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 ? 

jurb...@gmail.com wrote:

--------------------------


> 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.


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

 
> Is there something off the shelf for this ?


** 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 

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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On Friday, February 2, 2018 at 1:49:57 AM UTC-5, jurb...@gmail.com wrote:
> 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 ?

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

bloggs.fred...@gmail.com wrote:

-------------------------------

>
> > 
> > Is there something off the shelf for this ?
> 
> Sure. But if you want a specific part no., someone needs to know the
> VA rating of the transformer.
>

** 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 

>'Why not save yourself the bother and just buy another one?"

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. 

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

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

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

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

From your other post :

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

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 and strong (I am surprised there is no phosphor burn, I mena you could project 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 resistors 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 tubes so it probably won't makew any difference at all.

jurb...@gmail.com wrote:

------------------------

> 
> >"** I would try a 6.3V AC "wall wart" rated at say 0.5amps. "
> 
> Do you really think it will take that 3,500 volts ?
>

** 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.

http://www.cettechnology.com/wp-content/uploads/products/pst-pdst-split-bobbin-transformer-series/PST-PDST-crop-2-web.jpg

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



....  Phil 








 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 and strong (I am surprised there is no phosphor burn, I mena you could project 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 resistors 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 tubes so it probably won't makew any difference at all.

On Fri, 2 Feb 2018 21:21:47 -0800 (PST), jurb6006@gmail.com wrote:

>>"** Even fairly large CRTs used in scopes have 6.3V 300mA heaters."
>
>Yup, screen size has little or nothing to do with the heater power required. 
>
>>"The main thing for the OP's job is that it have unusually good primary to secondary insulation. "
>
>Which is why I used a hi-pot unti in the past which has now failed.
>
>From your other post :
>
>>"** I would try a 6.3V AC "wall wart" rated at say 0.5amps. "
>
>Do you really think it will take that 3,500 volts ? OK I'll take your word for it. 

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

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

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

>"** Ever take a close look at one ? "

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.