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[SOLVED] need repair tips for an ultron 553ap oscilloscope

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Newbie level 4
Apr 28, 2011
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I got one of those vintage oscilloscopes, however if i turn it on (and the CRT is already heated up), i see a line/moving dot for a second or so that then fades away quickly. The time base seems to work, as well i can change the position of the line in x- and y-direction. Any hints where i should start seeking the error? Any schematics available for this device? The inside parts are overlook-able and there are verbose prints on the boards.


I have a Tektronix scope several decades old. It developed a problem with the dot rapidly appearing and disappearing. A buzzing sound came from inside. The cause was in the high voltage output. It was intermittently shorting through transformer coils which provided 6 V heater voltage to the high voltage tube. I had a schematic to help me.

Sounds like your problem is in your high voltage output. Everything else sounds okay.

It might be a circuit which supplies power momentarily to the dot, but quickly loses charge (faulty capacitor somewhere)...

or a component overheats and cuts out...

or an overheated component diverts power from something else.

Capacitors are notorious for going bad as they age. Many technicians routinely replace all capacitors in old equipment.

Look for a tube which glows and then goes dark as your dot fades away.

Do you feel like applying a meter to the circuits? You'll need to work gingerly around high voltage. Getting shocked goes with the job. Fortunately it's low current and won't kill you. Unless something's really out of whack.

Thanks a lot for your suggestions!

Yes, i am able to apply a meter to the circuits, though i am not keen on getting shocked, i know already what it feels like :)

The high-voltage part in this oscilloscope is realized with two charge pumps as it seems, one gives -1400V, that is also where the intensity knob takes influence. I'd favor a defect in this part, as it is easy to reach :) There's also a glow lamp in that part, and it glows continuously when power is on.

The other part is a charge pump that delivers +1600V (all numbers written on the boards). That part is nifty to reach and it would mean quite some disassembling work, including desoldering of wires.

So far all parts look sane from their appearance, and there is no buzzing sound or such. I'd also measure all voltages from the power-supply as well, all values are printed on the board as well. So i can step by step exclude components.

At least none of the caps in the first charge pump is short and they seem to charge/discharge, all diodes in the pumps (cascades) are okay as well.

So the 15V, -68V, 50V and -50V from the power supply are okay and off by only up to ~1-2V. There is also a 300V line on the power-supply that has only 253V, no idea if that is still okay, and no idea what purpose it has.

So far i can't measure the 1600V and -1400V as they are out of range for my meter. Another thing i noticed: When you turn the device off, then a slowly fading away and unsharp dot appears in the middle of the screen. Doesn't hat mean, that actually there's enough high voltage present?

Sure i can order 22 high-voltage caps now and replace them in good hope, but if the observations made so far indicate a different failure, i'd be happy to know in advance :)

Everything you've done is progress.

Now we ask every possible question about what can make the beam fade.

The fact that it fades gradually suggests something is overheating. If it disappeared suddenly then you would suspect a rogue contact opening when hot and closing when cool. Or closing when hot and opening when cool.

There's a freeze-spray troubleshooters use to cool components. If you have infrared goggles that's even better.

Can you determine whether the beam fading is caused by something that happens because the beam came on? Example, something happening inside the CRT because the gun is shooting electrons?

Is something in the CRT suppressing the beam? Does a heater wire stop heating? Look for voltage drop in the heater circuits. The typical value is 6.3 VAC.

If you were to hear buzzing then you might suspect arcing somewhere on the path to the CRT.

Can you detach the wire that carries hi voltage? (Scope off). Then rig a clip so you can attach your meter and measure it directly when operating? Your multimeter should be able to handle this voltage.

Does something in a different circuit ruin the hi voltage? Draw too much power? Create oscillations?

Measure voltage at the intensity control. See if the input stays constant. Both at input and output.

See if you can set intensity low enough so the beam remains. Darken the room. Watch for anything lighting up your phosphors. The gun may be still be giving off a few electrons.

The power supply is always suspect. However you don't say another system is affected.
Nevertheless look for something happening elsewhere as the beam fades. Does it move in any direction as it's fading? Perhaps your deflection circuits are also dropping in voltage. On the other hand the supply may not be going so low that their regulation is affected.

Since the scope works at all, you might try feeding it a slightly lower AC voltage. At a level where the problem doesn't kick in. Do you have a triac rated high enough to run your scope? Start at just lower than normal. Beware going too low or circuits might act in a way that ruins something.

---------- Post added at 15:38 ---------- Previous post was at 14:03 ----------

I saw your post after writing mine.

You say the beam re-appears when you turn off power.

This suggests a circuit is sapping juice while the power is on.

Then when you power off, the problem circuit turns off. Then the hi voltage operates for a few seconds afterward.

The dot is out of focus because your deflector plates are not focussing it.
But since the dot is focussed while power is on, then your deflection circuits are good. I imagine those are for 50V.

