Bringing Mr. Dean back to life (Tek 2225 with screwed up power supply) - Page 1
Forum section: electronics / repair

author: The Guy
author_rank: Regular Contributor
timestamp: September 25, 2017, 09:36:00 pm
content: Well here I am repairing another 222five. This one does not power up though it`s in very good cosmetic condition.Power transformer its ok, but circuit voltages are all over the place....Here`s the schematic attached with the actual voltage readings in red.circuit around T902 looks highly suspicious!! Let me know your thoughts please.Thank you.

author: Toasty
author_rank: Regular Contributor
timestamp: September 25, 2017, 09:59:04 pm
content: Track down the 38V line to find what's killing it to 26V. Some of those voltages (+8.6, -8.5, +5.2)won't come up until T902 is being properly fed signal.Also, check the main cap C900 and the 38V cap C971 for capacitance and ESR.T

author: The Guy
author_rank: Regular Contributor
timestamp: September 26, 2017, 12:49:14 am
content: Quote from: Toasty on September 25, 2017, 09:59:04 pmThanks! will do...I forgot to tell that either Q950, Q980 or both get very hot! ...I`m not sure what that means.

author: Toasty
author_rank: Regular Contributor
timestamp: September 26, 2017, 01:16:17 am
content: That could indicate the lack of signal (sweep/pulses/oscillations) to the transformer causing them to turn on but not do anything. They are essentially grounding the DC from L910 through the transformer from Pin 7.U910 and or U940 (TL594) could be faulty...http://www.ti.com/lit/ds/symlink/tl594.pdfT

author: The Guy
author_rank: Regular Contributor
timestamp: September 26, 2017, 01:28:59 am
content: Quote from: T5asty on September 26, 2017, 01:16:17 amBut then that could happen because the voltages are upset right? ...at R943 (at the upper left corner of U940 in the schematic) there are 5V where there should be 23V. With 5v there U940 cannot properly operate if operate at all i think.... am I right?

author: The Guy
author_rank: Regular Contributor
timestamp: September 26, 2017, 01:44:34 am
content: Yep... in the PDF that you kindly linked it says that:"The TL594 has circuitry to provide an undervoltage-lockout functionality. A minimum recommended VCC voltage of 7 V is recommended for operation, but if the VCC voltage drops below 6 V during operation, then the device shuts off"So that indicates that at 5V U940 is effectively shuted down. Am I getting this right?

author: Toasty
author_rank: Regular Contributor
timestamp: September 26, 2017, 02:03:28 am
content: At U940 where you have 5V marked pointing to where it says +23.0V, I believe that is an error.There's no way for the voltage at U940 Pin 14 to be +5.0V and correct and the voltage at the top of R943 to be different. Pins 2, 6, 13, 14 are tied together.Where the +23.0V is referencing is the bottom of YR942 (15v Zener diode) and Pin 12 of U940: 38V - 15V = 23VVcc for U940TBTW: Since you have several of these, you can use the others to compare voltages to this one.

author: The Guy
author_rank: Regular Contributor
timestamp: September 26, 2017, 02:28:34 am
content: Quote from: Toasty on September 26, 2017, 02:03:28 amoh wow... you are so right! Didn`t see that.BTW, instead of those 23V there are 11V... just measured it.I`ve been comparing measurements back and forth... still can`t figure out what is happeningI don´t have an esr tester, so maybe I´ll just change those caps and see what happens.Do you think measuring ripple after C900 is going to tell me if that cap is bad?

author: Toasty
author_rank: Regular Contributor
timestamp: September 26, 2017, 03:42:12 am
content: It would be a good indication of -a- problem, but I believe -the- problem to be in the switching circuit that drives the transformer T902.T

author: The Guy
author_rank: Regular Contributor
timestamp: September 27, 2017, 12:22:54 am
content: So I measured ripple after C900 and it is exactly the same than one of the working 222five I have. Not sure if I measured it correctly... I grounded the probe`s ground wire to the chassis and conected the probe to the positive side of the cap. I`m not sure if that`s correct because c900 doesn`t have its negative lead connected to ground.I also removed all the jumper connections used to apply power to the different circuit boards just to isolate the power supply from possible shorts in other sectors, but nothing changed. Power supply is currently disconnected from the rest of the circuits.I think it is important to note that those 27 or so volts are already present instead the 38V right after Q913 (in orange in the schematic attached below). Maybe Q913, Q912 or CR912 are bad?Thank you!

