Tektronix 2225 very weird vertical position fault [solved] - Page 1
Forum section: electronics / repair

author: Musikermomo
author_rank: Contributor
timestamp: November 01, 2020, 01:10:21 pm
content: Hi all,I am trying to repair my scope Tektronix 2225 and I am lost.The problem it shows is that the vertical position of channel one is influenced by the vertical position knob of channel 2. Also channel one trace shows the inverted signal of channel 2, even when ch2 is switched off, but only if the channel 2 vertical position knob is not turned all the way up.Kind of hard to explain so I made two videos:https://youtu.be/AgEqlyDWqIwhttps://youtu.be/sZYH9mG9tBQChannel two seems to Work fine.I checked the front panel and the attenuator board and could't find any faults so I think the problem must be somewhere on the Main Board.Ch1 seems to work fine if i disconnect J2-1. I measured the voltages surrounding this part and further up to the delay lne driver and they are almost spot on.Appreciate any help.Thanks!Moritz

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 01, 2020, 03:05:39 pm
content: I would try to replace U180 (CA3102E)NB: Whenever you replace an integrated circuit, always install a good quality socket.Service manual:https://elektrotanya.com/tektronix_2225_oscilloscope_sm.pdf/download.html#dl

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 01, 2020, 03:30:23 pm
content: For a better diagnosis, a lot more information would be needed:1) CH1 only: Is the CH1 vertical position control working normally with the same deviation up and down.Is the CH1 signal normal? (amplitude, waveform)Does the CH2 position control have any influence?2) CH2 only: same3) BOTH: describe the operation in chopp and alt .... Is it different?

author: Musikermomo
author_rank: Contributor
timestamp: November 01, 2020, 03:45:09 pm
content: Quote from: akimpowerscr on November 01, 2020, 03:30:23 pm1) CH1 only: Yes to all.Ch2 position control has influence on Ch1 position and amplitude. Ch1 trace seems to behave normal, when Ch2 Position is set fully clockwise.2) Ch2 only: everything seems fine.3) chop and alt make no difference regarding this problems.If I feed a signal into ch2 it gets doublicated (but mirrored) on ch1 regardless of switches (Ch1/Both, Add/Alt/Chop). The amplitude changes when fiddling with Vert-Pos2.

author: Rollin Hand
author_rank: Regular Contributor
timestamp: November 01, 2020, 03:59:33 pm
content: looks like a scratchy or open potenciometer.check values

author: Musikermomo
author_rank: Contributor
timestamp: November 01, 2020, 04:06:41 pm
content: It is a bit scratchy, but I checked the component values and pcb traces around the position control (Front PCB) and they are fine.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 01, 2020, 04:12:38 pm
content: Interesting, I have never seen such a defect.With a slow time base, (50ms/div for example), in BOTH, what do you see on the screen in ALT and in CHOPP?To not waste too much time in diagnosis, I would start by replacing the two CA3102E (U180 and U130)Check also diodes CR183, CR189, CR186 and -8,6V rail

author: pbarton
author_rank: Frequent Contributor
timestamp: November 01, 2020, 04:27:05 pm
content: You can only get interaction between the Ch1 vertical position control and the Ch2 vertical position control if you have a fault in the alternating channel logic.Ch1 and Ch2 should never be selected at the same time. If they are, then you have the fault that you can see.The good news is that Ch1 and Ch2 amplifiers are both OK.The channel selection logic is U540A, pins 8 and 9 should alternate.

author: Musikermomo
author_rank: Contributor
timestamp: November 01, 2020, 04:43:54 pm
content: Quote from: akimpowerscr on November 01, 2020, 04:12:38 pmYeah, it is weird... I've been searching the Internet for 2 days and have not found any similar problem.The Scope is partly disassembled right now, but if I remember correctly, it behaves normal in slow timebases, exept the vertical position problem and ch2's signal on ch1.I measured the diodes in-circuit. CR183 and CR189 have threshold voltages of 0,6V and resistances of 1kohm.CR186has a threshold voltage of0,15Vand R of100ohm.Is CR186 dead?

