Author Topic: Tektronix 2221a repair project  (Read 770 times)

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Offline Fractal

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Tektronix 2221a repair project
« on: May 07, 2021, 10:41:57 am »
Hi!

I just acquired my first analog scope to complement my Rigol 1052

It's a Tek 2221a. Cosmetically apart from the dust it looks in good shape; no broken knobs or yellowing. Allegedly came from a school but picked up at a general electronics shop locally here in Taiwan

Focusing appears fully functional too, as my research showed this was a recurring issue with these puppies


Then when I came to test it I noticed a couple issues;

1- The screen membrane buttons don't work; a minor issue I thought, probably just a little oxidation

But 2- After a few minutes of jerking around the knobs, the OSD disappears! But traces remains visible somehow. Only the digital features seem to become inoperable.


...So obviously I told the shop it appears it's dead, but I'll take it for 30 bucks, which they agreed lol


Coming back home, the OSD was working again on powering up!


Now the reason for this topic is that I can't quite figure out the origin of the problems...

1-For the front button I cleaned the membranes and pcb, applied contact cleaner for good measure, but still no response on the interface

2-I'm confused as for the origin of the OSD failure; it seems to come up after heating up a bit, but I don't think it's a PSU issue as the traces remain useable. There is not all that much dust either that could choke cooling that much


Has anyone had similar issue with their 2000-series scopes?

Here are some pics:


The interface, after the OSD dies


Here with fully working OSD shortly after powering up


And here the top guts because pretty

Thanks!
« Last Edit: May 08, 2021, 01:48:14 am by Fractal »
 

Offline Fractal

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Re: Tektronix 2221a repair project
« Reply #1 on: May 07, 2021, 12:59:40 pm »
After some further investigation, I have now narrowed down the button fault to the flex pcb carbon pads; even after cleaning/sanding(on paper) and recharging with a pencil, no beans

There is some continuity between one 2 points of the same pad, but none from the pad to the pin.

I might need to recharge the pads with some carbon paint or something

 

Offline David Hess

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Re: Tektronix 2221a repair project
« Reply #2 on: May 07, 2021, 10:37:54 pm »
My experience is that the traces on the flexible laminated circuit board corrode to an open condition.

The storage trace and on screen display are controlled by the same intensity control which allows the on screen display to be toggled off and on by turning the control fully counterclockwise.

 

Offline Fractal

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Re: Tektronix 2221a repair project
« Reply #3 on: May 08, 2021, 01:42:43 am »
Thanks for your response David

Are those Tek flexibles commonly afflicted by this issue? What's the best course of action to bring them back to a functional state?


I have noticed that flicking the screen intensity dial down to none allows enabling/disabling the OSD, but this is something different; when it goes off, I can't turn it back on again with this control


Besides, I had a new issue while letting it run open, attempting to see if it could be a heat accumulation issue.... IT FREAKING BLEW!

Something from the PSU area blew off; white smoke, big bang, but the screen was still showing... Probably a bypass cap?


You can see a bit of it next to the main switch.

Looks like it came from whatever is hidden under the power socket
« Last Edit: May 08, 2021, 01:49:28 am by Fractal »
 

Offline bdunham7

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Re: Tektronix 2221a repair project
« Reply #4 on: May 08, 2021, 03:15:52 am »
The button problem will likely be open circuits in the flex ribbon.  I fixed mine with a CircuitWriter silver paint pen.

Your explosion was likely RIFA madness.  (Google it)

As for the OSD, what happens if you run it in 'STORE' mode?  Fix the RIFAs first, but then try the 'STORE' mode without twiddling any of the intensity knobs.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline David Hess

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Re: Tektronix 2221a repair project
« Reply #5 on: May 08, 2021, 03:27:04 am »
Are those Tek flexibles commonly afflicted by this issue? What's the best course of action to bring them back to a functional state?

They very commonly fail.  Some people have laid out new ones and had them produced.  It is of course the same part as on the 2232 and 2224.

Quote
I have noticed that flicking the screen intensity dial down to none allows enabling/disabling the OSD, but this is something different; when it goes off, I can't turn it back on again with this control

My guess then is a lose connection between the z-axis signals at the CRT and the vector graphics board.  I do not remember but there may be separate connections for the storage and on screen display intensity signals.  The schematics will show it and you can use the 2232 service manual which should be easier to find.  Reseating the connectors to the vector graphics board may fix it but access can be tricky because of how it is assembled.

Update: another thing which might cause it is a dirty or intermittent poteniometer.  If the intensity control momentarily disconnected, the CPU might interpret that as a turn fully counterclockwise causing it to disable the on screen display until toggled back on again.

Quote
Something from the PSU area blew off; white smoke, big bang, but the screen was still showing... Probably a bypass cap?

That was probably a RIFA X or Y class paper line capacitor.  Those should be replaced if you gain access to them but they are not needed for operation.
« Last Edit: May 08, 2021, 03:35:44 am by David Hess »
 
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Offline Fractal

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Re: Tektronix 2221a repair project
« Reply #6 on: May 08, 2021, 06:31:07 am »
Well that RIFA suggestion was bang on



There only appears to be one across the board. All the electrolytics I could see looked fine too.

it's a 28nF, I believe I found a modern KEMET equivalent on mouser


Next step then would be to order a replacement along with conductive paint, and meawhile remove the front panel so I may refurbish all the pots with contact cleaner; some are a bit glitchy indeed
 

Offline David Hess

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Re: Tektronix 2221a repair project
« Reply #7 on: May 09, 2021, 12:39:52 pm »
it's a 28nF, I believe I found a modern KEMET equivalent on mouser

That link does not work for me but the value is not critical as long as the proper type of capacitor is used.  When selecting replacements for parts like that, I usually start with the lead spacing.