author: Toasty
author_rank: Regular Contributor
timestamp: September 27, 2017, 01:30:50 am
content: Be sure to drain any charged caps safely. Check for voltage before beginning.Again, using another working unit for reference, start testing the transistors & diodes using your DMM in "Diode Check" mode. Disconnect the good PSU as you did the bad one.You can also take readings from chassis ground to the various test points. With the DMM in Diode Check, and the -positive- lead to chassis, take comparative readings at the numbered hexagonal points on the schematic. Make a chart with columns being the "good" machine and then the "bad" one.Also check, in the same manner, at Pin 12 on both PWM ICs.T

author: The Guy
author_rank: Regular Contributor
timestamp: September 27, 2017, 04:32:13 pm
content: Quote from: Toasty on September 27, 2017, 01:30:50 amokey dokey....I took all the other measurements you suggested and they read as follow:point number:30 reads 35.5V good scope and 26.7V bad scope31 reads 35.5V good scope and 26.8V bad scope32 reads 2V good scope and 2.3V bad scope33 reads 36V good scope and 27.5V bad scope34 reads 18V good scope and 10.5V bad scope (referenced to pin six)35 reads -0.2V good scope and -3.6V bad scope (referenced to pin five)pin 12 at U910 read 29V good unit 20V bad unitpin 12 at U940 read 20V god unit 12V bad unit

author: Toasty
author_rank: Regular Contributor
timestamp: September 27, 2017, 07:00:37 pm
content: Ummm, no.DMM in Diode Check mode, Not voltage - no power applied, not plugged in.T

author: The Guy
author_rank: Regular Contributor
timestamp: September 27, 2017, 08:47:46 pm
content: Quote from: Toasty on September 27, 2017, 07:00:37 pmDang! What was I thinking..... Alright here are the measurements in diode mode, no power applied:Point30 reads 3.2V good scope and 3.2V bad scope31 reads 3.2V good scope and 3.2V bad scope32 reads 0V good scope and 0V bad scope33 reads 3.2V good scope and 3.2V bad scope34 reads 0V good scope and 0V bad scope (referenced to pin six)35 reads 0V good scope and 0V bad scope (referenced to pin five)pin 12 at U910 read 0V good unit 0V bad unitpin 12 at U940 read 1.1V god unit 1.1V bad unitAlso replaced C971 and C915... but nothing changed. Still need to try replacing c927, c972, c953 and c952 to see if some of them are leaking.Thank you!

author: Toasty
author_rank: Regular Contributor
timestamp: September 27, 2017, 11:52:55 pm
content: ?? 34 reads --- (referenced to pin six)?? 35 reads --- (referenced to pin five)Why the "referenced to" instead of chassis ground?The readings can be taken from any of the points highlighted in the attached image.Please check resistor R907 for value & compare to good unit.T

author: The Guy
author_rank: Regular Contributor
timestamp: September 28, 2017, 12:25:05 am
content: Quote from: Toasty on September 27, 2017, 11:52:55 pmSorry bad use of language... i meant that points 34 and 35 were measured at said pins.Seems that resistor R907 has a problem because the measurement jumps around... no stable at all... in the working scope it´s dead stable at 0.4ohm

author: The Guy
author_rank: Regular Contributor
timestamp: September 28, 2017, 12:42:38 am
content: hmmmm.... took the resistor out and it reads stable at 0.4ohm now... weird. Will still replace it.

author: The Guy
author_rank: Regular Contributor
timestamp: September 28, 2017, 01:12:26 am
content: Changed R907 and reading is stable now but the voltages are still off... nothing changed really.

author: Toasty
author_rank: Regular Contributor
timestamp: September 28, 2017, 01:25:56 am
content: Okay. Dang it.DMM in Diode mode+ lead to any of these pins on U910: 4, 7, 9, 10, 13, 16- lead to U910 pin 12Good vs Bad unitsT

author: The Guy
author_rank: Regular Contributor
timestamp: September 28, 2017, 02:09:47 am
content: Quote from: Toasty on September 28, 2017, 01:25:56 amAlrighty.... will do that tomorrow... it´s late here....I just finished replacing caps c927, c972, c953 and c952 to rule out any leaking path, but nothing. Mr. Dean still´s deader than a plank of wood...

