Cheap AndEasy 6.3 Volts

I am playing around with this old scope in which the power transformer winding for the CRT filaments is arcing internally. In fact years ago I did put in a hi-pot 6.3 VAC transformer and wired it to switched AC internally and it worked for a long time.

But now the replacement is arcing.

The filament is elevated to about 3,500 volts negative DC. What is the possibility I could find a cheapo switched mode for this purpose. The insulation would be much easier for the higher frequency. In fact it could possibly be AC coupled.

Is there something off the shelf for this ?

Reply to
jurb6006
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** Much too likely to inject switching noise into you scopes' trace.

** I would try a 6.3V AC "wall wart" rated at say 0.5amps.

Normally, these have the primary add secondary wound on separate halves of a plastic bobbin and use plenty of insulation since they need to meet class 2 safety requirements.

Do not remove the tranny from the plastic box, just glue it inside the scope somewhere, oriented so that it causes no hum in the trace.

.... Phil

Reply to
Phil Allison

Why not save yourself the bother and just buy another one? CRT scopes - even decent spec ones - go for peanuts these days.

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Reply to
Cursitor Doom

Sure. But if you want a specific part no., someone needs to know the VA rating of the transformer.

Reply to
bloggs.fredbloggs.fred

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** Even fairly large CRTs used in scopes have 6.3V 300mA heaters.

The main thing for the OP's job is that it have unusually good primary to secondary insulation.

.... Phil

Reply to
Phil Allison

First of all that is not my way, plus I have more time than money. Also I have the dual timebase plugin for this one.

And it was a gift from my uncle who is now dead, I think the last thing I got from him. I also got a 310 from him that was the coolest thing I saw in some time. For service it had a hinge at the back o the cabinet and opened up like a book.

Reply to
jurb6006

Yup, screen size has little or nothing to do with the heater power required .

secondary insulation. "

Which is why I used a hi-pot unti in the past which has now failed.

From your other post :

Do you really think it will take that 3,500 volts ? OK I'll take your word for it.

However all I see is 5 volt AC ones, but it should work. The CRT is nice an d strong (I am surprised there is no phosphor burn, I mena you could projec t it on a wall !) so 5 volts shod be enough to at least be usable.

Worst that can happen is that it arcs. I'll also have to look through what I got gloating around, I might end up using a 12 volt unit with some resist ors to cut it down. Might take a bit longer to warm up but who cares ? this thing is tubes anyway and takes time. It has those little HV rectifier tub es so it probably won't makew any difference at all.

Reply to
jurb6006

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** Ever take a close look at one ?

The split bobbin places a couple of millimetres of insulation grade plastic between the primary and secondary windings.

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Being a simple AC to AC type, there are no other components like ceramic EM I suppression caps to cause trouble.

.... Phil

OK I'll take your word for it.

and strong (I am surprised there is no phosphor burn, I mena you could proj ect it on a wall !) so 5 volts shod be enough to at least be usable.

t I got gloating around, I might end up using a 12 volt unit with some resi stors to cut it down. Might take a bit longer to warm up but who cares ? th is thing is tubes anyway and takes time. It has those little HV rectifier t ubes so it probably won't makew any difference at all.

Reply to
Phil Allison

Usually tjose devices are rated at 2500 V and only tested for a short time (1 min ?).

Use an (auto)transformer to convert 5 V to 6.3 V.

Reply to
upsidedown

I will now. Well I will see if it works, I am pretty sure all I got is 9 volts, 12 volts and 5 volts. But resistors are my friend if I use a 9 or 12.

Reply to
jurb6006

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** Class II appliances require a 3,500VAC proof test - but in reality the rules concerning permitted insulation materials, minimum creepage and clearance distances that are at the heart of the matter.

FYI:

The dielectric strength of common plastics like PVC, Polypropylene and Polyethylene is around 20kV per mm.

The split bobbin places a couple of millimetres of insulation grade plastic between the primary and secondary windings.

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Being a simple AC to AC type, there are no components like ceramic EMI suppression caps to cause trouble - plus the iron core remains floating.

Bet it works a treat.