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 01, 2020, 04:57:47 pm
content: CR186 is in parallel with a resistance of 100R, what you are measuring is OK.If CR186 should be defect, you should measure a dead short circuit.Check also CR136 (same as CR186) and CR189.It is also possible that the select channel logic U540 is defect but the set/reset seems to work as you can select CH1 or CH2.As I already asked:"With a slow time base, (50ms.div for example), in BOTH, what do you see on the screen in ALT and in CHOPP?"NB: I am waiting you to answer me that in CHOPP, you see both traces and in ALT only one trace each time....Is this correct ?

author: Musikermomo
author_rank: Contributor
timestamp: November 01, 2020, 05:57:52 pm
content: It took quite a while to reassemble the thing, I'm sorry.Quote from: akimpowerscr on November 01, 2020, 04:57:47 pmBoth OK.Quote from: akimpowerscr on November 01, 2020, 04:57:47 pmThis is correct.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 01, 2020, 06:06:46 pm
content: Than, it seems that the select channel circuit is working....But replace also U540A (74LS74) to be sure.

author: Musikermomo
author_rank: Contributor
timestamp: November 01, 2020, 06:41:10 pm
content: Quote from: pbarton on November 01, 2020, 04:27:05 pmpins 8 and 9 do alternate in ALT-mode.

author: pbarton
author_rank: Frequent Contributor
timestamp: November 01, 2020, 06:43:53 pm
content: The oscilloscope may be displaying Ch1 correctly?However, if there is a select channel problem that enabled both Ch1 and Ch2 together (during the Ch2 display period), then the resulting Y signal would be the ‘sum’ of the two channel amplifiers, and there would be interaction between Ch1 and Ch2 vertical settings.The observer would still see two traces in CHOP and one trace each time in ALT (at slow timebase).That proves that CHOP and ALT modes work, but it does not prove that Ch1 and Ch2 are alternating independently.What you are not seeing is Ch2 (on its own) during the Ch2 display period.Could be U180 (CA3102E) or those diodes mentioned above.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 01, 2020, 06:47:16 pm
content: Replace the two CA3102E (U180 and U130)

author: Musikermomo
author_rank: Contributor
timestamp: November 01, 2020, 07:12:02 pm
content: Thank you very much for the help so far!I think I will swap u180 with u130 first to see if they cause the problem.Unfortunately these ICs are quite expensive.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 01, 2020, 07:26:01 pm
content: The CA3102 can be replaced with three matched pairs of transistors like 2N3904's if you are desperate, but not recomended.If you find CA3049, that's the same but in metal can.

author: pbarton
author_rank: Frequent Contributor
timestamp: November 01, 2020, 07:59:33 pm
content: If you look at the CA3102E datasheet, you will see that it contains two independent differential amplifiers. I would be using a meter to check the impedance, pin to pin, to determine if I can identify a faulty reading. You have two chips to compare (U180 and U130) before removing them from the board.The CA3102E is expensive, so I would not buy one unless I could identify an internal fault using a meter. Tektronix Part # 156-0534-00.https://www.littlediode.com/components/home.phpsame seller also on eBay.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 01, 2020, 08:22:22 pm
content: The funniest thing is that I sold a 2235 wreck two months ago for about the price of ONE CA3102E .....It was irreparable, and in particular the pre-regulator inductance was missing, buttons were missing or broken and the channel 1 input switch was completely worn.But with an external 43V, it was working ...

author: Musikermomo
author_rank: Contributor
timestamp: November 01, 2020, 08:45:11 pm
content: Quote from: pbarton on November 01, 2020, 07:59:33 pmThe only difference I could find so far is between pins 9/10 and 3/10U180:3-10 --> 1M7-10 --> 0,5MU130:3-10 --> 700k7-10 --> 300kQuote from: akimpowerscr on November 01, 2020, 08:22:22 pmThis is my first own scope. I scored it on ebay for 50€ in "fully working condition".Now the seller has dissappeared

author: pbarton
author_rank: Frequent Contributor
timestamp: November 01, 2020, 09:18:27 pm
content: The differences you have measured are small.If you had identified a short or an open circuit between pins, that would have been a different story.Select Ch1 (only) on the front panel, measure the voltage on each pin of U130.Then select Ch2 (only) on the front panel, measure the voltage on each pin of U180.Compare results.akimpowerscr: I use this document as a guide for those power supplieshttp://www.hakanh.com/dl/docs/troubleshooting_tips_on_2200_ps.pdf

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 01, 2020, 09:44:23 pm
content: Quote from: MusikermomoUnfortunately, there are people who say that an analog oscilloscope is only worth it if you pay it very cheap.But in life, there are no miracles.For 50 € you only buy wrecks .... and then some people come to complain that an analog oscilloscope is unreliable.Do you really think a seller is going to waste their time testing a device they sell for only $ 50 ?These devices were very expensive (2000, 3000 euros and even more) because there were expensive parts in them, like the cathode ray tube for example.A good analog oscilloscope costs at least 150 euros, unless it is sold broken down or for parts.You live in Germany, why didn't you buy a HAMEG HM203? They are simple and inexpensive.Hameg has made over 150,000 HM203s.They have sold it everywhere, industry, education, administration, hobby, ....

