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Old 13th Sep 2012, 1:36 pm   #101
Miguel Lopez
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Default Re: Building an oscilloscope at home

The sawtooth is generated by that circuit. On post #92 it is the input stage (Y amplifier) from which the sample is taken, from the wire named 7 (inside a circle).

I tested that signal by moving the pot and I notice that the sawtooth varied the point at which it started. But it also changed the amplitude.

I swap the valves to 12AU7 and it performed better.

Anyway there is a problem with this circuit as I have not been ablr to get sweeps faster that 100uS. Don't know if it is a problem with the simulation software or with the circuit and valves involved.
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Old 14th Sep 2012, 2:03 pm   #102
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Default Re: Building an oscilloscope at home

This is the complete circuit I intend to build. Y-Amplifier and X-amplifier. As said, I'm having problems with the X-amplifier as in some cases during simulation, the sweep is not linear (low frequency, see image of the LF sweep) and in other cases it simply does not oscillate (high frequency).

Y-Amplifier is working OK.

Any ideas?

Note: Mods, please remove the picture from post #92 in order to avoid duplication. Apologise in advance.
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Old 17th Sep 2012, 5:57 pm   #103
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Default Re: Building an oscilloscope at home

I have built the chassis for the EHT PSU. It consist of an oscillator (6N1P), and a power output valve (6N44S), driving a ferrite core transformer at 9 to 10 kHz to generate the 850V and 180V that I need to power the CRT.
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Old 19th Sep 2012, 1:56 pm   #104
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Default Re: Building an oscilloscope at home

I've studying about the time bases and I noticed that the Serviscope Minor uses a Miller time base. I'm understanding this time base, as I have read some books about this. If my problems with the time base of the PE65 remain, I'm gonna try this one.

I can not simulate the Miller time base in the PC as the software I'm using has not any pentode with the third grid separated from the cathode.

Anyway, I have to test physically all these circuits before make my choice.
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Old 19th Sep 2012, 3:11 pm   #105
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Ah, the Miller timebase, a superb simple idea, used by Sinclair in all of his CRT TVs. I was there when he did it, happy days.
 
Old 19th Sep 2012, 4:47 pm   #106
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Default Re: Building an oscilloscope at home

I found an interesting detail in the Miller time base of the ServMin. It uses a -600V rail. See the red circle. Why is this necessary?
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Old 24th Sep 2012, 5:28 pm   #107
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Default Re: Building an oscilloscope at home

I wired the EHT PSU this weekend. See attached picture. I had to remove the coil from the capacitor can because when it was inside, the oscillator did not oscillate. I think it was because the can acts as a short-circuited turn thus reducing too much the inductance of the coil. I placed it bellow the chassis with two wooden blocks.

But that leads me to another doubt. In radios all coils and RF transformers are inside aluminiun cans. Are not they affected by this? Then, why my reaction coil is affected?
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Old 24th Sep 2012, 7:27 pm   #108
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Default Re: Building an oscilloscope at home

Hi Miguel, I think it is to do with the frequency of operation. Somewhere, in one of the theory books (possibly Langford Smith) is a formula for the distance a screening can should be spaced away from a coil. It may also be related to skin effect.

Nicely wired EHT section, it does not pay to cramp things too much !

Ed
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Old 24th Sep 2012, 8:21 pm   #109
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Default Re: Building an oscilloscope at home

Quote:
Originally Posted by Ed Dinning
Nicely wired EHT section, it does not pay to cramp things too much !
Sorry mate, didn't understand this sentence well If you are concerned about how much close are the components soldered, don't worry. This PSU is only intended to produce 350V and 180V AC at 12 to 15kHz. The EHT will be got using multipliers.
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Old 24th Sep 2012, 8:21 pm   #110
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Default Re: Building an oscilloscope at home

The difficulty with a timebase is in getting it linear. The serviscope charges a capacitor via a very large resistor from a very high voltage (the -600v supply) Naturally it charges following an exponential curve, but in this case only a small amount of the 600v possible range is used, and the curve is straight enough.

Another way of putting it is that a high value resistor driven from a high value source is an approximation to a constant-current source.

Another way is to use a valve or transistor to make an active constant current source. Some of the older circuits were very inventive in combining current sources with oscillators. Mr Miller was reponsible for the idea of the feedback capacitor making an integrator circuit. Another way of linearly charging a capacitor.

