Tektronix 2236 Analogue Scope - Page 1
Forum section: electronics / repair

author: MFinney
author_rank: Contributor
timestamp: April 26, 2018, 10:42:18 am
content: Hi, I hope you can help. I have aquired a Tektronic 2236 oscilloscope and have a problem with the display output on the CRT. I have the instruction manual and have exactly set the scope up in terms of button positions to get a base line trace section 2-14. After a bit of adjustment I managed to get a horizontal flat line across the CRT screen. The line changed in intensity and the intensity level was erratic. I left the scope for circa 1/2 hour and the horizontal line then disappeared (no settings were changed in this period).I depressed the beam find button and the beam reappeared above the centre line length 25mm. Despite no amount of powering up/down could I get the beam back under normal base line settings.I ran the self check trouble shooting tests (displayed on the CTM) and no errrors were indicated.I am reasonably electronics literate and am quite happy to replace components if required but by no means an expert on these analgoue scopes. Based on the symptoms above could the forum suggest basic things to check.Many thanks.

author: bd139
author_rank: Super Contributor
timestamp: April 26, 2018, 10:45:14 am
content: Grab the service manual here first:http://w140.com/tekwiki/wiki/2236First thing to test is the voltages of the power supply with a DMM. Steps in the manual above and expected voltages.This line of scopes have a lot of rather old and dicky capacitors in them which can drag the voltages off. I did a 2225 recently which ate about £20 of capacitors before it worked properly.

author: MFinney
author_rank: Contributor
timestamp: April 26, 2018, 07:54:53 pm
content: Hi thanks I’ve checked with my meter and the values areTP961 -8.58W968 +5.03W960 +8.61W956 +29.87W954 +101.0The only one that is reading low is W968. The caps are all original scope appears to be working ok with the beam finder permanently pushed in but crt blank when out. Can you give advice where to look next?

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 26, 2018, 08:21:20 pm
content: Quote from: bd139 on April 26, 2018, 10:45:14 amWhat all types were giving out ? I have a Tek2215 with a problem I think in the horizontal/timing/trigger ? (I'm pretty new to scopes of any type)Sure is pretty inside, I'd rather replace some aged caps before they fail.

author: markce
author_rank: Regular Contributor
timestamp: April 27, 2018, 09:22:57 pm
content: Please check high voltage supply is 1900-2100V (point 2 under Power supply and CRT diplay).

author: bd139
author_rank: Super Contributor
timestamp: April 27, 2018, 11:14:28 pm
content: I wouldn’t go poking HV yet. Dangerous and the CRT itself acts as a good “visual voltmeter”Go through the check out procedure in the service manual I linked and note any differences.Does the intensity control do anything when you hold the beam finder down? Some of these have dodgy intensity pots.

author: markce
author_rank: Regular Contributor
timestamp: April 28, 2018, 12:01:01 am
content: It's point 2 in the checkout, point 1 is already listed Ok here (also see page 3-17 of service manual). And yes, you should be carefull with this voltage. It's still not the final Crt HV. I've also measured the 14kV final HV on my Tek 2236 when I refurbished it, but with a special probe that is needed to do this safely.

author: MFinney
author_rank: Contributor
timestamp: April 29, 2018, 10:44:53 am
content: Thanks for the info. I don’t have the equipment to check the hv side so I’ll have to leave this. Using beam finder I get a strong trace so suspect issue lies elsewhere. Looking at it I would assume the vertical circuit some where. Thanks for the help I am not going to let this defeat me !!

author: markce
author_rank: Regular Contributor
timestamp: April 29, 2018, 01:12:30 pm
content: My response was aiming at the intensity part of the problem. You could easily make a DIY probe with 2 HV resistors (Vishay) for the 2kV. Do the measurement hands-off.Also check the 510k series resistors in the Z circuit. They tend to change value over time.

author: MFinney
author_rank: Contributor
timestamp: April 29, 2018, 04:47:54 pm
content: Hi thanks for this I’ve checked the four in series r888, 889 890 891 and they range from 528 max to 518k I’ve also checked r892 477 k and r894 527k. Would these being off cause the symptoms above?? Thanks

author: markce
author_rank: Regular Contributor
timestamp: April 29, 2018, 05:26:45 pm
content: They seem to be Ok (within 5%). R892 has a 5M pot in parallel.For intensity problems, I would start at signal 48 (collector of Q840) and work back through the schematic from there. You will need a second scope for that.