The problem may be at the 300V circuit. 300v is a typical value used in tube amplifiers. But it's down to 253. See if you can disconnect the wire at 300v tap and measure the tap alone.

If its at 300v or more then re-connect the wire and see if it drops to 253. That may be where something is draining power. Could be thermal instability or thermal runaway or parasitic oscillations, etc.

It's a kind of problem can affect your hi voltage circuit even if it's indirectly related.

If my knowledge of tube circuits were greater I could give more recommendations.
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So checked a few more voltages:
X/y-voltages look sane, voltages for the y-deflectors are nearly equal at 130V, voltages for x deflectors rise and fall as expected, depending on the timebase selected.
Then there is another voltage (z?) that is also @ 130V. The 300V are okay, actually there is a divider that brings the voltage down to 250V. 250V is also the voltage noted at the board mounted at the crt's connector that is connected to the 300V part by a simple wire. So printed information on the boards was a bit unclear as it seems.

So far i can not see any signs of components running hot, all parts and their surroundings look sane. Over all it is still very clean inside the scope.

As i have no means to measure the high voltage parts i'll give that at least a try and desolder the caps and see if i can do some tests on them and in case replace them first. Wouldn't it be possible to achive the phenomenon of a reappearing beam, when the second accelerator-plate fails, so that electrons can't pass through the focus-ring? Just a quick idea, i might be wrong :)

It's probably okay to use your meter to measure the 1400 and 1600 V.

This is because the hi V source itself probably has high internal resistance. On the order of megohms.

Therefore your meter will load the circuit and pull down the value to a fraction of what it really is.

You can still watch how the reading changes under all conditions. Whether it drops to zero, or half, etc.

If your meter pins at the highest range, you can add a resistor in series to divide it down.
Attach a resistor to the end of your leads. Maybe 20 M ohm.

If your meter is 20,000 ohms per volt...
And your DC range is 1,000V...
Then your meter shows 20M resistance...
And you'll get division by two.


You'll find out a lot if you can get at the hi V wire to the CRT and measure hi V.

You may have to cut away a bit of insulation to get at the wire. Even though it creates a spot where arcing can occur later on.


If nothing else, you may resort to a homebrew hi V generator.

Tap off the highest VAC on the power transformer and run it through a voltage multiplier. Easy to construct using diodes and capacitors. A few stages should yield 1600 V (negative).
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Okay, measuring the 2 voltages worked fine, with my meter, it also showed values above 1000V without any problems and without arching or such :)
Picking up voltages was easy, as the 1400V is a test point and the -1600V is a soldered point on one of the boards. However both voltages look quite sane and take about 2 seconds to fully establish. The 1400V turns out to be 1348V in the end and the -1600V reach -1650V, so nothing suspicious i'd say :-(

---------- Post added at 10:17 ---------- Previous post was at 09:06 ----------

We are coming very close to the problem as it seems. After measuring also the intensity voltage (E of the power transistor of the hv-generator) and the test point at its respective collector (both fine), all of a sudden the beam appears permanently, but intensity can not be adjusted/decreased anymore. But seems like the problem is around or before this transistor (2SC515) then, that maybe went from bad to short after being loaded more than usual by my measuring. I'll check the affected components that are just a few :)
Thanks a lot for your patience and help!

So far there were two blown 2SC372, one is shorted between B and E and one is dead at all. The 2SC515A seems to be dead as well, and also a zener 1s1715 shows a strange behavior it has a forward voltage of 0,5V and a reverse voltage of 1,2V. For the zener (rated 6.0-7.1V) and the 2sc515a i need to find/order fitting replacements first. The 2sc372 got replaced with two BC547 i had at hand. The involved caps look ok.

Hehe, well, the board is single layer and the tracks are big and easy to follow. So just the kind of design when i first got into contact with electronics :) However the high voltage stuff first scared me off a bit, but now i am happy having success!

Very positive. You'll pat yourself on the back over this the rest of your life.

Like I do. My hi V arcing problem was caused when the old transformer windings developed a spot where arcing could occur. It's unique with many taps that go to all the circuits. No replacement available.

The hi voltage was attached to 6.3V windings. I solved the problem by installing a separate 6.3V transformer. Room was scarce and I had to turn it every which way to fit. Even though it was the smallest I could find.

But it worked. Later when my repair developed arcing I replaced it with another transformer. (Thank you Radio Shack.)

My buddy had the same kind of scope. He had got 2 and I bought one from him. When his scope developed the same problem I fixed his the same way.

I could hardly believe the manufacturer would run hi voltage inside 6.3V transformer windings. The varnish must be adequate so as not to break down.

However I believe I aggravated the problem when I stored the scope in a humid basement. Now since keeping it in dry living quarters the problem has not returned.

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