A Dead Tektronix 2235A

[wjcoll]

Hello everyone.

I've only just joined, so this is my first post and typically I'm looking for help.

I'm an absolute newbie when it comes to the use of oscilloscopes. I'm a life-long self-learner and thought it was time to educate myself. Have some basic knowledge, but not a lot.

Anyway, I did what many before me have done and bought a used scope off eBay for not too much. Did some homework and waited until I scored a Tektronix 2235A. The seller said that apart from powering up, he hadn't done any tests. Caveat Emptor!

Scope arrived the other day and it did power up and looked like it might be a winner. Then, about 15 minutes later it blew the main fuse, a 1.25A 240V. I'm in Australia (240v here). I let things cool and tried another fuse; no good, blew again. Only had a 1.5A left, so tried that.....bang.

Doing some homework I've found that they're susceptible to power supply problems particularly if they've been sitting around unused for a long time, like I suspect this one has.

I've sourced the manuals (including service and schematics) but need some advice on where to begin and if there's a common easy fix.

Any help would be greatly appreciated. As I said, I'm a newb' so simple explanations in words no more than two syllables would help.

Cheers,
Wayne (wjcoll)

 

[cowboybob]

Welcome, wjcoll!

Your scope may be 24 years old (nice scope, by the way - a "ruggedized" MIL spec rig).

Typically, with older gear, the biggest power supply (and other) problems are capacitor related, especially the electrolytics, leading directly to the symptoms you've related.

And those problems can often lead to futher down-stream problems, cascading into some really messy situations.

How comfortable are you tearing into the guts of the scope? How about reading schematics?

 

[rumpfy]

G'day WJ,
You're another lucky soul who has a Tek.
Theres been a few posts on Tek CRO's recently, and you might want to look them up.
The cowboy hits a couple of important points, but you'll need a decent volt/ohm-meter. Presumably you have this stuff?
Just to help us to help you, just give an indication of where you are. My guess is that if you're smart enough to buy a TEK, then it goes without saying you got enough smarts to do what is needed.

 

[KeepItSimpleStupid]

#1: Did you check the voltage select switch (if there is one) before powering up?

Do do an inspection of the electrolytic caps for any spilled electrolyte. This may be the series with bad caps.

Join the yahoo group Tekscopes. groups.yahoo.com. There should be info in the archives.

 

[wjcoll]

Hi Guys,

Thanks for the welcome. What a response. I did some digging around and found two suspect items. The first was a .068uF X2 cap which had totally disintegrated and a blown thermal resistor. I've got the parts on order from Mouser and hopefully that will solve my problems. I'll keep you posted on whether this is the fix.

From purely a visual, all the other caps in the power supply region look OK. No leaking or swelling that I can tell. Will be glad to get her working again.

Once again, thanks very much for the responses !!

Cheers,
Wayne from OZ

 

[wjcoll]

Sorry Cowboybob and rumpfy; didn't answer your specific questions.

I'm a newbie in terms of using a scope but have been tinkering and building kits for quite some time. At the age where 60 is looming and 50 fast disappearing, I've managed not to fry myself yet (a wet finger is no substitute for a multimeter when it comes to mains voltages ). I'm self-taught in a number of fields from machining to metal casting and too many other hobbies to mention. My rule is; take things slowly, THINK, make sure you understand, and if you don't, ask someone who does.

We'll see how I go. Thanks for the concern.

Cheers again,
Wayne

 

[rc3po]

Hi wjcoll,
I'm new here also. I hope that fixes your scope. I love my Tek 2465A DV scope that I got from e-bay. It's been going great so far.
Did you do as KeepItSimpleStupid said and make sure the voltage switch on the back is in the right position?

 

[wjcoll]

Hi rc3po,

Still waiting on parts at the moment so it's still fingers crossed. There's no voltage selection switch on the 2235A, so it either regulates to either mains voltage or it's a 240v design. It was bought from within Australia and had been used here so I don't think the line-in voltage was the issue.