author: markce
author_rank: Regular Contributor
timestamp: April 29, 2018, 08:35:05 pm
content: If I understand your initial posting correctly, you have two problems: one with intensity and one with increasing vertical offset.

author: MFinney
author_rank: Contributor
timestamp: April 29, 2018, 09:24:34 pm
content: Yes although I think the vertical offset is the main issue.

author: markce
author_rank: Regular Contributor
timestamp: April 29, 2018, 11:00:39 pm
content: Ok, vertical is less complicated. Do both input channel have the same drift? If so, the problem is most likely in the Y output amp (main board).

author: MFinney
author_rank: Contributor
timestamp: April 30, 2018, 08:09:38 am
content: Hi yes both input channels have exactly the same drift. I only have a DVM is it possible to point me in the direction of a few key things to check? I have all the schematics so will try and work backwards. Thanks for the assistance. Mark

author: MFinney
author_rank: Contributor
timestamp: April 30, 2018, 11:40:25 am
content: Hi one thing that I don’t understand is a reference in the trouble shooting guide flow diagram. It states ‘momentarily short across delay line leads’ schematic 2. Could someone explain how to do this as I’m unclear on which points I need to short. Thanks

author: markce
author_rank: Regular Contributor
timestamp: April 30, 2018, 01:02:53 pm
content: Without the 'momentarily short', you could start to check the voltage at input of delay line (collector Q206 and Q207) before and after the fault occurs. Should be 0V. You then know if the problem is in the first or the second part of the output stage.

author: MFinney
author_rank: Contributor
timestamp: April 30, 2018, 08:29:41 pm
content: Hi borrowed a test scope wave form a points 1 2 and 3 are correct. Wave form test points 4 and 5 look wrong. I’ve checked the collectors on q206 and q207 and they are not zero. -0.4V and +0.4v hopefully this narrows down the problem?

author: markce
author_rank: Regular Contributor
timestamp: April 30, 2018, 10:11:54 pm
content: What is the difference of collector voltage when you start with a cold sope, compared to the error situ after 1/2 hour?

author: MFinney
author_rank: Contributor
timestamp: May 01, 2018, 05:47:12 am
content: HI I’ve taken a few readings from cold and after 1/2 hour once running and there is no change? Does this narrow down the problem? Thanks for help mark.

author: markce
author_rank: Regular Contributor
timestamp: May 01, 2018, 09:58:04 am
content: So the vertical drift is apparently introduced after Q206/207. This narrow it down to Q230-Q256. With the scope setup in singe channel and collector Q206/207 adjusted to 0V with channel 1 vertical control, what is the vertical plate voltage (cold and after 1/2 hour) for both plates?

author: Old Printer
author_rank: Frequent Contributor
timestamp: May 01, 2018, 07:47:38 pm
content: Following along with great interest! I have a 2225, running fine at the moment, but you never know, and besides it's just plan interesting. Thanks Markce!

author: markce
author_rank: Regular Contributor
timestamp: May 01, 2018, 09:14:13 pm
content: Thank you Old Printer.I bought my 2236 some 5 years back, not really cheap but in very nice condition (outside). It only had the well known focus problem. In the process of fixing that I did a recap of the power supply. Not really happy with the result, I modified the Z amplifier. After an accidental short between +5 and +30 on the main PCB had to repair the trigger logic and time base (4-5 ECL and TTL IC's). I finished with doing a lot of the 2236A mod's, counter repair and a fan upgrade.It is a nice scope to service. Excellent documentation, not many special components and everything easily accessible.

author: MFinney
author_rank: Contributor
timestamp: May 02, 2018, 08:42:29 am
content: Many thanks Markce for the help and advice. I didn’t have a lot of time this morning to do a longer test but did manage to take some readings. I can perform more this evening when I return from work. I’ll try to describe my findings below:I put my DVM on the collector Q206/207 and the voltage moved around and it was quite sensitive to setting it at 0V using the vertical control. I then pressed in beam find momentarily and they settled down and I got it to 0V.I then checked the vertical plate voltages on each plate which were +21.77 volts and +2.363 volts. I then pressed beam find the horizontal trace appeared on screen and the lower reading changed from +2.363 volts upto circa 16 volts.I didn’t have time to take readings over half hour but hope it gives you some information.ThanksMark

author: markce
author_rank: Regular Contributor
timestamp: May 02, 2018, 11:14:04 am
content: So the output stages are unbalanced right away.Just some quick checks. What is the voltage across C292 ? C292 could have failed.And at the collector of Q231 ? Checks if Q230/231 are Ok.

