Tektronix 2213A Repair

[aaronsnoswell]

Hi all.

My Dad gave me his faulty Tektronix 2213A to repair. I thought I'd start a thread to document my progress / ask for help. The scope broke a year or two back when my dad was playing with some un-shielded radio circuits. The (CRO) screen started going fuzzy, not producing a clear picture, then at some point after that, the fuse blew (possibly from an un-related cause - maybe a lightning strike or something).

So far I've removed the case (which in itself was a mission - the screws were completely stuck), and purchased a new fuse. The ratings near the fuse holder say 250V 1A slow. I popped the new fuse in and today tried to plug the power in, but as soon as the cord was plugged in, I heard sparking and saw sparks from within the shielded power circuit area, then the loudest bang I've ever heard (and magic smoke came out). When my ears stopped ringing, I removed the cord and confirmed the new fuse is dead, but the bang was so loud I think a capacitor must have also died.

Given how it failed, I can only assume something in the power circuitry is defective. My next plan is to remove the power circuitry shielding and go from there. Any comments or suggestions welcome!

[tautech]

Presumably you have manual/schematics. If not go to Tek website and get them first!
Study & understand them or get help here. Post link or photo or Schematic of affected area and we will see if members can help. Good luck with your Tek.

[oldway]

First, do'nt plug the power in no more and do'nt worry about fuse and the preregulator.
It is far more secure to feed (with correct polarities) C937 (1800µf) with a 1A current limited 40V dc power supply.
So you can check the inverter, the HV and all the scope without risks.
If everything is allright, then you can try to repair the preregulator and the current limiting circuit.

[aaronsnoswell]

Hello oldway and tautech. Thank you both for your replies and my apologies for taking so long to get back. I'm looking into this scope again on tautech's recommendation.

So in early 2014 after posting here I replaced the fuse then had a look in the mains section of the board and discovered a varistor right on the AC input that had exploded. I was able to find a modern equivalent replacement part and the scope now powers on fine.

I'm now back to the 'fuzzy' display issue that I mentioned in my opening post. See the below pictures. These pictures show the 500mV pk-pk 1Khz test signal with AC, then DC coupling. Channel 1 settings there are 0.2V/div and .1ms/div. The display is exactly the same when I use channel 2.



These images were taken with the focus knob turned all the way up (or down - I don't know which way is which). When I turn the knob the other way the fuzziness just gets worse and worse. This makes me think the focus circuitry is damaged. I'm going to try and find/understand that part of the schematics and read some of the Tektronix service guides, then post back here.

Thanks again both for your suggestions.

[oldway]

Here we are again...

On the mainboard, you have a focus trimpot (R875).
Try to ajust it to have the right focus.
Try also to ajust the astigmatism trimpot (R887)

[jlmoon]

I'm going to lean towards a problem with that HV supply / flyback (actually focus tap which is part of that HV supply)  I do believe there is a focus diode embedded within the flyback on some of those devices.  Don't quote me on that.

[aaronsnoswell]

Hi again guys.

oldway - On the 2213A R875 is a regular resistor, not a trimpot. There is an astigmatism trimpot, but I haven't tried adjusting it yet. I've attached the relevant page from the manual that shows all the adjustment trimpots in this model.

---

I've been looking at the schematic for power supply circuit; see below picture.



I initially suspected that C893 might have failed and caused a ground/low resistance ground connection at the CRT focus wire, but I de-soldered C893 and it was 990nF which is in spec (supposed to be 1000nF).

Next I decided to check the focus potentiometer (R893). This resistor is specified to be a 5MOhm 10% resistor, but I de-soldered it today and tested it, and it was 6.63MOhm, which is out of spec. The 'replaceable electrical parts' section lists it as RES.,VAR,NONWIR:PNL,5M OHM,10%,0.50W. Does anyone know what NONWIR:PNL means? Could this resistor being 1.03MOhm too high be the cause for my focus issue? It's a funky looking potentiometer (it connects the whole way forward to the front panel) so I'm wondering if I could just solder another resistor in parallel to adjust it to the correct resistance?

[tautech]

Look at the current paths associated around all the HV stuff.
Several voltage dividers that the values also IMO should be checked.

I has seen several problem in CRT HV PSU's, crook or open high value resistors are common, DC restorer diodes, HV caps.
Did you get the Tek troubleshooting pdf?
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

The Focus pot can be left for now IMO, it's value might only change the range of adjustment.
A parallel resistor should do no harm, but I dont think it wil do a fix for you.

[siggi]

The focus pots range is set by the string of 510K resistors before, after and shunting it, plus the 180K resistor to -8.6V. The Q855 transistor presumably couples in an intensity related focus adjustment signal. It's range is limited by the 180K, as the transistor can at most pull the end of the 510K chain to -8.6V. I think the pot would have sufficient range even if Q855 is bad, but the transistor is also suspect....
Check R886/8/9 890/1/2/4 for reasonable values, should be doable in circuit.
Note that C893 might be leaky at high voltage only. It seems to be for decoupling, so I don't see the harm in running (briefly) with it out of circuit for testing.

Note also that as the focus pot is shunted by 510K, it's precise value is relatively unimportant. It's important that the wiper make contact, however .


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[siggi]

BTW: I'm assuming you've verified the low voltage supplies for value and ripple? You can get a decent idea of the ripple with a DMM in AC mode if you don't have another scope to look "properly".

This is worth doing as a first step. It's quick and easy to do, and failing electrolytic caps in the PSU are a common failure. Excessive ripple on the power rails can manifest in fuzzy traces (though I don't think that's happening here). Also, it's something you'll want to fix while you have'er open .

