I need help with fixing a Tektronix 2245A oscilloscope (long)

Hello all,

I bought this oscilloscope last weekend, it's the first one I've owned. I have a basic understanding of electronics, my interest mostly relates to keeping older stereo gear running, my own and the stuff on permanant loan to family. So I usually am only opening things up to change lamps and clean the pots. Anyway, I wanted to try to gain a deeper understanding of how circuits work so I decided to keep my eye out for a scope and now here I am.

The scope seemed to work fine with one exception: The intensity control for 'A' (it has seperate controls for 'A' and 'B') seemed wonky. When A was turned all the way down, there was still a trace visible, which didn't seem right. As you turned it up it would stay at the same intensity and then at about 40% it would suddenly get much brighter.

I did a web search and found a couple of people who had issues with the intensity control on the 22xx series of scopes (though none of them had the same symptoms as mine), the problems ranged from a defective/dirty pot, to a broken wire, to an open resistor, to a problem IC (LM324). I also learned that this scope actually does a POST when you turn it on and it will throw up an error message if anything is majorly wrong. I also read the advice to keep a fan blowing on it while the cover is off, which I have been doing.

I found a copy of the schematic (I have linked to excerpts here, the whole service manual is 121 megs) and decided to do a cursory examination; this also would give me a chance to see if the inside was filthy which I was planning on checking at some point (actually, it was very clean).

Well my problem wasn't a wire since the pot was soldered directly to the CRT control board. I checked voltages at the 'A' control pot, fine. R505 was nearby and was fine. It took a while to find LM324, it was a mile away on the main board, half of it was poking out from under a plastic cover that covered the high voltage section:

So I checked all the resistors in the above schematic associated with 'A' and they were all okay. However I did notice a resistor that was burnt. Checking the board layout, it was R1027. It was at this point I noticed the board layout in the service manual was different in places than my actual unit.

This resistor is on this schematic:

It's listed as a 0.2 W here:

On paper it is supposed to be 10 ohms, it measured at 27. I check R1026 to see if it matches the schematic and it is 10 ohms and it looks fine (it's right next to the two 10uf caps in the picture). To see what effect replacing this resistor would have, I put a couple of resistors in parallel until I had 10 ohms total. The only effect adding and taking away these resistors had was moving the trace slightly up and down on the screen.

Now here's the thing - at some point during all this the intensity problem seemed to fix itself. The trace would now go completely off with the dial turned down all the way and it would get progressively brighter as the control was turned. Okay, I was happy to leave it alone except that I figured I should replace the resistor. I assumed it got too hot being under that shroud. I took out the old resistor (which flaked apart) and cleaned the area - the trace at one end was blackened. I don't think I'm the one that caused this but I'm not sure. The old resistor was too burnt to tell if it was original, it was the same size as the others though.

Okay, now here's the actual problem - I replaced the resistor with a

I turn the unit on and it doesn't complain. The controls are still working fine. I check the new resistor and it's very hot. Measure the voltage across it: 4.6v. If my math is correct, that's 2 watts. Turn it back off and let it cool. Back on to check voltages: Incoming voltage to new resistor is +7.4v to ground. LM324 pin 4 to ground is 2.7v. Voltage pin 4 to pin 11 (across IC) is 10 volts. Voltage pin 11 to ground is -7.43 volts. Voltage after R1026 is -7.46 to ground. I desolder one end of R1026 and verify it is 10 ohms. C1006 (the yellow guy near pin 7 of the IC) measures 47 ohms with a multimeter. Shouldn't it read open? For reference, C1005 measures about 100 ohms.

I'd just like to have an idea of what the problem might be before I change any more components. Now I'm half wondering if it's the IC and the fact that it's facing up and has a fan on it is allowing it to work properly. Anyway, thanks for reading my long story and I'd appreciate any suggestions you have.

This is where I downloaded the service manual:

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Hi Chris, That is a capacitor and should read O/C on an ohmeter. Lift one end and re-check. You might find it's OK. In circuit measurements are not always reliable !

Chris B. prodded the keyboard

Hi,

Thanks for the reply. I was thinking about it last night and I figured it has to be C1006 leaking or shorted to ground. I just took one end off and checked it - 120 ohms DCR. I also notice it's slightly charred in one spot. So I will replace it (and I will probably replace C1005 at the same time. My next question is what to buy to replace it. It's listed as:

CAP, FXD, CER DI : 0.022 uf 20% 50V

Does that mean ceramic disc? I hope I can find one in that style locally.

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Chris B. prodded the keyboard

Its dead ! Bin it.

Its not a critical component. 0.022 uf is a common value and 50Vw is typical. Try RS components or Element 14, Mouser there's lots to go at.

Hello,

After reading your post I ran it with the cap still floating at one end: 0.1 v across the resistor, +7.5 volts at pin 7 of the IC. Interesting that it didn't mind being fed with less than +3v.

So presumably the cap failed and caused the resistor to char. Here's the thing - the main board power schematic shows 25 or 30 of these things. The processor board has 20 or so more. Are these things failure prone or is the fact that these components were covered and maybe got a little hotter than others likely to be a factor? I'm not going to replace anything other than these two for now, but I'm thinking of ordering a bunch of these:

which is the closest thing I could find.

I have another question, what is the purpose of the 10 ohm resistors? Is it to limit current if the IC shorts out?

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I do some work for my friends now and then and I just got a CB radio in at home base for the second time..

The first time the signal meter was pegging when turned on, that was caused by a 47uf 10v cap.

The second time, which is just last night, the squelch circuit failed to operate and open. That was caused by a shorted capacitor in the local by pass network, also a 47uf 10v cap.

Both these caps failed shorted and were the same values. Something tells me I should replace all of these caps in the radio before returning it.

P.S.

These caps failed shorted down below 10 ohms.

Jamie

Yes. It's fairly unusual for that type to fail, but not unknown.

no

no

That's fine. Don't worry about the others

Chris B. prodded the keyboard

That's about the size of it !

I wouldn't ! The link shows 22 pf as the value.

Remember for your troubleshooting pleasure, you have two identical channels, makes it easy to compare known good to the defective one. Mikek

Here:

Yeesh, how embarassing. Can you tell I'm not used to working in units smaller than micro? Thanks everyone for your help, I'm going to put an order in and buy some other stuff from them that I can't get locally (like decent solder wick). I'll post an update when I get it done.

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The problem is you bought the scope without the manual that should accompany it as it travels through life. I lucked out and got my $225

Discharged caps show increasing resistance as they charge, then show open. If they read 0.0 Ohms they are shorted.

I embarrass myself way worse than that, don't sweat it!

dave prodded the keyboard

Yes thats true ! I still have my Tek scope manual somewhere kicking around... Unfortunately the scope got pinched.

dave prodded the keyboard

I agree an electrolytic would, but 0.022uf !!!

Wasn't there a place to hold the manual in many of the scopes? I can't remember, but when I got my 545 (with some plug-ins) I seem to recall the manual was in some slot or place in the case. Certainly the hospital discards came with manuals. I was lucky, someone I knew grabbed it all before it was tossed out

Michael

A couple other sources:

This is the Tekscope group on Yahoo where people who collect and repair Tek scopes can be found:

snipped-for-privacy@yahoogroups.com

There was a removable vinyl pouch that screws to the top of that family of scopes. Most get removed so you can stack other equipment on top.