Dead scope.

[Hero999]

I turned the scope on, it made a buzzing sound, a green line appeared briefly on the screen briefly then it stopped working. I've noticed that the internal fuse has blown, now I'm going to replace all the capacitors on the supply, and check the rectifiers too.

It's a very old scope, it only goes up to 1MHz and it's all discrete componants but no valves so I suppose it can't be that old. One unusual thing is it doesn't have a flyback, instead it has a multiplier made from eight 450V 4.7:mu: f capacitors, is this normal? Should I replace them all to?

Is there anything else I should check?

I'm concerned I may've caused this, when I got the scope out I noticed a piece of dirt had sliped under the screen so I gave it a very gentle tap to dislodge it. There again I didn't hit it very hard and it's definitely been banged around more than that in it's life. Just taking it home in the car would've subjected it to more vibration and it's survevid that.

EDIT:
It's a Gould Advance OS250B.

 

[on1aag]

Scope problem.

Hi Hero999,

There's only a doubler circuit for the anode voltage, there should be
approx. 2,8 kV on the anode of the crt. There will most likely be a
negative voltage on the cathode, the four 4µF capacitors are all connected
in series ! Without resistors to evenly divide the voltage between the
capacitors. The total accelaration potential will be the sum of both.
I think you might need a manual ?

on1aag.

 

[tunedwolf]

oops...well...time to shell out on that nice new Tek you have been hankering after for years...

 

[philba]

oops...well...time to shell out on that nice new Tek you have been hankering after for years...

or buy another used scope on ebay. 100 mhz dual channel analog scopes are running pretty cheap these days.

 

[mramos1]

I have been on Ebay looking, there are some deals for sure. Dual trace 100Mhz storage scopes with recent calibration date (1 year) for a couple hundred.

But be careful, I almost convinced myself (birthday coming up) I need a DS2024 (I think it was). It went for $1700 but I went up way more than I wanted to spend (but it was so nice).

I would do anything my wife told me for a couple days to own one (but I would want to see the list she had first).. Oh and I can not afford it

 

[ljcox]

I turned the scope on, it made a buzzing sound, a green line appeared briefly on the screen briefly then it stopped working. I've noticed that the internal fuse has blown, now I'm going to replace all the capacitors on the supply, and check the rectifiers too.

One trick you could try is to insert a 100 Watt light globe in place of the fuse. The filament has a positive temperature coeficient so it has minimum resistance when cold.

It will glow if there is a short but no further damage should be done and so you can trace the fault.

As you wrote, a faulty cap or diode is the most likely cause.

Len

 

[awright]

An old scope like that may not be worth a lot of investment of time or money, although I understand the attachment one develops for old, familiar, instruments. Is this an instrument you have some history with, or was it newly acquired?

Checking the capacitors is a good start. If you lack a capacitor checker, remember that some DVMs come with a capacitor check function and they are often quite inexpensive. Alternatively, some crude tests are possible with just a DVM or VOM. Having several identical capacitors for comparison greatly facilitates such cheap and dirty tests.

Set your DVM/VOM on a range that charges the capacitor over a perceptible period of time. Apply your tests leads to the capacitor and observe the dynamic response of the pointer. (It is much more difficult to to this test with a DVM because looking for a trend in a series of flashing numbers is difficult. Additionally, the current supplied by the DVM in the resistance mode may be so low that the capacitor does not charge up in a reasonable period of time.)

At the instant you apply the leads to the capacitor, you should see the Ohmmeter pointer dip to nearly zero, then gradually climb back up to near infinity. If the voltage indication does not climb up to a very high resistance, the leakage is probably too high. If the pointer doesn't dip to near zero initially, the capacitor may be open. If the needle dips to a low value and stays there, the capacitor is probably shorted.

You are lucky that you have several of each type of capacitor on the board. This allow you to observe the typical charging time of each capacitor of interest and compare the needle dip characterists of each.

Better capacitor tests can be performed with higher voltages and more sophisticated equipment, but the tests described above can tell you most of what you want to know about your voltage multiplier capacitors. If you do use high voltages to test the capacitors, be very careful.

