Tektronix 2213 - 2nd Channel Problem - Page 1
Forum section: electronics / testgear

author: dan3460
author_rank: Frequent Contributor
timestamp: October 31, 2015, 05:29:25 pm
content: I have been working on this scope for a while. I have been posting on the yahoo groups for Tektronix too. here is the manualhttp://www.hellspark.com/dm/ebench/tools/Analog_Oscilliscope/tektronix_2213_service.pdfThe second channel produces a stable line when you just turn, after a minute or two the line starts to move up and eventually it goes of screen. It was a suggestion to change U220 which I have done but did not fixed the problem. My next step will be to change the transistors around the U220, this are Q222, Q225, Q233, Q234 and Q239. I have not been able to find the transistors. Does anyone knows if they could be replaced with other transistors?Does anyone have an idea that may be different from changing transistors?Thanks.

author: PA0PBZ
author_rank: Super Contributor
timestamp: October 31, 2015, 05:42:41 pm
content: Q222 - J300Q225 - MPSH10Q233 - MPSH10Q234 - MPSH81Q239 - MPS6523http://www.reprise.com/host/tektronix/reference/transistor.asp

author: Andy Watson
author_rank: Super Contributor
timestamp: October 31, 2015, 05:46:48 pm
content: Since this is a steady-state fault you ought to be able to cut-out the trial and error and home-in on the fault with a DVM.

author: dan3460
author_rank: Frequent Contributor
timestamp: October 31, 2015, 06:00:05 pm
content: Thanks for the quick response.I have looked at the voltages around all the transistors, they all start pretty good but all of them move out on voltages. I know that the problem is with something that gets warm and looses its specs.

author: Andy Watson
author_rank: Super Contributor
timestamp: October 31, 2015, 06:26:04 pm
content: Can you rule out (or in) U245? It's a bit trickier with the remaining transistors because it looks like (sodding diagram over two pages!) they form a feed-back loop back through U220. U220 would be my next test - what are the voltages on its two input pins (2 and 3), and on its output pin 6?

author: alsetalokin4017
author_rank: Super Contributor
timestamp: October 31, 2015, 08:48:51 pm
content: You should be able to track this problem (DC offset error) down with a can of "Freeze Spray" since it is pretty clearly heat-related. Spray the suspect components one at a time and see if the trace comes back down to where it should be.I had a similar problem (not heat related but a hard DC offset that took the trace off the screen permanently) in my 2213a and it turned out to be the JFET (your Q222) and the associated PIN diode. Replaced the JFET with a generic MPF102 and the PIN diode with a plain old 1n4148 and the scope is working perfectly again. The hard part was disassembling the attenuator switch to get to the circuitboard underneath to replace the diode.I just put the 1n4148 in there as an experiment while I looked for the actual equivalent replacement (which turned out to be NTE 555a) but why replace it if it works? Anyhow I have a pair of the NTEs now in my parts stash, waiting.....You may find this link useful:http://www.sphere.bc.ca/test/tekparts.html

author: dan3460
author_rank: Frequent Contributor
timestamp: November 01, 2015, 02:53:16 am
content: New problemhttps://www.eevblog.com/forum/Smileys/default/angry.gif.As I was probing voltages I found that when I pressed around VR222 the voltage changed, so I decided to take the board out and see if I had a loose connection. As I tuned the board upside down, two of the tops of trimming pots fell off. Now they will not stay on.

author: dan3460
author_rank: Frequent Contributor
timestamp: November 06, 2015, 02:43:01 am
content: I have changed all the trimpods from the board. I also got Freeze spray and started testing the components. It was no change when I hit the transistors. It returned to normal when I hit U220 and U245. U220 I already changed, could I have damaged the replacement?U245 comes in a board, on ebay someone is asking $39 for it, more than the scope is worth. I order the IC that is in the board to try to change it, it was only $3 from China.Why would it returns to normal when I hit U220 with cold? Been a new IC, is throwing me out.

