heat sink, so had to salvage from replaced device and solder in place.Q205 detailBefore removing from chassis, shorted CRT high voltage connector to ground and, paranoid, kept it in place.Detail of melted horizontal deflection driver connector replacement. Also replaced all electrolytics and the 470pf which had burn marks on the case.Test setup outside of the case. Use extreme caution here! High voltage and lots of loose stuff.First image. Yes, there are a few issues...The CRT (a bright flashlight helped in this) appears to have an inner and outer glass casing. The outer glass offers a flat screen. On the inside of this glass, or perhaps impregnated somehow, is a gold/brown strip which begins about half way up from the bottom and extends to the top of the screen (fades as it gets near the top). So, looking at a trace (green in this case) we see green at the bottom and a sudden colour change near the middle, and back to sort of green near the top.I'm uncertain if this is a feature.. but the sudden and precise horizontal axis where it starts suggests so.Below are some photos of the issue. Blurry image was the only way to show the colour shifts as camera compensated and wiped them out.Anyone have a clue what this is?

author: ArcticGeek
author_rank: Regular Contributor
timestamp: August 18, 2016, 02:07:56 pm
content: Looks to me like you have a bad LCD shutter assembly. Tek made these displays but attaching a color LCD on the to of a B&W CRT; they called this LCD a LCD shutter assembly. I've seen several fail in this way where they give odd colors on either the top or bottom. Sometimes it can be caused by a loose connection on the ribbon cable where it attaches to the CRT driver board, but that is rare. Most of the time it is caused by a defective shutter assembly, and there isn't much you can do about it other than replacing either the entire CRT or the LCD shutter.Those LCD shutter can also fail with odd looking blue or pink splotches scattered throughout the viewing area, or even a separation of the gel that they put in between the CRT and the LCD assembly.At any rate, it doesn't hurt to check the ribbon cable connections to see if there is anything loose.

author: Jwalling
author_rank: Unknown
timestamp: August 18, 2016, 02:48:12 pm
content: Since they appear horizontally and not blotchy, I suspect the connector where the shutter connects to the shutter controller, or the shutter controller itself. Inside the connector there are two connectors, one may be pushed out a bit. Seen that happen before.Jay

author: k1mgy
author_rank: Regular Contributor
timestamp: August 19, 2016, 01:32:55 am
content: OK. So the CRT has an LCD in front? Wild. Will check connectors...What's the purpose of the LCD?

author: casinada
author_rank: Frequent Contributor
timestamp: August 19, 2016, 01:56:37 am
content: There are LCD replacement kits that get rid of the CRT. I don't know if you are willing to invest that much money as they are not cheap.http://simmconnlabscom.ipage.com/store/page5.htmlorhttp://www.km5tz.com/TDS.htmI'm sure you can make your own kit with parts from ebay for cheaper.It will make the scope lighter, cooler......

author: Jwalling
author_rank: Unknown
timestamp: August 19, 2016, 09:47:05 am
content: Quote from: k1mgy on August 19, 2016, 01:32:55 amGives it color. The tube is monochrome. More info on NuColor:http://www.electronicproducts.com/Test_and_Measurement/Oscilloscope_makers_ride_the_color_bandwagon.aspx

author: Jwalling
author_rank: Unknown
timestamp: August 19, 2016, 02:06:13 pm
content: PS: in that picture where you have an alligator clip shorting the anode clip to GND?That doesn't do anything - voltage is stored in the CRT tube itself. It acts like a capacitor.Jay

author: k1mgy
author_rank: Regular Contributor
timestamp: August 19, 2016, 07:38:46 pm
content: QuoteRight! Well, I did try to show that I had bothered to drain any charge from the hv supply itself. But, yes, I discharged the tube thoroughly... although aware that charges can build up again, just like a capacitor.Worse, I was operating the tube disconnected from chassis ground. Had I touched the metal shell of the tube, Iprobably would not be writing this just now. Learned this from a video on the subject... all after the fact.Despite having been through worse: 4200 Volts between thumb and nuckle... and once removing the plate cap of an 807 while it was still powered up, I would not recommend my cavalier behaviour to any "young players".Lesson: "There are old pilots and there are bold pilots, but there are no old bold pilots"

author: Jwalling
author_rank: Unknown
timestamp: August 19, 2016, 08:32:15 pm
content: I remember (as a dumb kid, of course) making a jacobs ladder out of an old oil burner transformer with my best friend. Had all sorts of fun by placing paper (and other things) into the path of the arc where it would catch fire, etc. Then I held a #2 pencil point-first into the arc. Of course I was holding the other end with the metal thing that holds the eraser!I'm not sure what the HV supply on these puts out, I never measured it. But when discharging them, I've never seen a spark or heard a crack. First time I worked on one, I didn't think to ground the CRT band to the chassis either; I could hear the hiss and quickly powered it off with no harm done.Jay

author: k1mgy
author_rank: Regular Contributor
timestamp: August 19, 2016, 10:23:25 pm
content: Well, a success story!As it's somewhat time consuming, removing the processor board and the aluminum shields each time, I checked two items with the video driver exposed:1. That connector to the LCD Shutter Driver (it looked fine, but I pressed the two connectors firmly into the housing and then re-attached, just to be sure...2. The socketed ROM. I removed it carefully, examined the pins (they were clean), then re-inserted it.When re-assembled the artifact was gone, and the colour looks just fine!So thank you, thank you, thank you to everyone who lent their experience and ideas. I wish I could take you all out for a coffee and muffin as a small token of my thanks. If there's a tip jar, let me know!I think the display is bright enough to be useful. My Tektronix 2440 certainly is much brighter, but its capabilities are totally eclipsed by this 744a.After I use it for a few months, I'll venture into the upgrades documented in this EEVBlog.Thanks again all, and to Dave Jones for this forum!

author: Jwalling
author_rank: Unknown
timestamp: August 20, 2016, 09:53:31 am
content: Quote from: k1mgy on August 19, 2016, 10:23:25 pmCongrats!When I work on those, I have two extended length cables that allow me to test and adjust the pots outside of the chassis. A major time saver.Jay