Cheap AndEasy 6.3 Volts

it works. I used the 12 volt.

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 reall y hefty cathode. It does go very bright and stays sharp, though at really h igh intensity it has somewhat of a haze to it.

It still shows up before the sweep starts just like it always did, so as fa r 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 i n 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 lik ely to work right. First of all the frequency i s way down, plus it has alo t 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 pape r 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 cons umer equipment. In typical Tektronix style it seems to only have one refere nce 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 tha t 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 th e gain ? At least they are common collector (don't say it) stages so it sho uldn'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.

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

NT

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** 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

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value.

stead of continuously - causing a much higher peak voltage drop. Add in bri dge diode losses of about 0.8V each and you may not gain anything when deal ing with voltages like 5 or 6V.

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 filt er 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 boas ts 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

e. "

That yields a bit over 7 volts unless you have intentional ESR in the circu it. 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 war m up before the HV rectifiers, but it also beats the sweep.

ng 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 ci rcuit I will be satisfied. Problem is I don't have a hell of alot of experi ence with scopes other than the basics. This thing has the channel switchin g and all that, not just a regular amp.

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

NT

Still that is two components instead of one. And if you use a cap like that it is not advisable to use a regular lytic. Something like a speaker crossover cap would be more suited, though in this application does not need to be bipolar.

It works now, all I have to do now is to goop the wall wart to something inside the cabinet, out of the way hopefully.

On Monday, 5 February 2018 15:46:08 UTC, snipped-for-privacy@gmail.com wrote: nt:

why? Do you expect to exceed ripple current spec at 100hz?

yep!

NT

Filters are supposed to filter. They are not supposed to be used as reactiv e components. First of all the tolerance is too loose, the capacitance can be double he rating and it is a pass. So you really can't predict what you' re going to get.

Crossover caps have an AC rating. I am not sure about the ripple current is sue but it seems the value is low enough that it might be an issue. But I d o know you can't use regular lytics for a crossover. They won't last.

And if you think about it, why not just use such an AC cap in series and let its Xc do the job ? With rectification it would be about the same anyway. But one resistor suits me just fine.

Actually if it was a high frequency switched mode I could capacitor couple the output to the filament and use caps that are rated more than 3,500 vol ts DC. Then the transformer doesn't have to take any DC.

But this is all academic. What was a bit surprising is the filament current . It must be around 600 mA which I think is high. Perhaps they have a diffe rent coating that needs to be hotter to work. If not, it seems like it wou ldn't last all that long.

But I got more fish to fry. I got one channel clipping and the other deflec ted clean off the screen. Next is a quick look at the power supply, see if it is regulating. I learned the hard way that in a scope when they say 15.5 volts they mean 15.5 volts, not 17, not 14, 15.5. Somehow, even though the circuit is completely complementary a voltage that is off can cause an off set. Maybe because of slight differences in the transistors. I should find more suitable ones. The originals have a gain of 5 and a bandwidth of 300 M Hz, the ones I stuck in have a gain more like 200 but the bandwidth is low, something like 2 MHz. Its intended application was audio. Hell, they use a higher bandwidth transistor for audio now. In fact I do't know if you can even get one with a bandwidth that low.

Of more concern would be finding one with gain that low. And I don't know o f any way to decrease effective hfe in a transistor. Any bright ideas on th at ?

i wonder what is the lowest current gain transistor available now. i know s ome of the switching transistors are that low but those are not linear so t hey are probably not suitable. You should have seen when I tried to use a s witcher as a video amp. The gamma was off in a funny way.

Luckily it is used common collector (don't say it) so high hfe probably is not a concern, but not THAT much. 200 vs 5 ? I don't hold much hope for it working right. I was just looking for DC balance, getting rid of the clippi ng. That would tell me nothing alse is wrong. But I consider the indication s to be inconclusive.

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

** Power supply "filtering" refers to removing *supply frequency* ripple fr om a rectified AC wave in order to create a smooth, DC rail. Capacitors do the job of SMOOTHING by storing a small amount of charge when the voltage i s at it highest and delivering it back when the voltage drops away.

The process is fundamentally different to linear filters used in speaker cr ossovers etc.

In fact, caps used in DC supplies are often called "smoothing caps".

** And crossover ( bi-polar) electros have no place in DC supply smoothing. ** Cos that will not allow you to boost a voltage, only drop it. e ** Many scopes use DC to DC inverters instead of a full SMPS.

