Are very old CRT oscilloscopes possibly dangerous because of their X-rays?

Question

I'm a novice who's looking for a very basic oscilloscope. I found some on Ebay. Probably I'm much too careful, but I'm wondering if cathode ray screens from oscilloscopes of the 80s may leak dangerous levels of X-rays or if a broken case could increase the radiations.

Answer 1

That's not "very old" in terms of CRTs, but anyways:

your concerns aren't unfounded. CRTs do leak X-rays. However, the amount and hardness of the radiation scales with size of the CRT. And with an oscilloscope display, you're at the least end of all this, so, even if the protection isn't as good as in a consumer TV, you'll be fine.

Also, billions of people literally spent years in front of CRT TVs, and the population-wide effects are benign. How many hours a day do you intend to spend in front of that scope while it's running, for how many years?

Financially, buying a CRT oscilloscope almost certainly makes no sense for a beginner. Knowing how to operate one takes time, and some guidance, and the older they are, the more likely you'll need some calibration for it to be certainly useful. There's cheap digital oscilloscope modules, which are maybe analog-wise less nice than these old-time oscilloscopes, but much easier to deal with, use nearly no power, and being much smaller and harder to damage in transportation, much cheaper to ship.

Answer 2

The anode voltage in a cathode ray oscilloscope is a few thousand volts. X-rays aren't produced until the that voltage exceeds 20 kV or more. That only happened in color televisions. Your oscilloscope will not produce X-rays.

Answer 3

You are not in danger. For occupational exposure (e.g. 8 hours a day, five days a week), the limit is often given as 50 mSv (or 50,000 μSv) in a single year with a maximum of 100 mSv or (100,000 μSv) in a consecutive five-year period. One study using CRT monitors concluded that because of the low dose rate, accurate dosimetry was difficult. It estimated the effective dose (ED) to an operator working in front of a monitor was 454 μSv yearly. That's less than one percent of the dose limit. It reduced to 16 μSv yearly after adding a conventional leaded glass sheet. I can't imagine that a CRT oscilloscope is going to vary wildly from this order of magnitude.

X-ray dose estimation from cathode ray tube monitors

Answer 4

Generally not an issue- the second anode voltage on oscilloscope tubes is only a few kV (resulting in negligible X-rays), compared to B&W and color TVs which have more like 15-30kV and commercial X-ray tubes which use more like 60-100kV accelerating potential to produce useful amounts of X-rays. Image from here

Answer 5

The case leakage may not be an issue, because if a CRT does emit X-rays, you are looking at the exposed screen anyway.

If the operating voltage of the CRT tube is low enough then it won't emit X-rays.

And the scope CRT is smaller than computer monitor and television CRTs which were relatively safe too.

Answer 6

As mentioned in other posts, it depends on:

• Anode voltage
• Shielding implemented in the device.

Anode voltages in CRT oscilloscopes vary extremely widely - from a below 1kV eg on small units with DG7-32 or similar tubes, to 24kV on units like the Tektronix models 502, 519, 7904. Very specialized devices (bespoke devices, obscure military gear) might possible go higher.

The reasons to use a higher voltage are:

• Sharper trace (that is certainly what you wanted in the 502 - it is low bandwidth)

• Higher brightness (especially with events that have a low repetition rate, where you rely on extreme brightness to make them visible to the eye or to photographic film)

• Higher bandwidth (more voltage, faster beam - faster beam, less time spent to go through the deflection plates. If the beam "stays" in the deflection field longer than it takes for a voltage change you want to record, there would be a conflicting hence and hither of deflection voltages applied to it, you would just thrash around the beam, but not properly deflect it! That's why you want it in the high-speed 7904 and 519, these are 500MHz/1000MHz devices. Other engineering tricks (deflection plates that are segmented and fed delayed versions of the input signal to each segment, so the deflection travels along the beam) were used in these very high speed designs in addition... interestingly, the 1000MHz 7104 model is only around 12kV, likely because the CRT in it is a special design that can easily be damaged by too much brightness.

There are also reasons to use a lower voltage:

• Cheaper build, easier to manage safety (high voltage, X-rays, ...) and reliability.
• Less chance to create so much brightness, when setup by an inexperienced user, that the phosphor or other parts of the CRT can get damaged.
• More sensitivity - the faster your beam, the more voltage you have to put on the deflection plates for the same deflection. Building amplifiers that can output 10s of volts at UHF frequencies was not trivial back then and isn't today. Also, that can become an EMI nightmare - you essentially are feeding the deflection plates with a transmitter, and you cannot shield them fully because anything except vacuum and glass will be in the way of getting a picture.

For comparison, 32kV was used in most color TVs, sometimes even higher in CRT projectors. There is lead in the glass envelope of these for a reason.

The bad rap early color TVs got for excessive X-ray production, by the way, is not entirely related to the CRT - early models use a parallel stabilizer circuit in the high voltage supply, based on a very big vacuum tube (eg the PD500 tube) that had much thinner glass (and usually was shielded in a lead enclosure for a reason...) and actually looked a bit like an X-ray tube... and behaved a lot like it.

For most CRT oscilloscopes you will find a manual online - since there were laws in place even then requiring manufacturers to document unwanted X-ray output of devices, you will probably find data in it.

Mind that X-rays from deceleration will go perpendicular to, not in the direction of the beam -- so you will be comparatively safe in front of the device.

Mind the elephant in the room - high voltage, when actually working inside these devices. Since they are designed as professional test equipment for trained users, these devices tend to be easy to open and run with the covers off. Be careful. But: the 12kV/24kV devices (even some 1950s era models!) will usually use some kind of inverter power supply for the anode voltage, like a non-ancient TV. These tend to have low current capacity and are usually regarded to be more likely to fling you across the room than plum kill you (does not mean they are harmless). Some older 2-4kV models use a conventional power supply that simply uses a high voltage from a winding on the mains transformer, and a conventional rectifier and filter circuit. Consider these circuits electric chair grade lethal!! There is a reason they stopped using that kind of circuit in TVs already in the 1940s....

By all means, do not discard or destroy old equipment out of fear - but be aware of what accelerating voltage etc apply to your gear!