Dead scope.

[Someone Electro]

Yea whith DC you cant let go.Thats why i never ever toch my cap bank when charged.Its 11 000uF 400V thats 880 J.So if i toch somthing in a way i cant let go im prety much a goner then.And if you meaby have sweaty hands then it would make a huge curent flow.

Whith HV caps there isent much of a mather of cant letting go cuse the cap wil probobly discharge in a few miliseconds and the curent is huge

 

[fingaz]

I once got a HV shock from a TV. . . It actually burned a hole in my thumb! Certainly an experience i don't want to repeat

 

[Nigel Goodwin]

I once got a HV shock from a TV. . . It actually burned a hole in my thumb! Certainly an experience i don't want to repeat

Burnt holes are common place, particularly if you get a shock from a charged up capacitor - they seem to blow/burn two holes, one for each contact point.

 

[fingaz]

Yeah, I seem to remember two holes in my thumb. . . It was a few years ago, but i remember it well enough to keep my hands away from HV caps until they've been discharged

 

[Hero999]

I've replaced the 250mA slow blow fuse, powered it up slowly using a variac and it works but it's behaving very strangely. When I turn the brightness down a bit it oscillates, the waveform flashes on and off at about 1Hz. When I turn the brightness up until it stops flashing, the waveform's amplitude changes slightly about once a second. It's also horribly off calibration (voltage wise). I set a signal generator to 50Hz sinewave so the oscilloscope read 16Vpp but the multimeter said it was 4.15V, it should read 5.66V - that's 26.5% out!

All of the controls work and both channels exhibit exactly the same behaviour. I haven't seen anything like this before; does anyone know what the problem might be?

I suppose this is better than having no oscilloscope at all - at least I can see the overall shape of a waveform.

 

[awright]

Check your power supply voltages.

Whenever you have problems in several different functional areas of an instrument, suspect the function that is common to them all. That's usually the power supply. It's highly unlikely that a variety of different problems would occur in unrelated areas of the instrument without some common cause. The symptoms you describe are also probably related to low power supply voltages.

Be very careful checking high voltages. Use a High Voltage Probe like the Fluke DMM accessory. You may still have to be aware of the loading of the probe pulling down the measured HV. If you have a manual for the scope it will often tell you what instrument (i.e., what probe resistance) was used to measure the indicated voltages. Try to match that. Be sure to connect the ground lead BEFORE probing the HV, as instructed in the probe manual. Keep one hand in your pocket!

If you don't have or can't find a manual for the scope, consider looking up the CRT specs and being sure the voltages are in the ballpark of their recommended operating conditions.

Let us know what you find.

Have fun!

awright

 

[Hero999]

Thanks for the reply.

I did think about replacing the capacitors but I decided I'd have one more final go at powering it up. What you've just said makes perfect sense and having thought about it now it's obvious. One of the capacitors might be bad so it's acting like a relaxation oscillator; this is just a hypothesis but would you agree?

I don't have a manual and have searched on Google.

I suppose the next question is which capacitor is it most likely to be? I suppose I'll have to check every electrolytic capacitor in the power supply to answer that one. I'm tempted to replace them all but that might be worth more time and money than the scope is.

 

[mramos1]

No schematic is not good thing.

Maybe look for swelling tops or bottoms on the caps. Replace them first if any.

 

[Hero999]

The capcitors all look in good shape.

Luckilly it seems to be improving, it the waveform seems to jitter less and it now only occasionally flashes on the dimmer settings. I think it's improved because running it has repolarised the capacitors slightly. I'm less tempted to mess around with it than before because I have a feeling I wiil break it, all it takes is a slip of the meter.

I've been thinking about making a high voltage probe for awhile now, though it will take alot of those small ¼W resistors, fortunately I've got a huge reel of 1000s them in my assorted box of resistors that I haven't sold yet. I just need to get hold of some potting compound and a suitable enclosure.

 

[awright]

It's possible that a capacitor is failing and healing repetetively, but I think it is more likely that the problem is a circuit going unstable due to low power supplies or some feedback phenomenon due to high power supply impedance(low capacitance).

Unless you are unusually clumsy, I wouldn't worry too much about measuring the voltage on the main power supply capacitors (not the HV power supply caps which will look a lot different from those in the main power supplies). Just be sure to use a meter/probe good to 1KV or so. Without a schematic you won't know what value you should measure, but you would be able to detect large low frequency oscillations to narrow your attention down to fewer components rather than replacing things blindly.

What brand/model of scope is it? Tube or transistor or IC circuitry? If it is a tube model, you can get some idea of appropriate voltages by looking up the recommended operating conditions for the tube types.

If it seems to be improving, the problem was probably capacitor depolarization. However, it's a brute-force method of re-polarizing the capacitors to hit them with full voltage after a long sleep. But apparently they have survived so far, so it makes sense at this poing to let it soak at normal voltages to see if the problem goes away. If the caps were going to blow due to being depolarized when hit with the HV it would have happened early in the reawakening of the circuit.

Apropos slipping of the probe: It is not a bad idea when working with HV circuits to turn off power, connect your meter to the circuit with clip leads (being sure that clip lead wires with low voltage insulation are dressed away from ground and other circuit components and supported on insulators), then turn the device under test on, read the meter, and turn the DUT off. A bother, but much safer for you and the DUT.

awright

 

[mramos1]

I too have spools of resistor in the garage.. And tubes of chips, big boxes of caps, etc.

Far as that probe I would take care.. I had a HV probe way back and was still nervous about using it.

 

[Hero999]

It's all transistors, no valves or ICs.

I haven't used it for a couple of days, I'll check the low voltage side before I even think about the high voltage side.

