Cheap AndEasy 6.3 Volts

jurb...@gmail.com wrote:

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

> 
> 
> > "Your best bet would be a transformer constructed with the primary 
> > and secondary 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 did find a 9 volt and a 12 volt. If the CRT filament is 300 mA that
> means 21 ohms. 
>

** 21 x 0.3  = 7 volts 

> I doubt it is less than 300 mA so that is where I'll start. I'll find 
> or make a 21 ohm resistor and use the 12 volt one.
> We'll see how that goes.


** Maybe put 3 x 6.8 ohms, 1W MF in series.

The filament will light up more slowly than usual. 


....  Phil

it works. I used the 12 volt. 

>"** 21 x 0.3  = 7 volts "

So I had a 27 and put it in. It dropped 10.7 volts, not leaving enough for the filament. I changed it to a 10 ohms and that yielded about 5.8 volts at the filament. Must be more current like 600 mA or so. Maybe it has a really hefty cathode. It does go very bright and stays sharp, though at really high intensity it has somewhat of a haze to it. 

>"The filament will light up more slowly than usual. "

It still shows up before the sweep starts just like it always did, so as far as I am concerned that part of it is done. 

Now I get to play with the vertical. It is stuck in channel 1, putting it in chop, alternate or add makes the trace disappear. I sweep the deflection plate voltages change. And channel 1 clips the top of the waveform. At the 20 mV setting it clips at about 37 mV. Also channel 2 is off the screen to the bottom, which is the same direction. I found as shorted transistor, one of a pair of 2N711s. I stuck in a 2B1193 but found out later it is not likely to work right. First of all the frequency i s way down, plus it has alot more gain, perhaps too much. I thought maybe it would get it going and at least give me some sort of channel 2 but no dice. 

Sure is nice to have the paper manual. I got it in DJVU format but the paper is much easier to follow on a folded page. 

But at least it is working. The time base is fine, I am glad not to have to deal with that as well. Pretty soon it will be time to goop that wall wart in place, but I think I'll give the power supply a quick check. I learned the hard way if it isn't regulating right you get symptoms unlike most consumer equipment. In typical Tektronix style it seems to only have one reference for all the regulated supplies and they all reference to that. Actually that is a good design because all the power supplies come up together, but it can make troubleshooting a different experience...

One of these days I might just buy a scope. Though I would like to have the features of a digital scope I would rather have a standard CRT scope for a few reasons. 

Anyway,thanks for the idea. I had no idea those wall warts could handle that level of voltage. It works and it doesn't chock you, that is good. Now to find some PNP transistors with a gain of 5 ! Is there a way to decrease the gain ? At least they are common collector (don't say it) stages so it shouldn't be all that critical.

2N1193, not 2B...

Gain of a couple hundred but bandwidth is very low. 

Perhaps a clue - when the unit is operated with those transistors removed the channel oscillates at a few MHz. 

Might as well fix it, or I can look for a 525 volt rated lytic for the Eico capacitance bridge. I can measure ESR just fine but i have nothing to accurately measure capacitance, those little caps DO go bad sometimes. 

On Saturday, 3 February 2018 10:47:10 UTC, jurb...@gmail.com  wrote:

> >"Usually tjose devices are rated at 2500 V and only tested for a short time (1 min ?). "
> 
> I'll let you know what happens. 
> 
> >'Use an (auto)transformer to convert  5 V to 6.3 V. "
> 
> Don't have one and have no idea where to get one. 

easier to rectify the ac and add the right size cap to raise the rms value.


NT

tabb...@gmail.com wrote:

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

> 
> > >'Use an (auto)transformer to convert  5 V to 6.3 V. "
> > 
> > Don't have one and have no idea where to get one. 
> 
> easier to rectify the ac and add the right size cap to raise the rms value.
> 

** Only works out if the winding is lightly loaded. 

Rectifying and filtering an AC wave means current is drawn in pulses instead of continuously - causing a much higher peak voltage drop. Add in bridge diode losses of about 0.8V each and you may not gain anything when dealing with voltages like 5 or 6V.  



....  Phil 

On Monday, 5 February 2018 02:12:04 UTC, Phil Allison  wrote:
> tabby wrote:
> 
> -------------------------
> 
> > 
> > > >'Use an (auto)transformer to convert  5 V to 6.3 V. "
> > > 
> > > Don't have one and have no idea where to get one. 
> > 
> > easier to rectify the ac and add the right size cap to raise the rms value.
> > 
> 
> ** Only works out if the winding is lightly loaded. 
> 
> Rectifying and filtering an AC wave means current is drawn in pulses instead of continuously - causing a much higher peak voltage drop. Add in bridge diode losses of about 0.8V each and you may not gain anything when dealing with voltages like 5 or 6V.  
> 
> 
> 
> ....  Phil

yes, on reflection it's too low V for that.


NT

tabb...@gmail.com wrote:

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

> > > 
> > > easier to rectify the ac and add the right size cap to raise the rms value.
> > > 
> > 
> > ** Only works out if the winding is lightly loaded. 
> > 
> > Rectifying and filtering an AC wave means current is drawn in pulses instead of continuously - causing a much higher peak voltage drop. Add in bridge diode losses of about 0.8V each and you may not gain anything when dealing with voltages like 5 or 6V.  
> > 
>> 
> yes, on reflection it's too low V for that.
> 
> 

FYI:

Not long ago, I needed to add a buffer stage to the output of a 1970s valve AM tuner. The original circuit included a pot-core based 9kHz whistle filter driven straight from the detector diode - so quite high impedance and a poor match to most SS audio.

What I came up with was a LF351 JFET op-amp connected as a voltage follower (1Mohm  resistor, input to ground) powered from the 6.3V heater line - one side of which was grounded. A pair of BAT46 diodes fed two 220uF electros producing +/- 8.5VDC rails. 

There was about 100mV of residual 50HZ ripple on each rail, but the IC boasts over 80dB of PSRR, reducing 100mV to below 10uV. 

The high impedance load brought the audio up to over 1V peak, so I added a 10kohm pot at the output to trim the level, hidden on the back.



....   Phil 

>"easier to rectify the ac and add the right size cap to raise the rms value. "

That yields a bit over 7 volts unless you have intentional ESR in the circuit. Much easier to use a 20 ohm resistor and have the CRT run on 5.8 volts. It still warms up before the sweep so I lost nothing. i expected it to warm up before the HV rectifiers, but it also beats the sweep. 

>"Add in bridge diode losses of about 0.8V each and you may not gain anything when dealing with voltages like 5 or 6V. "

I got some iN58XX series Schottkys that would do it. 

Fuck all that, it works right now the way it is. Once I fix the vertical circuit I will be satisfied. Problem is I don't have a hell of alot of experience with scopes other than the basics. This thing has the channel switching and all that, not just a regular amp.

On Monday, 5 February 2018 11:07:38 UTC, jurb...@gmail.com  wrote:

> >"easier to rectify the ac and add the right size cap to raise the rms value. "
> 
> That yields a bit over 7 volts unless you have intentional ESR in the circuit. 

What it yields depends entirely on the size of the cap.


NT