Tektronix 2236 Analogue Scope

[MFinney]

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.

[bd139]

Grab the service manual here first: http://w140.com/tekwiki/wiki/2236

First 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.

[MFinney]

Hi thanks I’ve checked with my meter and the values are
TP961 -8.58
W968 +5.03
W960 +8.61
W956 +29.87
W954 +101.0

The 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?

[lordvader88]

Grab the service manual here first: http://w140.com/tekwiki/wiki/2236

First 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.

What 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.

[markce]

Please check high voltage supply is 1900-2100V (point 2 under Power supply and CRT diplay).

[bd139]

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.

[markce]

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.

[MFinney]

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 !!

[markce]

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.

[MFinney]

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

[markce]

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.

[markce]

If I understand your initial posting correctly, you have two problems: one with intensity and one with increasing vertical offset.

[MFinney]

Yes although I think the vertical offset is the main issue.

[markce]

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).

[MFinney]

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

[MFinney]

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

[markce]

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.

[MFinney]

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?

[markce]

What is the difference of collector voltage when you start with a cold sope, compared to the error situ after 1/2 hour?

[MFinney]

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.

[markce]

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?

[Old Printer]

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!

[markce]

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.

[MFinney]

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.

Thanks

Mark

[markce]

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.