Tektronix 2440 issue

[adrianni]

Hello,

Just got a Tektronix 2440 from ebay. I live in Europe so first thing I selected the proper voltage.
It ran ok for about 5 minutes and then there was a hissing noise coming from inside the scope and then some smoke.
Next second I turned it off. So I opened it up because I wanted to look inside to see what burned. It is kinda crowded there, I managed to see a slightly burned wire coming from the fuse to inside the guts of the scope.
Anyway, I decided to turn it on again but this time running at 120volts from a transformer. It ran ok, everything is good, passes all tests, it is in CAL, no errors. Hmmm...
I selected 230 volts but this time I also powered it through the 120 volts step-down transformer. It didn't start, only the GPIB Led's were blinking so I'm guessing voltage selector works.

Now it runs at 120 volts. It has been running for two days, like 5 hours a day with me near it just in case.
What do you suppose has happened? Now even the burning smell disappeared because in the first day I could feel it coming out of the vent grill...

[tggzzz]

Almost certainly the infamous mains filter X/Y rated capacitors, and maybe some series resistors. There are many many descriptions of the problem on this forum; if nothing for the 2440, then there are similar caps in the 2445/2465 scopes, and other equipment.

The solution is to clean any carbon deposits from inside the scope, remove all the X/Y rated capacitors and replace them with new capacitors with the same X/Y rating.

[David Hess]

I agree; the failure was with the X and Y class input capacitors or maybe the capacitors in the line side EMI filter.  The external capacitors have low value resistors in series with them which were destroyed as well.

[adrianni]

Thank you guys, I'll look into it and let you know what's going on.

[adrianni]

Update on the Tektronix 2440 scope repair.

You were right on the money. Indeed, one of the X2 capacitors failed and also one 68ohms resistor was fried. I rushed to the electronic parts shop and bought the necessary parts. After I replaced the two twin capacitors and the resistor I tried the scope and everything works perfect at 230 volts.

One thermal resistor gets quite hot during operation and so does the 68 ohms resistor. I mean not exagerated but to the point when you cannot hold your finger on it for more that a couple of seconds.

In case I power it on 115 volts, only the thermal resistor gets hot. I understand from the service manual that there are different ways of operation based on the voltage selector switch.

I have to keep it running for a couple of hours or so to make sure the problem will not reappear.
I will keep you posted.

Pictures...

[David Hess]

Good, you changed both capacitors.  The resistors in series with these capacitors should be flame proof units and there is nothing wrong with increasing their power rating.

These capacitors are known to fail with age.  Water gets in causing the paper to expand and brake the case.

[adrianni]

I am replacing all X2 capacitors in all my scopes. Good thing they are all the same value, 0.068uf, I only have Tektronix scopes. This way I may avoid burning other series resistors.
The 2440 scope is built in such a way that you can have the low voltage power supply in hands in less than 10 minutes. Takes 30 minutes to get it up and running with new capacitors.

Thank you again for your advises.

[Bushougoma]

The infamous epoxy potted Rifa X caps strike again .

The epoxy bodies get fine hairline cracks in them which allows moisture inside this corrodes the plates once the corrosion bridges the dielectric layer they short out.

I remember reading somewhere the reason for the cracking was the epoxy was poorly selected and the rate of thermal expansion of the epoxy and the capacitor element itself (foil / dielectric assembly) wasn't close enough.

When the capacitor element heats up and expands the rock hard epoxy doesn't expand with it so it forms hairline cracks on the casing, which leads to moisture ingress, and the eventual failure.

One failing:

https://youtu.be/Jmvnli40gRk?t=5

The construction looks like a cost cutting measure to me. Instead of molding a separate plastic case, placing the capacitor body inside, and filling the bottom with epoxy (like most X caps). They placed the capacitor body in a mold and filled that with epoxy. Bet it made some bean counter very happy.