author: dan3460
author_rank: Frequent Contributor
timestamp: November 14, 2015, 10:11:27 pm
content: I think I found the problem. The U245 is mounted in a little board that seems to be ceramic (or a really hard plastic) and one side of this little board is connected to the attenuator board. In preparation for the new chip I started working on de-soldering the little board. I was setting up my magnifying lamp and I saw a hairline crack on the U245 board, after de-soldering I can see that the crack also shows on the chip side. This make sense, as the chip warms up it expands separating the 2 sides of the board and the traces on the board lose contact.I was able to find the part new on ebay, it was $40, with the other few parts and capacitors I have expended more than the scope is worth, but I do this thing for fun. At the end this scope will go to a kid that is learning electronics, I think is well worth it.I'll post again when I get the part and see if this fix the problem.Thanks to everyone.

author: dan3460
author_rank: Frequent Contributor
timestamp: November 19, 2015, 02:16:27 pm
content: Well, that did not fix the problem.. I put the new U245 in, turn on the scope, got a line. Turned around to do something in my computer, turn back to the scope and with horror a see the line starting to move up.I quick blast of freeze to the U220 brings the line down. So far I have changed U220 (still have one more IC new, jut in case), U245, the trimpods and a couple of lytics on the board.Because the behavior is the same, before and after U220 was replaced, I have the feeling that something around is making U220 hot loosing the ability to control circuit. Pin 2 and 3 of the opamp supposed to be at 0v, pin 2 is never at exactly 0 and start to go up when the line starts to move up.I have the same feeling that I got several years ago when on one of my firsts repairs I got stuck with voltages on a power supply, it wasn't until someone told me how it was supposed to work that the light bulb turned on and I was able to fix it. I'm hoping for the same here.

author: Andy Watson
author_rank: Super Contributor
timestamp: November 19, 2015, 03:01:56 pm
content: Quote from: dan3460 on November 19, 2015, 02:16:27 pmI'll repeat what my mentor once told me, "fault-find; don't witch hunt."U220 restores the D.C. level to the signal. The voltages at its input seem to be correct (under the circumstances). If U220 is working, its output, pin 6, should be attempting to correct the charge on C221 - given your description of the of U220's inputs I would expect the voltage on pin 6 to be significantly negative - probably at the negative rail. Have you measured pin 6? Since you've already replaced U220 I think it is reasonable to assume that it is working - but check it!When working, U220 restores the D.C. level on C221 via R219 - something appears to be preventing this. Either:U220 is broken - but this has been replaced, so unlikely, but check the output voltage.The output of U220 is shorted to ground (or other rail) - this would account for it getting hot. Check the voltage on pin6.R219 is open circuit. Difficult to test 100M!There is some leakage into the node between R219 and Q222. Could be CR219 or Q222, or other contamination of the PCB. I would suspect Q222 since the voltage appears to be drifting positive.

author: fivefish
author_rank: Frequent Contributor
timestamp: November 19, 2015, 03:13:45 pm
content: QuoteI had the exact problem on a 2465A.QuoteNope... I didn't touch any chips or transistors in my debugging.QuoteYes, replace all the caps on your power supply. These are old scopes, several decades old... caps go bad/dry out/go out of spec, especially in this very hot environment inside the scope chassis. Some caps may have leaked at the bottom, and not obvious to naked eye until you desolder the caps.After replacing all electrolytics on my power supply, all traces become stable.

author: dan3460
author_rank: Frequent Contributor
timestamp: November 19, 2015, 03:30:14 pm
content: Thanks for the answers, I would try those steps.As for power supply, I have changed all caps as you said they have been there for many years.I'll post my results.

author: dan3460
author_rank: Frequent Contributor
timestamp: November 21, 2015, 01:38:14 pm
content: Ok, I have changed Q222 and that solved the problem.The scope has been on for 10 min and the line still there, still turning back now and then to make sure donow i'm going to calibrate this thing.Thanks everyone for your help.