A standard iron core mains transformer feeds a linear PSU using regulator I Cs to produce positive and negative rails of 15 to 20VDC. This DC supply ru ns a most of the internal circuitry including an inverter, running at about 30kHz, that drives a ferrite transformer with a square wave.

The secondary of the ferrite transformer has multiple windings covering the high voltage needs of the scope. One winding, with only a few turns, suppl ies 6.3V to CRT.

Because of the use of square waves, DC rails derived from these windings ne ed very little filtering or smoothing, plus they are all *regulated* since the DC supply powering the inverter is regulated. The scheme is simple, reliable and rivals or betters SMPS efficiency.

My own 50MHz dual trace scope is built like this, its total power consumpti on is 14.5 watts.

.... Phil

On Tuesday, 6 February 2018 01:31:20 UTC, snipped-for-privacy@gmail.com wrote: NT:

ive components.

that's meaningless

rating and it is a pass. So you really can't predict what you're going to get.

sure, measure it. Or for anyone that lacks an RMS meter, just stick larger caps on until it glows the right colour & emits ok. The CRT, not the cap :)

issue but it seems the value is low enough that it might be an issue. But I do know you can't use regular lytics for a crossover. They won't last.

aye but that has nowt to do with it

he same anyway. But one resistor suits me just fine.

you could do lots of things

e the output to the filament and use caps that are rated more than 3,500 v olts DC. Then the transformer doesn't have to take any DC.

???

nt. It must be around 600 mA which I think is high. Perhaps they have a dif ferent coating that needs to be hotter to work. If not, it seems like it w ouldn't last all that long.

ected clean off the screen. Next is a quick look at the power supply, see i f it is regulating. I learned the hard way that in a scope when they say 15 .5 volts they mean 15.5 volts, not 17, not 14, 15.5. Somehow, even though t he circuit is completely complementary a voltage that is off can cause an o ffset. Maybe because of slight differences in the transistors. I should fin d more suitable ones. The originals have a gain of 5 and a bandwidth of 300 MHz, the ones I stuck in have a gain more like 200 but the bandwidth is lo w, something like 2 MHz. Its intended application was audio. Hell, they use a higher bandwidth transistor for audio now. In fact I do't know if you ca n even get one with a bandwidth that low.

of any way to decrease effective hfe in a transistor. Any bright ideas on that ?

Add a D from b to e?

inverting them usually gets you low gain

inear so they are probably not suitable. You should have seen when I tried to use a switcher as a video amp. The gamma was off in a funny way.

s not a concern, but not THAT much. 200 vs 5 ? I don't hold much hope for i t working right. I was just looking for DC balance, getting rid of the clip ping. That would tell me nothing alse is wrong. But I consider the indicati ons to be inconclusive.

NT

Oops, bad continuity in writing there. I do not have o boost it because I am using a 12 volt wall wart. I don't have a 5 volt one. And this is better anyway. One resistor.

Still, I do not believe it is good for a "smoothing" type cap to constantly charge and discharge like it would with a rectifier and limited by less than adequate filtering.

ctive components.

that's meaningless "

Don't be so sure. Why can't you keep running a car battery dead and recharg ing it ? I'm thinking something similar might happento lytics.

r caps on until it glows the right colour & emits ok. The CRT, not the cap :) "

Can't see the filament in the CRT due to shielding that looks like it doesn 't come off easily.

ny bright ideas on that ?"

"Add a D from b to e? "

Well the transistor I got is germanium and I think so is the original, so m uch for that.

I think the voltage is too high for that. If you invert the transistor you need to stay under Vebo which is usually about 6 volts i a silicon, probabl y quite a bit less in a germanium. The working voltage is much more than th at. Plus, what happens to the bandwidth if you do this ?

that is its job.

NT

eactive components.

rging it ? I'm thinking something similar might happento lytics.

It doesn't

ger caps on until it glows the right colour & emits ok. The CRT, not the ca p :) "

sn't come off easily.

Any bright ideas on that ?"

much for that.

you not got any germanium diodes?

u need to stay under Vebo which is usually about 6 volts i a silicon, proba bly quite a bit less in a germanium. The working voltage is much more than that. Plus, what happens to the bandwidth if you do this ?

Veb is much higher with germanium, 20-30V. And inverting was done precisely to increase bandwidth, albeit at the cost of gain.

NT

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** Yep - charging and discharging an electro through it full voltage range 100 or 120 times per second is something you very rarely see as it approaches or exceeds the ripple current limit. The part will likely run warm and have a short life.

..... Phil