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.

 

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.

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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!

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Len

 

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

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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

 

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

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Il give you shocking experience.

 

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

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'Intellectuals solve problems. . . Geniuses prevent them.' . . . Albert Einstein

 

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.

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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

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'Intellectuals solve problems. . . Geniuses prevent them.' . . . Albert Einstein

 

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.

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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

 

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.

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No schematic is not good thing.

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

 

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.

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Please ask on the open forum if you have a question and I'll be happy to help, if I know the answer.

 

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