I'll keep the thread posted on any success!

Cheers,
Wayne

 

[rumpfy]

had a look at the power supply and the extract is attached.
You said a 'thermal resistor' and I took this to be the item RT901. There is a VDR item VR901 which maybe you referred to. My guess is that it is unlikely that VR901 is faulty
I couldnt find the 2 x 0.068, which sounded to me like a noise filter arrangement in the mains power supply leads. The drawing shows C902/903 which are often a 2 x capacitor with 3 leads.
Assuming that the blown thermal resistor is the RT901, then it may have failed because of a fault. I would suggest you go through the power supply and check out the bridge rectifier for shorted diodes before reapplying power after you get your parts. By using a series resistor of say 20 ohm in series with the power supply leads when you power up after the repairs, you will limit any fault current to a manageable value. This is just to get it started to check for an insanely wrong condition. If things look OK, then remove the resistor.

 

[wjcoll]

Thanks rumpfy.

Yes you're right, it was RT901 on the filter board. The 0.068 cap that died was C904 on the main board. a couple of the other paper film caps in the region look a little bit suspect with their outer casings showing heat/age cracking, but no swelling. I'm going to go through the whole region and replace the caps just to be sure.

Thanks for the current limit tip on power-up, I'll be sure to follow your advice.

Many thanks again,
Wayne

 

[wjcoll]

Hi again all,

Well it took a while but all the bits and pieces I was chasing finally arrived. I replaced the dead 0.068uF cap (C904) and the thermistor (RT901) , then tentatively push the power switch. Another blown fuse !!!!!!!!!! Exactly the same condition exists. I'm suspecting whatever took out the first two components didn't stop there. These two were easy to spot with some help from advice here as they were physically damaged. So where to from here? Any suggestions would be greatly appreciated. Feeling a bit deflated at the moment. Murphy's law dictates it could never be as simple as I first thought.

Thanks again,
Wayne

 

[rumpfy]

Go back to post 9. From memory there is an input filter which is 'disconnectable' from the down stream components. try disconnecting it and try applying power to try to isolate the fault.
Frankly, to blow up these components suggests there is something else clearly at fault and I would be suspicious of the power supply diodes or filter capacitors. My view is the problem is VERY close to the mains input point. faults further into the electronics would be unlikely to destroy the RT901.
In the short term, I would use a series resistor in place of RT901.

 

[wjcoll]

Hi rumpfy,
My message might have been misleading. The same condition exists in as much as the fuse keeps blowing. The two components I replaced (C904 & RT901) appear fine. The fuse goes once the unit is switched on (and instantly) but not when main power is applied, so the fault is after the switch. Mind you that doesn't eliminate much.

Thanks for the tip. I'll revisit the earlier post and go from there.

Cheers,
Wayne

 

[rumpfy]

1. check the forward and reverse resistance of the diodes in the bridge rectifier cr 901, 2, 3, 4.
2. check the resistance to ground across c906.
Note that the ohm meter sends a small current when testing for resistance. This current has a polarity which is that the black test lead is positive with respect to the red lead on 'OHMS' for a conventional analog volt/ohm-meter. With a digital volt/ohmmeter, the polarity is sometimes positive on the red lead.
When testing for resistance you MUST have the 'positive' lead of the ohmmeter connected to the positive side of the power supply. I usually check resistance in these circuits with the test leads both ways, and take the highest reading. With the test leads connected to the power supply in the reverse poarity condition, you often see reverse connected diodes etc. So take the highest reading.
If this looks OK, then this would suggest that the problem is on the secondary side of T948. So go to all the secondaries and check for resistance to ground for each supply.
Testing the output around the high voltage supply might be a challenge. I would disconnect the transformer lead into U975. One of the other guys here had a problem with his voltage multiplier on a TEK 2245?
Take your time and sort it through and get back with the results before you power up again.
It was a quick reply last time cos I happened to be here when your post came through.