The highly compacted "Miller-Transitron" timebase to give it its full name relies on putting a pentode into some modes which are simply not included in models of valves... the people doing the modelling were mostly targeting amplifier designers.

David
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Old 24th Sep 2012, 8:57 pm   #111
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Default Re: Building an oscilloscope at home

Quote:
Originally Posted by Miguel Lopez View Post
Sorry mate, didn't understand this sentence well .
Fortunately, I think Ed meant the opposite of what you suspected. In other words, he liked the way that you'd left a lot of space between components
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Old 25th Sep 2012, 7:57 pm   #112
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Default Re: Building an oscilloscope at home

Hi Miguel, yes it was a complement on your layout and wise spacing.

Ed
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Old 9th Oct 2012, 1:26 pm   #113
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Default Re: Building an oscilloscope at home

I have done some tests with the EHT PSU. I've got the sine wave in the secondary of the ferrite transformer. Need further adjustments which are ongoing.

I found something that I can not explain.

Using one 6N1P, the oscillator works fine. I could get 14kHz, 18kHz, and 25kHz changing the capacitors. No problem on that. But with another 6N1P, I only g3t the oscillation if I reduce the heater voltage to 4V. What's up with this?

On the other hand, I intend to replace the 6H44C by an EL84, because I know the later better than the other. Could the EL84 work OK at 18kHz? I guess it does, as it is an audio valve and 18kHz in in the audio range, but I would like to know your opinions.

Thanks
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Old 9th Oct 2012, 2:25 pm   #114
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Default Re: Building an oscilloscope at home

I think an EL84 should be fine at 18kHz. It's within the traditional audio frequency range, anyway.
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Old 9th Oct 2012, 3:10 pm   #115
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Default Re: Building an oscilloscope at home

Hi Miguel, please could you show us a sketch of your circuit. It sounds to me like either the thing is latching up or more likey it is biased wrongly for that valve. Some measurements of kathode, anode, grid voltages might help.

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Old 9th Oct 2012, 8:21 pm   #116
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Default Re: Building an oscilloscope at home

The segment with the 6N1P is the circuit that I built. The segment with the EL84 is the one I intend to build.

When oscillating, I've got 35Vpk in the 33K potentiometer.
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Old 10th Oct 2012, 11:32 am   #117
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Default Re: Building an oscilloscope at home

It would help if you could measure the voltage at the anode and also at the junction of the 27k and 6k8 resistors with the good valve and the bad valve. I'm guessing they're both used valves...

Also you could try putting a 220 ohm resistor in parallel with a 22uF capacitor in sereis with the cathode of the first valve. The valve might be drawing a bit too much grid current before oscillating, biassing it off slightly might cure it? I have to admit when I build oscillators they seem to require a lot of tinkering to make them reliable!

D
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Old 10th Oct 2012, 12:44 pm   #118
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Default Re: Building an oscilloscope at home

Looking at it again there is no auto-biassing of the first valve so if you have quite a large voltage at the grid it will be drawing a lot of grid-current (i.e. going very positive). It might be worth trying a 100k in parallel with a 1n in the grid lead...
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Old 15th Oct 2012, 1:39 pm   #119
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Default Re: Building an oscilloscope at home

Quote:
Originally Posted by Dominic Beesley
It would help if you could measure the voltage at the anode and also at the junction of the 27k and 6k8 resistors
Valve 1 (1981): 196 V (oscillation OK)
Valve 2 (1975): 194 V (oscillation OK)
Valve 3 (1973): 190 V (no oscillation)

Quote:
Originally Posted by Dominic Beesley
It might be worth trying a 100k in parallel with a 1n in the grid lead...
I did it. No oscillation with any valve. I increased the cap up to 10nF and it oscillated but the sine wave was highly distorted.

Quote:
Originally Posted by Dominic Beesley
Also you could try putting a 220 ohm resistor in parallel with a 22uF capacitor in sereis with the cathode of the first valve.
I didn't. I'll try it soon.

Thanks.
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Old 16th Oct 2012, 1:20 pm   #120
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Default Re: Building an oscilloscope at home

This are the circuit to supply voltages to the CRT (focus, brightness, filament reference, etc) and the voltage multiplier for -1500V .

The blue caps in the voltage multiplier were sent by Jim (thank you mate)
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