author: MFinney
author_rank: Contributor
timestamp: May 02, 2018, 04:27:50 pm
content: Hi voltage across C292 is +8.07 volts. At the collector of Q231 I have -1.238 volts.

author: markce
author_rank: Regular Contributor
timestamp: May 02, 2018, 07:40:05 pm
content: Just to be sure, base of Q230 is 0V, base of Q231 is 0V, base of Q284 is 0V?Also base of Q254 is -0.5V and base of Q255 is -0.5V and emitter of Q255 -5.6V ?

author: MFinney
author_rank: Contributor
timestamp: May 02, 2018, 09:08:59 pm
content: Hi readings as followsBase q230 +35.8 mvBase q231 -30.4 mvBase q284 + 3.0 mvBase q254 -5.73 VBase q255 -3.657 VEmitter Q255 -6.47VI re checked collector q231 and was -0.825vHope this helps,

author: markce
author_rank: Regular Contributor
timestamp: May 03, 2018, 10:52:56 am
content: Q255 must be defect or bad solder joint (base).Before you replace/repair it, check the base voltage of Q284, should be close to 0V.Also check b-e voltage of Q257, should be close to 0.7V.

author: MFinney
author_rank: Contributor
timestamp: May 03, 2018, 11:12:07 am
content: Markce,thanks for your assistance in getting me this far, Im glad the readings make sense and point to where the fault is. I will re check all readings again tonight to confirm. Are spares/replacements readily available? I note Q255 is a SRF3188 NPN transistor.

author: markce
author_rank: Regular Contributor
timestamp: May 03, 2018, 11:35:32 am
content: Some of the semiconductors are obsolete after 30 years! The SRF3188 will be hard to get, but the BFR96 which it replaced is available as NOS, at littlediode.com for example.

author: MFinney
author_rank: Contributor
timestamp: May 03, 2018, 01:03:26 pm
content: Hi it looks like BFR 96 transistors are still available so good news. For my own interest and education can you explain from the readings I have gathered what indicates to you that Q255 is not working correctly?Thanks Mark

author: markce
author_rank: Regular Contributor
timestamp: May 03, 2018, 02:23:32 pm
content: The base voltage of Q230/231 is nearly identical and close to 0V. The plate output is alive (not 0 or 30V), but unbalanced. o the problem must be in the driving of the output stage.Base-emitter voltage of Q255 is nearly 3V (-3.657 - (-6.47)), should act as a diode and be close to 0.7V. So if your measurements are correct, I conclude the transistor is defect (or not correctly connected).

author: MFinney
author_rank: Contributor
timestamp: May 03, 2018, 05:04:08 pm
content: Hi I’ve checked all readings and in summary:Transistor. Base volts. Collector. Emitter voltsQ230. +35.8mv. -5.627V. + 1.129VQ231. -30.4mv. -0.825V. +0.766 VQ254. -5.73 V. -2.08 V. -6.47 VQ255. -3.657 V. +9.29V. -6.47 VQ256. -1.406 V. +2.91V. -2.114VQ257. +9.82V. +21.83V +9.31VQ284. +1.8 mVI’ve located the BFR96 transistors and ordered 4 so hopefully I can get this lovely scope working again.

author: markce
author_rank: Regular Contributor
timestamp: May 03, 2018, 05:11:18 pm
content: Please check Q230, as you measured it, it is also suspect.Edit:Not so in this case (?), looked up the datasheet of the AST4261.

author: lordvader88
author_rank: Frequent Contributor
timestamp: May 03, 2018, 05:54:08 pm
content: Quote from: bd139 on April 26, 2018, 10:45:14 amWhat about recapping a Tek2215, I barely looked around inside when I "improvised/fixed" it's power button, I must have been busy.

author: markce
author_rank: Regular Contributor
timestamp: May 05, 2018, 12:57:17 pm
content: On a 30 year old scope, some preventive recapping of the smps is advisable. I should start with the elco's at the primary side of the power supply. Tektronix uses quality capacitors, so be careful to select a comparable series.

author: MFinney
author_rank: Contributor
timestamp: May 08, 2018, 05:35:06 pm
content: Hi Markce, just a quick follow up email. I replaced Q255 with the Replacement bfr96 transistor and morning he scope is now drawing a straight line without beam find pressed in!! I still need to do some testing but at least it’s working!! Thanks for your assistance. Mark

author: markce
author_rank: Regular Contributor
timestamp: May 09, 2018, 05:51:57 pm
content: Hi, good to read that your repair of the Tek 2236 vertical amp was successful.