.... Phil

Reply to
Phil Allison

e (1 min ?). "

I'll let you know what happens.

Don't have one and have no idea where to get one.

People may be figuring out I am trying to do the maximum with the minimum h ere. I fixed a couple of equalizers lately. I need them because my hearing is going. It was nice to crank up that 16 KHz control and hear timbre again . And now I got the TV equalized so I can understand that.

I like good sound, there is a discussion right now at a high end audiophile (audiophool ? almost, mainly a love for sound and too much money on their hands) forum about hearing aids that are made to reproduce music. Learned a few things about them lately as well, they make them but they cost. Of cou rse some of these people have six figures into their stereo system.

Reply to
jurb6006

Wow, I would've never thought it that high. What is that about 20 times that of free air ? (dry free air that is)

Reply to
jurb6006

Found a 9 volt and a 12 volt. I'll have ot see if I have or can make the right resistor for it. I was sure I had a 5 volt one but it disappeared with many other things, including my sanity LOL.

Reply to
jurb6006

What is the make and model of the scope?

Reply to
tom

Try to avoid anything switch-mode for this application.

Switching power supplies can't stand very much in terms of high voltage when the high voltage stress is being applied permanently. The hi-pot test is only a minute (type test) or even less (production test) and the components across the insulation can only withstand it for a short time.

Switching power supplies have several components across the insulation, including "Y" capacitors and opto-couplers for the feedback path of the control circuit. They are designed to withstand mains voltage and the occasional surge (as is typical in public power networks) but not the continuous high voltage stress of a circuit that "sits" on a 3.5 kV potential permanently.

Also, switching supplies are strongly capacitive (between primary and secondary) due to the "Y" capacitors and they can generate a whole lot of noise. Both of these "qualities" are no good when the intention is to operate a CRT filament (while maintaining the CRT in a good shape as well as keeping the trace clean).

Your best bet would be a transformer constructed with the primary and secondary windings on separate bobbin halves.

If you could somehow get some "TEX-E" type (triple insulated) wire, you could rewind the secondary of a somewhat larger (compared to the small wattage you need) mains transformer for 6.3 V with the TEX-E wire. The main problem is that this particular wire type is hard to come by, and (to a lesser extent) that the insulation is relatively soft and requires taking care to avoid sharp edges, denting, or scratching.

Reply to
Dimitrij Klingbeil

It is a Tektronix 561A.

It is also the only scope I have with dual timebase. Not much bandwidth but I could pick a video signal apart and a few other things. I could borrow a scope with a dual timebase but it is better to have one lurking in the corner.

Reply to
jurb6006

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** Correct.

** Correct again.

** What for?

The secondary winding only has 6 volts between its ends no matter what.

The 6.3V transformer the OP fitted to fix the old scope likely had paper insulation and the core grounded.

.... Phil

Reply to
Phil Allison

I would have to agree. It doesn't have much bandwidth but like 100 KHz coul d cause some aliasing. It would have to operate above several times the 10 MHz (mc ?) bandwidth of the scope.

ondary windings on separate bobbin halves. "

As Phil described a wall wart with an AC output.

I couldn't find the 5 volt one, and I know I don't have a 6.3 volt one. I d id find a 9 volt and a 12 volt. If the CRT filament is 300 mA that means 21 ohms. I doubt it is less than 300 mA so that is where I'll start. I'll fin d or make a 21 ohm resistor and use the 12 volt one. We'll see how that goe s.

Reply to
jurb6006

insulation and the core grounded. "

OH DEFINIELY !

The one in there is a typical I and E job and screws to the chassis. theref ore when it is powered by a non-grounded circuit you get a shock off all th e metal parts of the scope, and they are just about all metal parts.

Maybe it would make a good alarm clock. With cardiac arrest you'll never be late again.

What's more, right when I discovered the fault it blew the chip out of my P C to TV convertor. I used to like watching videos on my bigscreen, but that thing is history now. Anyone need a Sony SD-186B that is very strong ? And it pisses me off that I had two of them but gave one away. But I guess I a m lucky it didn't fry my old PC. Thing runs like lightning.

Reply to
jurb6006

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