author: Musikermomo
author_rank: Contributor
timestamp: November 02, 2020, 01:53:52 am
content: Quote from: akimpowerscr on November 01, 2020, 09:44:23 pmI wanted an analog scope. I use scopes mainly for ampstuff, so no need for modern fancy-pancy scopes.My father bought a very nice scope some years ago for 50€ from ebay, it was a Hameg, that was in working condition, as advertized.But call me naive, I didn't know that lying is ok if you sell a tek scope...2225ves go for just about the same prices as hm203s...

author: Musikermomo
author_rank: Contributor
timestamp: November 02, 2020, 10:59:18 am
content: Quote from: pbarton on November 01, 2020, 09:18:27 pmI measured each pin and these are the only major differences. (All other pins are just a few mV off.)Ch1(only):U130: Pin4/8 --> 0VU180: Pin4/8 --> -550mVCh2(only):U130: Pin4/8 --> -10mVU180: Pin4/8 --> -80mVBOTH:U130: Pin4/8 --> -5mVU180: Pin4/8 --> -320mV

author: pbarton
author_rank: Frequent Contributor
timestamp: November 02, 2020, 12:00:42 pm
content: Can't see anything wrong there. The manual contains lots of sample waveforms to verify correct operation.Do you have access to another scope?

author: Musikermomo
author_rank: Contributor
timestamp: November 02, 2020, 01:41:39 pm
content: Quote from: pbarton on November 02, 2020, 12:00:42 pmI'll try to check the waveforms next. I don't have a second scope, but maybe mine can examine itself.Another fault (maybe they are related) is, if Ch2 INVERT is activated, the trace jumps up (Ch2 down) about 3 divisions, even with Ch2 input set to GND. And it's not inverted.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 02, 2020, 02:02:56 pm
content: QuoteThis is not a fault, it can be adjusted by trimpot R84 (Bal).With symetrical waveform, how are you sure it is inverted or not ?

author: Musikermomo
author_rank: Contributor
timestamp: November 02, 2020, 07:22:47 pm
content: Quote from: akimpowerscr on November 02, 2020, 02:02:56 pmI calibrated it. That works now.

author: Musikermomo
author_rank: Contributor
timestamp: November 03, 2020, 03:20:47 pm
content: A friend might let me use his scope this week.I made another video with a signal attatched to ch2 and showing only ch1. Maybe it gives you any clues...https://youtu.be/sZYH9mG9tBQ

author: pbarton
author_rank: Frequent Contributor
timestamp: November 03, 2020, 05:34:45 pm
content: From your video, I can see that its Ch2 that is working correctly when Ch2 is selected.However, It appears that Ch2 is not being ‘de-selected’ when you switch to Ch1.You need verify that Ch2 is ‘de-selected’ once switched to Ch1.It appears that Ch2 is wrongly selected/enabled in both Ch1 and Ch2 positions.I assume that Ch1 is correct, in that it is selected in Ch1 switch position and ‘de-selected’ in Ch2 position.EDIT: Are you sure that the front panel Ch1/BOTH/Ch2 switch is providing the correct signals?You need verify that Ch2 is ‘de-selected’ once switched to Ch1.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 03, 2020, 08:01:52 pm
content: It would be necessary to measure pins 10 and 13 of U540A (74LS74) for each of the 3 positions of the switch CH1-BOTH-CH2.Indeed, if the two Pin Preset (10) and clear (13) of U540A are simultaneously in the low logic state (L), the two outputs Q and _Q can both be in the high state (H), selecting both channels.But I do not understand how such a defect could cause a variation in the amplitude of the signal with the change of position on the screen.This makes me think that it would rather be one of the two CA3102E which would be defective.But without spare parts, you have to exhaust all possibilities, the simple solution of changing the two CA3102Es and seeing if that solves the problem is not possible.

