Safety

[Rob]

John,

You bring up a valid and important point about pets and exposed tube electronics which use high voltages. I have two housecats and when I am working on one of my vintage sets in the house, especially one of the floor level 21" vintage color sets I am especially careful not to leave the room even for 5 seconds if the back is off and the set is operating.

That ventilation screen under the chassis you speak of is missing on a couple of my consoles but not operating ones, so no worry, but something that could easily be overlooked, especially if such a set were to be restored and re-sold to another pet lover.

Rob

[jshorva65]

In all cases, I make certain that the sets I restore are enclosed to NEMA-1 specifications. Back and bottom covers, if missing, are replaced. With missing bottom covers where I do not know how the original was constructed and lack access to a sample upon which to base a design for a reproduction, I have been known to cut a piece of ordinary window screen to size and secure it with staples. In other cases, where an outline of the original cover is visible, I design a cover with the proper shape and an array of 1/4" diameter ventillation holes spaced 1/4". For makes that used slots for ventillation on back and/or bottom covers (Zenith is one), I design the reproduction part to have an array of 1/4"Wx4"L slots spaced 1/4" between slots and 1/2" between rows. Reproductions of masonite backs are under development. I need to find a manufacturer with the proper tooling to fabricate masonite (cutting to size and punching of ventillation and other holes). I'm also researching plastic parts such as CRT neck caps.

[Charlie]

Cats definately love "investigating" anything open! Years ago, my ex-wife had a cat that decided to come in the room where i was working on the Curtis Mathes. I didn't see her, and left for a moment to find a tool. The set was on with the back off and the high voltage cage open. When I returned a few minutes later, guess where I caught her about to put her nose! Well, I shouted at her and she took off like a rocket. I suppose it was better than she "going up" like a rocket!

After that experience, I would leave the cabinet door under the kitchen sink open while working on a set. That empty cabinet would keep her entertained for hours!

Kinda reminds me of the movie Christmas Vacation where Randy Quaid says, "If that cat had nine lives, it just spent them all!"

[Steve D.]

I'm not sure about pets, but for us accident prone humans, a little family CPR instruction goes a long way.

[jshorva65]

Flyback-type post-war hv supplies are significantly less dangerous than their 60-hz pre-war counterparts and probably would have resulted in a wiser but angry cat if she had gotten her nose into the hv. Nike has certainly learned a lesson and typically avoids open tv sets like she avoids our dog (50 pounds of playfulness). I don't allow the dog in the basement because she seems to thinks it's "outside" and I'm tired of cleaning up messes. The cat lives down there, though, to minimize her effect on my gf's granddaughter's allergies. Getting back to the safety topic, the 60hz B+ supplies in tube sets are more hazardous than 15khz hv supplies, although not usually deadly like 60hz hv supplies. I was once knocked to the floor while trying to resolder +325V screen grid connection in a large (unplugged) PA amplifier. Discharge those electrolytics before poking around in the circuitry for any reason!!! It also bears mentioning that the hv flter condensers in pre-war sets are larger and pack a more hazardous zap than their 500pF post-war counterparts. Remember to discharge them before working on the set's circuitry. I use a test probe and wire with 30kV rated insulation, a 2K wirewound series resistor and a clip to the chassis at the other end. I tap each point twice and then measure the voltage to be sure there are no surprises waiting for me. It takes a little extra time, but it's easier than picking myself up off the floor. Use insulated tools during alignment, close hv cage covers and access the adjustments inside through the access holes.

Use caution when reaching inside the cabinet during alignment. Brushing against a tube will cause a nasty burn. I keep a product designed to treat sunburn in my First Aid kit.

Also, be very careful when handling picture tubes. Tubes larger than 8" are especially hazardous. Never handle a CRT without first discharging the capacitor that is formed by the Aquadag coatings. The shock from the residual hv is not usually dangerous in itself, but if it results in the tube being dropped it could lead to a dangerous implosion of the tube's glass envelope. I remember reading a news article in July of 1986 about a woman from Texas who died as a result of cuts from a picture tube implosion after picking up a tv that had been dropped. Never strike or scratch a picture tube, never use metal tools or abrasives when cleaning one. In general, handle all CRT's and any other glassware with caution. It is not a good idea to be fearful, just have a healthy respect for the fact that the glass is supporting several hundred pounds of pressure from the atmosphere and don't appy any more pressure than you would apply when handling a large window pane.

CRT's and other vacuum tubes may contain hazardous substances. Remember that these sets were built before the hazards of materials like Asbestos and Beryllium were fully known. Clean all cuts thoroughly and seek medical attention. A First Aid kit is highly recommended. Also, do not eat, drink or smoke at the workbench, wash your hands thoroughly before going back to other activities. Lead poisoning is a common occupational hazard of the electronics industry.

Fire is a very real possibility. A fire estinguisher with an A/B/C rating is highly recommended and will safely extinguish small fires from flammable solids (A), flammable liquids (B) and electrical equipment (C). Most hardware stores and department store hardware departments carry extinguishers.

Common sense, good knowledge of workplace safety and proper preparation are our best defenses. Keep rescuing those old sets, but remember to work safely.