Since you say that you heard a buzz just before the 'scope failed, you should test all rectifiers in the power multiplier circuit and other parts of the power supply. The test is most reliable if you disconnect one end of the diode from the board to avoid influence from nearby components. You should see a low resistance (<100 ohms) in the forward direction and a very high resistance (tens or hundreds of megohms) in the reverse direction. Here, again, testing at a few volts out of the ohmmeter is not as revealing as testing near diode reverse voltage rating, but it is readily available, cheap, and capable of revealing ordinary defects.

Don't overlook simple, very close examination of the circuit board for signs of overheated components, bad solder joints, cracked conductors, etc. A magnifying glass is very helpful here.

Be sure that the unblanking pulse is getting to the proper pin on the CRT.

Check the CRT filament pins for continuity. If the tube filaments are shot, you are pretty much up the creek. Unless you stumbled across a replacement CRT at very low price, it will probably not be worth replacing the CRT, even if you can find one.

Use a high voltage probe to confirm the presence of the proper high voltages on the CRT. You will not be able to measure the exact voltages due to loading by the measuring probe. However, you should be able to confirm the presence of a high voltage at the HV terminals.

Check for the presence of appropriate voltages on the power transformer. Old 'scope power transformers are usually specials and not worth the cost of replacement.

Have fun, and be careful.

awright

 

[Hero999]

An old scope like that may not be worth a lot of investment of time or money, although I understand the attachment one develops for old, familiar, instruments. Is this an instrument you have some history with, or was it newly acquired?

Yes, I got it from work just over a year ago it was destined for the bin. I don't plan to spend a large amount of time and money on it but I won't through it away if it's something cheap to fix like a bad capacitor or rectifier.

Thanks for your advice I'll try out your suggestions and get back to you with the results.

Another annoying thing is the capacitors aren't fitted with bleeder resistors so they don't discharge when the supply is disconnected to discharging them manually will be a pain.

 

[awright]

"Another annoying thing is the capacitors aren't fitted with bleeder resistors so they don't discharge when the supply is disconnected to discharging them manually will be a pain."

Well, at least that's very easy to fix.

awright

 

[Nigel Goodwin]

"Another annoying thing is the capacitors aren't fitted with bleeder resistors so they don't discharge when the supply is disconnected to discharging them manually will be a pain."

Well, at least that's very easy to fix.

Just place your hand across them!

Been there, done that!.

 

[CanonMan75]

Just place your hand across them!

Been there, done that!.

Me too. I wouldn't recommend it to a beginner, though

 

[tunedwolf]

ain't it funny how yer arse puckers up within two secs after ya do ?

 

[mramos1]

It takes a lot less than 2 seconds for me

 

[Nigel Goodwin]

It takes a lot less than 2 seconds for me

But it's two seconds before you notice!

 

[mramos1]

But it's two seconds before you notice!

Good point.

 

[philba]

reminds me of when I was a kid - there was a surplus store where I kind of hung out and owner tolerated me pestering him with stupid questions (how little has changed...). anyway, he got a shipment of huge military surplus caps. they were from radar installations, I believe. he said, "Hey kid, always bleed your caps off". He took a screwdriver and bridged the terminals of one. It made a very loud pop and the tip of the screwdriver actually welded itself to one of the terminals. That made a very distinct impression and gave me a healthy respect for electricity.

 

[Hero999]

I've had shocks from the mains but I've never had a shock of a huge capacitor. I don't that HV DC is mre dangerous than AC so I treat it with a great deal of respect.

 

[ljcox]

I've had shocks from the mains but I've never had a shock of a huge capacitor. I don't that HV DC is mre dangerous than AC so I treat it with a great deal of respect.

My father was an electrical linesman and he had many serious shocks.

He said that DC throws you off but AC grabs you and you can't let go. So he felt that AC was more dangerous. But both can kill!

 

[Hero999]

I thought that was the other way round. Doesn't DC cause your muscles to lock while AC causes spasms which through you off?

 

[tunedwolf]

yup, ac will throw you away, dc will make your muscles contract making you grab onto whatever you are holding, including the buss bar that's sizzling ya