Tektronix TDS 224 Oscilloscope repair. [UPDATE: I fixed it!] - Page 1
Forum section: electronics / repair

author: cyang13
author_rank: Newbie
timestamp: January 09, 2016, 07:52:18 pm
content: Hi guys!Recently I scored a "for parts or not working" condition Tek TDS224 100Mhz 4 channel scope on ebay. I tried to trouble shoot it and finally got stuck on some point.I am posting this mainly for sharing my trouble shooting experience and asking for help or discussion. I would appreciate it if anyone could share your ideas.First of all, let me do a quick introduction of the schematic of TDS224. It contains majorly four parts: The screen assembly, the main board, the power supply and the control interface board. Though being one member of the TDS 200 series, its main board is very different to the main board of TDS220 which Dave has tortured in the Mud Run:) Its interface to screen and power supply are basically the same as TDS220.Now here is what I did to trace the problem. For its initial condition, it does not power up when I plug in AC main power. There is frequency stable "click" sound from the power supply board, and the screen keeps flashing with random horizontal strip and back light of the screen is off.Therefore naturally the first thought was the power supply was broken. Luckily I have another working power supply board so I changed it but the problem still exists. So I decided to isolate different module and trace which part is wrong. And I found several interesting facts as follow:1. If powering up the original power module only, it works perfectly. Every voltage rail is in spec.2. If powering up the power module with connection to the high-voltage screen back-light. It also works well. The screen light is on.3. If powering up the power module with main board. The problem appears again!Now things are getting clearer. There must be something wrong with the powering to main board -- probably a short-circuit or something and it triggers the self-protection on power module thus the system keeps restarting and causing the click sound and strange screen. The power module and screen assembly might be fine (at least not causing short circuit).According to the service manual from tek website. The powering connection to main board mainly contains the following DC voltage: +5V, -5V and -23V. The -23V is probably for the screen because the connector from main board to screen is having -10V and -23V for driving the LCD. My guess is that there must be at least one of the voltage being malfunction.Therefore, I use a current limit voltage source to power each of the rails separately with the limit being set to the spec of the original power module. It turns out that when I am only powering +5V and -5V volatage to mainboard. It works! Though I didn't connect the screen, the probe compensation port on main board is having a normal 5V square wave output! This indicates that the main board stops never-ending rebooting and at least works to some degree!The only problem is with the -23V. When I am powering it with the claimed current limit, the power supply only gives -2.6V and reaches the limit. So the problem is which part on the main board is not working properly and I have located it to a relatively small area of the power supply for the screen from main board. (i.e. -10V and -23V)And then I got stuck. I have reverse-engineered a little bit and located three transistors marked Q130,Q131,and Q132 and one missing resistor designated R133. Please see the attachment for the location of R133 and forgive me for not having a super clear photo. The R133 is on the top side of main board. It is very close to the probe compensating output area. The three transistors are on the bottom side and under the ground metal cover. I tried my best to read the laser mark:Q130 reads: 2A and an "H" which is rotated for 90 degrees (UPDATE: This is probably a 3906,https://www.fairchildsemi.com/datasheets/2N/2N3906.pdf)Q131 reads: 1AM and an "H" which is rotated for 90 degrees (UPDATE: this might be a MMBT3904LT1 NPN transistor:http://pdf1.alldatasheet.com/datasheet-pdf/view/140113/AVICTEK/1AM.html)Q132 reads: 8F and a right arrow and a "|", like ">|"(This might be a MUN5216:http://www.onsemi.cn/pub_link/Collateral/DTC143T-D.PDF)I have tried my best on searching for this transistors but I have to admit that I am really bad at it. Could anyone please provide any help on this? Thank you very much!In addition, if anyone happens to have a TDS 224 or know the schematic, I would appreciate it if you could take a look at the value or mark on R133 and share it with us. That will be a great help.Update: By replacing all these transistors. The unit is still not on since R133 is still missing. Based on my reverse engineering where there is a resistor of 21.2K in serial with R133 and perform as a part of a voltage divider between -23V and -10V rail. I put a 10K in R133 and it works! There is nothing wrong with other parts of this unit and all four channels are in perfect working condition! Beauty!Special thanks to nctnico who inspired me on checking transistors.If anyone is interested in more details I am very happy to recall more about this repair experience.It is always so much joy at the moment when you see the lights up and everything works. I feel like I saved Earth from Aliens:)

author: nctnico
author_rank: Super Contributor
timestamp: January 09, 2016, 10:16:35 pm
content: I'd measure the transistors first to see if you can measure the diodes between base/emitter and base/collector and if there is no short between emitter and collector.

author: cyang13
author_rank: Newbie
timestamp: January 09, 2016, 11:43:48 pm
content: Quote from: nctnico on January 09, 2016, 10:16:35 pmThank you very much! I have tested and found that Q131(3904) is shorted! That might be the problem:)

author: nctnico
author_rank: Super Contributor
timestamp: January 10, 2016, 12:07:37 am
content: Now unsolder and test again after unsoldering and check if there is no short in the other circuitry.

author: cyang13
author_rank: Newbie
timestamp: January 10, 2016, 12:52:23 am
content: Quote from: nctnico on January 10, 2016, 12:07:37 amThank you for your advice! I think there is no other short. Because it powers up with a square wave output on probe compensation. I have unsoldered all three of them and ordered new parts. Hopefully this could fix the problem.

author: cyang13
author_rank: Newbie
timestamp: January 15, 2016, 07:48:20 pm
content: Quote from: nctnico on January 10, 2016, 12:07:37 amThank you very much. Based on your suggestion I fixed this scope! Yeah!