Quote
Next step then would be to order a replacement along with conductive paint, and meawhile remove the front panel so I may refurbish all the pots with contact cleaner; some are a bit glitchy indeed

With burned traces, I use wire and solder to effect repairs.

The potentiometers may recover with deliberate working.  They can be disassembled to gain access to the resistive element and wiper but the work can be delicate.

 

Offline Fractal

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Re: Tektronix 2221a repair project
« Reply #8 on: May 09, 2021, 01:35:01 pm »
Well, I just found a silver conductive ink pen and a similar specced cap at my local electronics shop of all things!




Idk what's going on with that link; here's the direct link to datasheet; it's also a paper dielectric https://www.mouser.com/datasheet/2/212/1/KEM_F3011_PME271M_X2_275-1101254.pdf

However the one I found is something else, although it appears to show similar characteristics? It's a PP film from Carli http://www.sjohm.co.kr/products/img/pdf06.pdf

I guess I will just send it and find out


I wish it were easier to solder a wire on a flex cable with those contacts; as you may have seen on the pic above I applied the paint already, will see how that goes
 

Offline David Hess

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Re: Tektronix 2221a repair project
« Reply #9 on: May 10, 2021, 04:31:16 am »
I wish it were easier to solder a wire on a flex cable with those contacts; as you may have seen on the pic above I applied the paint already, will see how that goes

I tried using conductive "glue" to fix one of mine but it was just not practical.
 

Offline Fractal

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Re: Tektronix 2221a repair project
« Reply #10 on: May 13, 2021, 09:20:58 am »
Hello!

Update time:

  • I managed to fix the flex ribbon using the conductive silver pen! All keys working perfectly again
  • Successfully replaced the RIFA capacitor; seems good
  • Fully cleaned up the front face, knobs and contact lubed the pots and buttons as much as I could without removing the front panel (requires taking the whole damn thing apart; don't want do do it yet



Everything is working great now, apart from a couple remaining issues....



  • The edge-lit incandescent bulbs broke after too much manipulation...I measured the voltage on that bulb and it goes up to 30V. Picked up a pack of 24V 5mm bulbs from the shop, but the light is too damn weak... Any idea on what bulbs should get in there?
  • The power... Before reassembling the shell it did run perfectly for an hour; no more OSD disappearing, all good. Since I recased it though now the whole screen goes down after about 15mins of running...I'm guessing this could be a thermal shutdown thing? Any ideas? I did clean the PSU and fan, it's extracting the heat out just fine... the Sensor is also placed properly where it was, right in front of the fan.


Any pointers for that recurring issue?

Thanks!
 

Offline bdunham7

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Re: Tektronix 2221a repair project
« Reply #11 on: May 13, 2021, 02:09:45 pm »
I don't recall the bulb number, but the bulbs are wired in series, so take that into account.  If you have one remaining that didn't break, you can put a bench PSU  on it and see what it takes for voltage and current.

As for the other issue, take the case back off and see if it works.  If you can get it to run consistently without the case, then you likely do have a thermal issue, but possibly not in the power supply.
A 3.5 digit 4.5 digit 5 digit 5.5 digit 6.5 digit 7.5 digit DMM is good enough for most people.
 
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Offline David Hess

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Re: Tektronix 2221a repair project
« Reply #12 on: May 14, 2021, 01:56:49 am »
The two series connected graticule illumination bulbs are Tektronix part number 150-0077-01 which is 14 volts, 0.08 amps, wire leads, #2182D, T1-3/4.

https://www.mouser.com/ProductDetail/JKL-Components/2182?qs=gp8goBkfC5Ha6gzQzENy6Q%3D%3D
 
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Offline Fractal

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Re: Tektronix 2221a repair project
« Reply #13 on: May 17, 2021, 09:15:55 am »
Hi!

So I'm glad I managed to get the brownout issue on camera.

This time I turned it on, left it there to measure the ripple on some PSU, and after a while ~10mins the OSD disappeared, then a few mins later the whole screen went off.

I let it run anyway and surprisingly everything came back up!

...Except not very consistently...


In that video you can see the screen flicker then die then come back finally without OSD...

Note: the reading difference came from my probing points that moved.




Any pointers now? Thanks!
 

Offline David Hess

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Re: Tektronix 2221a repair project
« Reply #14 on: May 17, 2021, 04:33:30 pm »
I do not see the deflection changing so the problem is not with the high anode or cathode voltage.  It might be a problem with the z-axis CRT circuits some of which operate at high voltage.  It could also be a problem with a loose connection between the z-axis CRT circuits and the intensity control circuits.

Does the entire screen blank out when in analog mode and the on screen display is active?
 

Offline Fractal

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Re: Tektronix 2221a repair project
« Reply #15 on: May 17, 2021, 05:09:26 pm »
I see; well it's a fully intermittent issue regardless of the store/non-store mode

Sometimes only the osd disappears, sometimes both osd and trace

You can notice the trace coming back without OSD towards the end, in analog fist then in store mode (as confirmed by the "T" trigger marker
 

Offline David Hess

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Re: Tektronix 2221a repair project
« Reply #16 on: May 18, 2021, 02:24:37 am »
I see; well it's a fully intermittent issue regardless of the store/non-store mode

Sometimes only the osd disappears, sometimes both osd and trace

You can notice the trace coming back without OSD towards the end, in analog fist then in store mode (as confirmed by the "T" trigger marker

That makes me think the problem is with the intensity control potentiometers or the connections to them.
 


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