 

[wjcoll]

Thanks rumpfy,

That's given me a lot to sort through. The help is really appreciated.

I'll keep you posted on progress.

Cheers,
Wayne

 

[tester272001]

Wayne, How'd you make out on this one? You have the schematic? Wondering if you found your fault This guy has the PDF of the service manuals: http://www.ko4bb.com/manuals/index.php?dir=Tektronix

 

[wjcoll]

Hi tester,
I've got the operation and service manuals off the net and have been taking it slowly to let it soak in. I've decided to replace all the 'lytic caps in the power supply region as well as the MOSFET which has been known to cause some problems. I'm also replacing the rectifier diodes. After doing a lot of reading of other people's experience with similar problems I decided to replace all the caps with either Rubycon, Nichicon or Panasonic and all rated at 105 degrees due to the known heating problems. I'm also fitting a cooling fan utilizing the jumper that's already on the board for one. Just needed a 270uF cap to fill an empty space on the board.

Sourcing the right spec and brand parts has been a slow process. I've bought a lot off eBay but have tried to avoid suppliers from dodgy regions. Things are taking up to 6 weeks to arrive at the moment and the MOSFET is coming from Israel.

On top of all that my son has decided to go and get himself married this Saturday. So plans and large volumes of money have gone out the window.

I wouldn't be surprised if after ordering all these parts it will turn out to be a simple power diode, but apart from the fact that it doesn't work it's in really nice cosmetic condition and looks like its been looked after. In the end I'm hoping I've got a really good scope that will have been worth the wait.

Just on that point. I try to avoid the temptation to rush in and take a punt on stuff I'm not sure of. Slow and steady should see me win this one (I hope).

Cheers for asking,
Wayne

 

[rumpfy]

First things first; If your youngster wants to go off and marry then good luck to you. Now he can feed and cloth himself so you should get back on your feet in no time.
With the ratings of capacitors, the data sheet should give you all you need. The temperature rating is quite important because a higher temperature capacitor will have a larger can size compared to a comparable lower temperature one. Try however to get a diagnosis of your problem. Remember too that the temperature requirement for a capacitor is a function of the AC current through it. I dont have a problem with your approach, except that this 'replacing everything' should not be your fault diagnosis procedure.
I cant say your apparent difficulty in finding parts is something I can relate to. I use both of the british suppliers with complete success. Price is secondary to me because the cost amortised over a long time is meaningless to me. The important thing is to be able to get what I want.
Good luck with the wedding.

 

[wjcoll]

Hi rumpfy,

Yes, speaking to other 'empty-nesters' it seems getting your kids married off is like discovering you've had a two inch hole in your fuel tank for 25 years and only just getting it fixed.

On the CRO front, I don't suppose I'm really trying to just replace everything in the hope I'll stumble upon the fault. It's more of a case of, having read a lot of various forum posts about the susceptibility of the 22xx series to heat problems in the power supply region I'm taking more of a restoration approach rather than simply repairing.

I'm interested in your comments about suppliers. I found any number of suppliers of the parts I was looking for but most were in the order of $45 for delivery, even on half-a-dozen components. I couldn't find a cheaper postage option on their check-out page. I'm talking about Farnell, Mouser, Digikey. Towards the end I did source a few parts from RS Components who I had dismissed because of the same experience some time ago but have now found they're offering free delivery for online purchases.

May I ask who you've been sourcing through?

I'm going to feel very torn if all my parts arrive in Friday's post. Saturday arvo would've been perfect for a tinker, but now I've got a prior engagement

Thanks again. Will keep the posts coming once I've got something useful to add.

Cheers,
Wayne

 

[KeepItSimpleStupid]

I really would try to fix first and restore second. A reminder that leaky capacitors can destroy traces and vias.

With a power supply issue, you may be better off troubleshooting it out of the scope. Loads are generally required.