author: pbarton
author_rank: Frequent Contributor
timestamp: November 03, 2020, 08:47:23 pm
content: Assuming that the fault is that Ch1 and Ch2 amplifiers are being selected together and that their differential outputs are joined in parallel….Assuming that there is a square wave on Ch2 only (the condition, as shown in the video).Assuming that the front panel vertical position controls are in their mid-position, then a square wave will appear across the center of the CRT.Both Ch1 and Ch2 amplifiers each contribute 50% to the ‘Y’ positionNote: Ch1 has no signal applied.Both the Ch1 and the Ch2 vertical position controls will move the square wave up or down.However as Ch1 does not carry a signal (a flat line) moving either vertical control will change the relationship (or percentage) that each amplifier will contribute to the final Y signal.If Ch1 (the flat line) is more dominant than the square wave on Ch2, then the displayed square wave will reduce in amplitude.I wish that I could think of a better way of explaining it.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 03, 2020, 10:29:46 pm
content: QuoteIf pins 8 and 9 of U540A do alternate in ALT mode, both CH1 and CH2 CAN'T be selected at the same time in this mode.Then, how do you explain this:Quote

author: pbarton
author_rank: Frequent Contributor
timestamp: November 03, 2020, 11:01:53 pm
content: Quote from: akimpowerscr on November 03, 2020, 10:29:46 pmI agree with you. I can’t explain that.I thought this was consistent with Ch2 not ‘de-selecting’ when the switch is in the Ch1 position?The mirrored bit, could be an artifact of the scope’s ALT switching rate and the square wave signal frequency?It could give the impression that one Channel signal was 180 degree out compared with the other?“The amplitude changes” as explained previously.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 03, 2020, 11:50:38 pm
content: But I have a completely logical explanation for what Muzikermomo found:The switching of channels CH1 and CH2 does not only depend on the logic signals, but also on the correct functioning of the two CA3102E.Hence my first diagnosis: one of the two CA3102Es is defective.It is very easy to check this, just measure the logic states of pins 8 and 9 of U540A with the selector in CH1 and CH2.One of the pins must always be at the high level and the other at the low level, never both at the same level.

author: Musikermomo
author_rank: Contributor
timestamp: November 04, 2020, 12:34:12 pm
content: Quote from: akimpowerscr on November 03, 2020, 08:01:52 pmQuote from: akimpowerscr on November 03, 2020, 11:50:38 pmBoth traces middle Y POSITION, CHOP, SEC/DIV 1µsCH1BOTHCH2U540/80,3V1,9V3,5VU540/93,5V1,9V0,3VU540/100,2V5,0V5,0VU540/135,0V5,0V0,8V-U130/62,5V2,7V3,0VU130/80,0V0,0V0,0VU130/110,7V0,1V0,6VU130/142,6V2,8V3,0V-U180/63,0V2,8V2,6VU180/80,5V0,3V0,1VU180/110,6V0,1V0,7VU180/143,0V2,8V2,6VSeems OK to me exept maybe U540/13 - CH2?Maybe I can use my friend's scope later this day.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 04, 2020, 01:17:30 pm
content: Quote from: Musikermomo on November 04, 2020, 12:34:12 pmThank you for this.CH1 and CH2 are correctly selected and there is nothing wrong with the selection logic circuit.Only solution possible: one of the CA3102E is defective.

author: pbarton
author_rank: Frequent Contributor
timestamp: November 04, 2020, 01:46:37 pm
content: Agreed, the selection switch and U540A are both operating correctly.This reading is suspicious U130/8 0,0V 0,0V 0,0VNeed to check that the diodes CR 138, and CR139 are OK. (if I have read the schematic correctly).

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 04, 2020, 02:04:21 pm
content: I wrote:QuoteBut it was CR139 and not CR189....

author: Musikermomo
author_rank: Contributor
timestamp: November 05, 2020, 04:58:51 pm
content: You were right from the start. I swapped U180 and U130 and now the Problem is the other way around!Thank you very very much for your help! I learned quite a bit about my scope in the process.I hope you had a bit of fun at least, guiding me to the solution, even if the first reply was already the solution.

author: akimpowerscr
author_rank: Regular Contributor
timestamp: November 05, 2020, 06:03:01 pm
content: All's well that ends well, but it's true that when you don't have spare parts, everything becomes very difficult.....

author: mbarszcz
author_rank: Contributor
timestamp: August 27, 2022, 10:33:29 pm
content: I stumbled upon this thread and have an identical fault with a Tek 2210.Was it simply a bad U130 / U180 ? All my channel selection logic seems to behave the same as yours (correctly), yet Channel 2 is bleeding into channel 1, and the channel 2 vertical position switch seems to affect channel 1.I have replaced the two CA3102E, but I'm still seeing the same fault.