[Rob]

John,

Excellent post! To further highlight the implosion concerns of large picture tubes I should mention that your pressure estimate was considerably on the low side. At sea level atmospheric pressure is 14.7 pounds per square inch of surface area. The pressure on just the faceplate of a 21" round faceplate TV picture tube is 14.7 X 346 square inches = 5,091 lbs. Yes Folks, that is 2.5 tons!!!!

Vintage color TV restorers should take special note. This is why any attempt to remove cataracts from an otherwise electrically good 21FJP22 color CRT by physical removal of the laminated faceplate is such a potentially hazardous and delicate operation, to be attempted with the greatest care, a containment vessel around the tube for possible flying glass and with safety clothing and face/eye protection.

Rob

One more precaution: Don't use the anti-static alignment tools on these older sets. They are conductive, and I found out the hard way when using one to align the IF on an old radio, and touched one of the terminal lugs on the IF can. I kept getting zapped, and soon found out why. Besides, I found that they detuned the circuit slightly when inserted in the slug. Do yourself a favor and get the red and yellow plastic tools instead.

Also never trust bleeder resistors, especially in HV supplies. They are frequently open. Remember too that older oscilloscopes have 60 Hz HV supplies just as dangerous as the prewar TV's.

[jshorva65]

My estimate of several hundred pounds was for a smaller tube (10BP4, etc.). Thank you for doing a better job of pointing out that the pressure is proportional to the size of the tube. Even the small 7JP4 and the smaller, bulbous 5" pre-war tubes can hurl shards of glass with surprising force. For that matter, even an ordinary tube or light bulb can shatter violently. Most light bulbs larger than 40W are filled with a gas which neither conducts electricity nor supports combustion. Light bulbs of 40W or smaller often will be evacuated and may implode if they are broken. Fluorescent bulbs will also implode if dropped since they contain a partial vacuum and a small amount of mercury vapor. Fluorescent bulbs also have phosphors on the inner surface of the glass which are often toxic. Cuts must be cleaned thoroughly.

[Rob]

John mentioned that the phosphors in fluorescent lamps are toxic.
Although true the hazard is not what it once was. Since I believe sometime in the 40's or 50's the use of berillium oxide (BeO) as an additive to the phosphor in fluorescent tubes has been discontinued. It was also added to the phosphor in early B&W projection CRT's to allow the phosphor to take more punishment from beam bombardment without burning. I know for a fact that the 5AZP4 is a tube containing such a berillium oxide mixture phosphor! Other types of the same era such as the 5TP4, 3NP4 and TP-400, and 7NP4 and 7WP4 if you can find one, may be equally hazardous. Berillium oxide is one of the most carcinogenic substances known. If trace amounts of the dust are inhaled cancer can result. One manual for a video projector I once owned that used the 5AZP4 warned that in the case of a CRT implosion during operation, the cooling fan in the projection head should be disabled by shutting off the power immediately so as to reduce the spread of the phosphor dust, and the room, or theater should be evacuated immediately!

Extra care should therefore be exercised in the handling and disposal of such projection CRT's beyond the concerns merely for flying glass.

Projection CRT's are usually operated at 27KV and higher, 40 kV on the 5AZP4. This makes these tubes good X-ray generators as well. All lead shields should be in place when servicing these projectors. I owned and serviced events projectors that operated a 6" CRT at 50 kV. The big 7NP4's ran on 80 kV. A half hour exposure to this tube without shielding was lethal.

Rob

[Jonathan]

Chuck, I'm glad you are ok and you never gor seriously hurt or killed. Just look at it this way, you got a 2.5kV zap from one of the coolest prewar sets. I'd love to own a TRK-5 or a TT-5, but I pretty much have a better chance at winning the lottery then even getting an oppertunity of owning one, or even getting the money to pay for it.

I think this is an important thread. I handle the 10BP4 I have with great care. An implosion would pretty much be leathal. This is quite un-nerving to me, but if I'm careful, I'll be ok. The HV cage is open as well, but I want to sand it and paint it with some metallic black gloss paint. Once it's done the HV cage goes on. The 60Hz HV windings are more dangerous, but a prewar HV supply or a postwar HV supply can both be lethal. I am more concerned about CRT implosion than HV, but I've worked with HV more than I've worked with CRTs.

With the 21" color CRTs. I'm getting one and I'm glad I read this thread. I will be extra careful when I handle the CRT, as if it implodes not only a good 21" round color CRT not exist anymore, I wouldn't either. Thanks so much for your posts and these safety tips.

[peverett]

I also read the story about the TV CRT imploding in Texas that killed a woman. If I remember correctly, this TV had not just been dropped, but had been thrown about in some kind of fight. Still, CRTs, especially the older ones, can be very dangerous and should be handled very carefully. The newer ones have some implosion protection built in.

I also recommend using an isolation transformer at all times. This is especially true of any series string type tube radios and TVs. These transformers protect both you and your test equipment.

As a final note, there are some non-series string TVs that do not have isolated B+ power supplies-I have two 1953 Hoffmans built this way.

[andy]

...

[Carmine]

I caught a HV shock off a roundie color Zenith. (Discharge, it wasn't powered up.) I don't remember the exact circumstances, as it was years ago.

What I do remember, was being "forced" to stand-up, while inside the cabinet... I hit the top of the cabinet hard enough to nearly knock the set over. Ouch!