 

[Hero999]

The 'scope has made a full recovery.

I've got a better 'scope so the old one got shut away in the cupboard for a couple of years.

Recently I've been thinking about getting rid of the 'scope. I didn't have the heart to throw it out so I was going to give it away.

I switched it on to see if it still worked, the intensity oscillated as it had done before. I took the cover off and took a photograph of the circuit board. I replaced the cover, turned on the power and to my astonishment it had made a full recovery!

It works perfectly now, no oscillation, no buzzing, all is well!

Why has it suddenly started working perfectly?

Could it be a loose connection or a pot setting?

I put it away on top of the cupboard (a producure that involed subjected it to some minor banging around) and got it out again, put it on my bench and turned the power on and it still works fine?

I'm at a total loss as to why it's made such a good recovery.

It turns out that I was wrong about the bandwidth specification, it goes up to 15MHz. I assumed 1MHz because I was using it with an uncompensated probe before which limited the bandwidth.

I don't know what to do with it now, it makse sense to get rid of it but I feel some stange attachment to it. I don't feel comfortable selling it because I wouldn't trust it not to fail pretty quickly.

Here are some pictures, it's a great newb 'scope as it's simple and very easy to use, with not complicated features.

 

[mramos1]

I've got a better 'scope so the old one got shut away in the cupboard for a couple of years.

Recently I've been thinking about getting rid of the 'scope. I didn't have the heart to throw it out so I was going to give it away.

I switched it on to see if it still worked, the intensity oscillated as it had done before. I took the cover off and took a photograph of the circuit board. I replaced the cover, turned on the power and to my astonishment it had made a full recovery!

It works perfectly now, no oscillation, no buzzing, all is well!

Why has it suddenly started working perfectly?

Could it be a loose connection or a pot setting?

I put it away on top of the cupboard (a producure that involed subjected it to some minor banging around) and got it out again, put it on my bench and turned the power on and it still works fine?

I'm at a total loss as to why it's made such a good recovery.

It turns out that I was wrong about the bandwidth specification, it goes up to 15MHz. I assumed 1MHz because I was using it with an uncompensated probe before which limited the bandwidth.

I don't know what to do with it now, it makse sense to get rid of it but I feel some stange attachment to it. I don't feel comfortable selling it because I wouldn't trust it not to fail pretty quickly.

Here are some pictures, it's a great newb 'scope as it's simple and very easy to use, with not complicated features.

It's funny, 2 plus years later and it works after removing the cover.

I've seen that with other electronic items. Pot/something was corroded and cleared, cracked solder joint mated up when you locked down the lid, etc. I've picked up a dead computer, got it to the office and it worked fine, returned it and never heard back about it.

If you have a new one, I would sell that one. Sell it cheap and the new person might get a first real project tacking all the solder joints.

 

[Hero999]

I spoke too soon, the oscillation problem has returned.

It must be a loose connection, it's the only thing that would expain it.

I'll have another look at it, if there's nothing obvious then I'll give it away.

On second thoughts, even if I fix it I think I'll give it away. I don't trust it and there's no point in having a piece of test equipment I don't trust.

Don't get me wrong, it's still functional as it is and would be fine for a first 'scope.

 

[mramos1]

If you tap on it (good tap) does it clear up for a second or two?

If not I would think a bad cap somewhere.

Too bad you're in England, I would give you a few US bucks for it, but the shipping would outweigh that probably. If you are going to give it away, let me know the shipping and handling cost .

I used the same Gould scopes years ago. We never really had a problem with them. You got the machine that was made on a Friday I think.

 

[Hero999]

If you tap on it (good tap) does it clear up for a second or two?

Not quite: I've tried tapping it and I can't get it to stop oscillating.

If not I would think a bad cap somewhere.

Possible but that wouldn't explain why it works for awhile after the case being remove then replaced, then suddenly stops oscillating.

Too bad you're in England, I would give you a few US bucks for it, but the shipping would outweigh that probably. If you are going to give it away, let me know the shipping and handling cost .

I can't see myself dragging it all the way down to the post office to get it weighed.

I used the same Gould scopes years ago. We never really had a problem with them. You got the machine that was made on a Friday I think.

It is very old so what do you expect?

It's still very usable, the oscillation is only a real problem at low intensities and it caused the waveform to jitter very slightly. I imagine that someone reading my posts here might think that it's worse than it really is. Unfortunately I don't have a video camera to record it or I'd post a video.

 

[mramos1]

It's an old scope, but a scope is a scope outside of the CRT being burned or it being really out of calibration.

The problem with yours sounds like it just bugs you, but it works. I would give it to a local for cheap and tell them keep the brightness up

Someone new in electronics would not have a problem with that. Like I said I would take it if shipping was not a pain just to have one in a place I do not have one.

Sounds like a CRT drive/circuit issue. Something breaking down with the pot on intensity. But the fact it sat then worked for a bit, probably not the pot.

 

[Hero999]

Well it started working perfectly again so I decided to have another look at it.

I took the bottom cover off; something I haven't done before. I noticed that a 350V 100µF capacitor had gone (the big blue one pictured in the attachment), the safety valve had blown and there was a small spot of gunge on it.

I replaced it with a Rubycon 400V 100µF 105°C capacitor from an old SMPs. The replacement capacitor is much smaller and the safety is on the other end so I left a small space between the capacitor and the case in case it blows.

I powered it up and the 'scope still works perfectly. I suspect the big red capacitors may have gone bad too so I'll replace them when I put my next Farnell/RS order in.

 

[Hero999]

I've had another problem with it oscillating then cleared up again.

I've had a go at calibrating it but it's not possible without the manual. I'm loathed to replace all the electrolytics only for it to still keep doing this.