author: Toasty
author_rank: Regular Contributor
timestamp: September 28, 2017, 02:22:49 am
content: Replacing parts in a shotgun fashion doesn't solve anything.Where is "here"? Your profile is missing a flag/location.Good night!T

author: The Guy
author_rank: Regular Contributor
timestamp: September 28, 2017, 02:54:53 am
content: Quote from: Toasty on September 28, 2017, 02:22:49 amI know it´s just that I thought that if there was any cap leaking along that 38V line (26V currently) i might fix the problem replacing them one by one. Had I a cap tester I would´ve checked´em outI´m a little weary about giving my location on the internet... dunno.... I´m just sort of a private person I thinkI really appreciate your time and help.Thanks a bunch and talk later!Good night

author: Toasty
author_rank: Regular Contributor
timestamp: September 28, 2017, 03:10:08 am
content: USA, China, Outer Mongolia, North Pole? Jeeze!

author: oldway
author_rank: Super Contributor
timestamp: September 28, 2017, 07:56:54 am
content: I have not read all the posts but I give you my first impression: I believe that + 38V consumption is excessive (to check).This would come from the inverter, as seems to be demonstrated by the fact that Q950 and Q980 heat up too much.Possible reasons:U975 defective (try to disconnect it from T902)T902 defectiveSecondary overload of T902.

author: The Guy
author_rank: Regular Contributor
timestamp: September 28, 2017, 05:23:06 pm
content: Quote from: oldway on September 28, 2017, 07:56:54 amSoooooo................Disconnected U975 and nothing changed... then swapped T902 for the one in the 222five I have for parts andBoooM! Got a working scope!!!Though now i need to get those sweeps on the screen cuz they´re completely deflected to the left side.Huge thanks oldway and Toasty!you guys rule... Mr. Dean sends his greetings

author: Toasty
author_rank: Regular Contributor
timestamp: September 28, 2017, 06:16:02 pm
content: Great! Good job!T

author: The Guy
author_rank: Regular Contributor
timestamp: September 28, 2017, 11:43:07 pm
content: Been trying to fix the horizontal amplifier issue and now i am puzzled again! I jumpered both leads (in red "A" in the schematic) from the horizontal output amplifier right before the CRT and the dot does not center! ...there is no circuit after that... it should´ve centered. In the working scope it centers just fine. That I don´t understand.If I put the SEC/DIV selector in X/Y mode the dot centers. Also if I push the beam find button it also appears at the side of the screen.The voltages recommendations are all in spec except for the voltage readings in red in the schematic which look a little weird.Any ideas?

author: The Guy
author_rank: Regular Contributor
timestamp: September 29, 2017, 02:32:50 am
content: Fixed it! It was a bad contact in one of those ribbon wires... It works as good as new nowI just added two great working 222five tek scopes to my collection ha!Cheers and thank you.

author: oldway
author_rank: Super Contributor
timestamp: September 29, 2017, 09:58:33 am
content: Congratulations .... Having at least two analog oscilloscopes of the same model is a solution that I recommend.One serves as a donor if necessary, this greatly simplifies the diagnosis and repair.I like very much the hameg hm605, it is the oscilloscope that I use every day and I confess.... I have 3 of them in good working condition ....

author: tautech
author_rank: Super Contributor
timestamp: September 29, 2017, 10:11:13 am
content: Just as oldway says, congrats.Charlie and Mr Dean are goers but what about poor Ben, poor poor cannibalised Ben.Along to Sphere for bits to fix him ?http://www.sphere.bc.ca/test/tek-transformers.html

author: The Guy
author_rank: Regular Contributor
timestamp: September 29, 2017, 11:38:50 pm
content: Quote from: tautech on September 29, 2017, 10:11:13 amThank you!Problem with Ben is that some sort of gorilla tried to fix it at some point and did a real lousy job... lots of lifted traces, screwed up stuff.. really not good. That doesn´t make me very exited about repairing it to be honest. I think I´ll just leave it as a spare parts scope for fixin future failures of the ones I have working.... or maybe even for repairing another one I could score along the line... I just payed about 40 bucks each. Not big deal I guessWe´ll see....All I want now is a 465 or something like that. I already have a 422 and Iloveit!

author: The Guy
author_rank: Regular Contributor
timestamp: September 29, 2017, 11:44:13 pm
content: Quote from: oldway on September 29, 2017, 09:58:33 amThanks! They are pretty good scopes... though they can´t compare with my awesomest Tek 547But hey, they work just fine!I was thinking about getting a Hameg hm 203 but I don´t know much about them.... how do they compare to their Tektronix equivalents?Cheers

author: oldway
author_rank: Super Contributor
timestamp: September 30, 2017, 10:35:49 am
content: For the Hameg oscilloscopes, I know them well because I have different models.I have three HM605, one HM1005, one HM1505, I had an HM204-4 but it was irreparable (transformer defective) and I sold it for parts and I have already repaired HM604.Without any hesitation, my preference goes to the HM605, by far.NB: repairing an HM605