[oldway]

I had only the 2213 service manual and not the 2213A 
You post schematics of the 2213A, that's easier now.

One important question: what side of the wipper of the focus give the best focus ? side of R891 or side of R893?

[aaronsnoswell]

tautech - Yep, I got the PDF and had a read thanks.

oldway - I'll solder it back in and let you know, it's still sitting on my bench sorry

siggi - First up, how would I verify the low voltage supplies? I've never poked around inside a running CRO before and don't know what to do to protect myself adequately from the nasty parts. Do I need a special multimeter cable / scope probe? Should I wear gloves? Thongs / Flip flops? Any advice you can give would be greatly appreciated! I've eyeballed all the big caps and none of them seem to be bulging visibly at least.

As for the resistors - thanks for explaining that circuit for me, it helped a lot. I think you are right about Q855 coupling in intensity - I read about that earlier in the service manual. I checked the resistors you mentioned in-circuit and got the following;

Component	Resistance	Wattage	Measured	Valid?
R886	180k	0.25	179k	Y
R888	510k	0.5	17-20M	N
R889	510k	0.5	745k	N
R890	510k	0.5	610k	N
R891	510k	0.5	565k	N
R892	510k	0.5	514k	Y
R894	510k	0.5	600k	N

Note all these resistors are supposed to be 5%.

I de-soldered R888 and even then it would only read 1M at the lowest, when I scratched some oxidation of the legs to get a good connection with the probe. I'll start by trying to replace this resistor, then replace the others if needed too.

These resistors (except R886) are all chunky 0.5W resistors. I only have 0.25W at the moment - is there any way I can use some of these (in series/parallel) instead?

Also, when de-soldering I tried using hot-air to free up the resistor and managed to make the PCB bulge I think the traces are still intact though. I need more practice at de-soldering...

[tautech]

Replace all 510K 1/2 W resistors.
As I've mentioned, these HV divider chains are known to fail.
Yours is typical.

Get youself a plunger style solder sucker, and use the hot air for SMD

Once the resistors are replaced check all the LV PSU rails for correct spec & ripple.
The ripple can be checked with a DMM set to a low range on AC.
Dave did a video recently on a spec check on a Tek. 
EDIT
It's not a Tek but a Hameg. 
However well worth the watch:
http://www.eevblog.com/2013/08/03/eevblog-502-19-hameg-analog-oscilloscope/

The HV stuff should have a sheild on it and after all you need only be concerned with LV ATM.
Manual should specify test points, voltages and ripple.

All easy stuff, just take your time.

[siggi]

siggi - First up, how would I verify the low voltage supplies? I've never poked around inside a running CRO before and don't know what to do to protect myself adequately from the nasty parts. Do I need a special multimeter cable / scope probe? Should I wear gloves?

Start by fixing the focus problem - it looks like you've found your culprit(s). Once you've refitted the HV shield, there shouldn't be anything too nasty exposed. Anything past 50V DC may tickle or sting, but is unlikely to hurt or maim.
If you're worried, work with one hand behind your back, and/or attach the probe to test points with the power off, then apply power, do the reading and turn power down.

These resistors (except R886) are all chunky 0.5W resistors. I only have 0.25W at the moment - is there any way I can use some of these (in series/parallel) instead?

For testing you can parallel two 1M 1/4W, or you can use two 270K 1/4W in series, it'll be near enough.
Mind that each of those 510K resistors is dropping ~1/6th of your cathode voltage, so the voltage rating of the resistors is probably starting to matter.

Replace all 510K 1/2 W resistors.

I'd replace all the 510K resistors as tautech suggests. You can still test with 1/4W, but I wouldn't leave it at that as repaired in your shoes.

[aaronsnoswell]

Hi all!

Great news - I replaced all 510k resistors as suggested and the focus issues are now gone! The image is now focussed when the knob is roughly in the centre of it's range.

Now that the focus issue is fixed I can see there is a slight ripple where the display should be flat (tops and bottoms of the test square-wave). I'm guessing this is some low voltage supply issue so I'm going to have a look around those test points. The manual says not to probe within the device unless it is plugged in using an isolation transformer - should I get one of these first? They look fairly pricey

Thanks for all the help this far!

[tautech]

Sweet. 

In what V/div ranges does it show up?

AC ripple on the LV supplies can be checked with a DMM on AC mV.

Have you a capacitance tester to check the PSU cap values?

[siggi]

Hi all!

Great news - I replaced all 510k resistors as suggested and the focus issues are now gone! The image is now focussed when the knob is roughly in the centre of it's range.

Awesome, congrats.

Now that the focus issue is fixed I can see there is a slight ripple where the display should be flat (tops and bottoms of the test square-wave). I'm guessing this is some low voltage supply issue so I'm going to have a look around those test points. The manual says not to probe within the device unless it is plugged in using an isolation transformer - should I get one of these first? They look fairly pricey

You can safely probe the scope's low voltage supplies without an isolation transformer. It's mostly the primary side in switch-mode supplies that you'll want an isolation transformer for.

You can even use the scope to probe it's own low voltage supplies. If the primary side reservoir cap is marginal, you'll see line frequency ripple. If the secondary side reservoir/filter caps are gone, you'll see switching frequency spikes and ripple.

Be careful on the primary side...


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[AMR Labs]

Hey guys, sorry to revive this old thread, but I've got a 2213A on the way and I am looking for the correct service manual. So far I got the 2213 (plain) service, and the 2213A operator. But I know for a fact there are substantial differences in the PS of the A version, so the SM I've got is not a very good match to it. Anyone?

Thanks in advance.