Homebrew Valve Scope. Version 2.0

[Miguel Lopez]

Hello everybody

I'm here again with the valve scope. Hope not to bore you with this. Well, once the scope was finished, it worked OK for a couple of months (+/-). Then some day, it smoked and smelled burnt varnish. I checked it out and I found the EHT transformer was burnt. As to wind this transformer was very hard and time consuming (about 22000 turns + 5500turns) I decided that it was not a good idea to wind it again and expect for it to get burnt again.....well, I don't like to be a Sisyphus.

I decided to use another variant and I thought on the voltage multiplier that I had used at the beginning. As this voltage multiplier injected noise to the scope signal (it used the same HT PSU as the whole circuit), this time I need a diferent PSU. To avoid even to use the same power transformer, I rectified the mains directly and supply the heaters of an ECL82 directly from mains using a capacitor.
Remember: https://www.vintage-radio.net/forum/...=101895&page=2

The ECL82 triode was the oscillator and the pentode was the power output for the ferrite transformer. This circuit worked fine but it injected noise to the signal too. So, I thought that I had to obtain the EHT without oscillators, but without EHT transformer too. So I bet on for a 60Hz voltage multiplier.

As building a 4kV multiplier from 60Hz required (for me) a big amount of components, I decided to use PCB to build the multipliers. Then, as I had "betrayed" the Gods of point to point layout, I decided to betray at a bigger degree, so this time I used PCB for everything. So the scope would need a total refurbishing. Here some of the changes that I did:

1.-) Generation of EHT is throught 60Hz voltage multiplier. Using 1N4007 diodes and 10uF/450V caps, supplied from a 120V winding in the mains transformer. +4kV, -1,5kV and -600V are now generated with these multipliers.
2.-) PCB for everything.
3.) All supplies are regulated (except those for EHT).
4.) According to above mentioned:
- HT regulated at 290V-
- HT2 for input stage, regulated at 150V-
- Heaters: regulated at 18,9V CD (three valves in series on each section of the circuit. Series supply to decrease the total current.
- Heaters supply elevated about 35V- from -HT
- CRT heater: Regulated at 6,3V-.
- To further noise reduction, the X and Y circuits are shielded from each other, with a new steel made chassis.
- Ranges sellector are now attached to a PCB with the required component, to avoid the use of wiring and so reducing possibility to induced noise.
- Vacuum diodes from the sweep generator are now replaced by FR107 S-S diodes. (this is a betrayal to the Vacuum Electronics Gods too, I'm a heretic)
- The circuitry for CRT supply is also mounted on a PCB and all required potentiometers for adjustment of CRT characteristics are now directly wired to the PCB.

In general, the circuits for X and Y amplifier are the same as in version 1.0, but this times they are mounted on PCB. Now I will be showing some pictures of the new version.

[Miguel Lopez]

Here some pictures of the new version of the scope.

In the first picture you can see the PCB that supply the CRT voltages from the -1,5kV PSU. The potentiometers above adjust the characteristics of the CRT trace such as curvature, max. amplitude, etc.

The second picture shows the different PSUs bellow the steel-made chassis. I used S-S regulators cause they are easier to work.

The last picture shows the steel chassis, which was made with 1mm thick steel sheet and rivets to join the parts.

[ValvoStef]

Nice and very impressive project, Miguel.

[mole42uk]

As you have done all the way along this project, it's another impressive solution. I have always held in high regard people who 'think outside the box' and develop elegant solutions to problems using materials that are readily available.
Well done!

[Miguel Lopez]

Thanks for your kind words.

This is the chassis from the other side (opposite to the CRT).

The transformer is the same as in version 1.0 but this time the heaters winding is 18V~ instead of 6,3V~, and it has another winding (120V~) to supply the multipliers.

Those 3 big capacitors are the HT filter, connected through 100 resistors. Ripple is negligible. They are mounted on a double sided PCB. On the bottom side of the PCB it is the bridge rectifier (1N4007) with their respective paralleled capacitors and a bleeder resistor

[Miguel Lopez]

I had a problem to upload pictures but I think it is solved now.

Let's go on. This is the Y amplifier. As said, similar to the Telequipment S51

[Miguel Lopez]

This is the basic circuit that I used for the HT PSU. I did some changes. Instead of LM317, I used LM7815, and instead of LM329, I used a 15V Zener diode with a 1N4007 in series, for temperature compensating.

Resistors are adjusted for my purposes. The circuit is supplied with 335 to 350V DC and it delivers 290V DC regulated. Two MOSFETs are connected to the output. one for the Y amplifier, and the other for the sweep generator. A voltage divider with suitable resistors and another MOSFET at their junction, provides the 150V which are also required for the Y amplifier.

This circuit is from National Semiconductor Linear Brief 47 (1995)

[Diabolical Artificer]

Well done Miguel, like others have said it is ingenious how you have solved the EHT problem you had by thinking outside of the box. You have obviously put in a lot of research and to find your solutions.

Did you have to thermally couple the 15v zener to the 1N007 ? By putting in close together?

Andy.

[Miguel Lopez]

Well, have to rectify something. I used a 1N4148 diode instead of a 1N4007. Sorry :red face:

Yes, they are very close together to be at the same temperature.

[dominicbeesley]

A very impressive effort.

Miguel, please could you put the pictures on the forum using the Go Advanced button - if you have problems with them you may need to reduce the resolution using a photo editing program...I always do that as it reduces the upload time.

Mods, the ImageBam site has just tried to install a trojan on my computer - I think it would be wise to remove those links and put the pictures on the forum. It is a notoriously virus riddled site...

[AC/HL]

I've uploaded the images. Although there was no malicious activity this time, we do prefer them this way.
If not for this reason, then because the subsequent loss of the third party site can render the thread useless.

[Miguel Lopez]

Sorry if Imagebam gives problems. Not my intention. the problem is that the "Manage Attachment" button on the advanced reply window is not appearing on my computer, so I can not upload the images as I did in the previous post.

Don't know the reason. I apologise, but then how can I upload the pictures?

[Paul Stenning]

Originally Posted by Miguel Lopez

Sorry if Imagebam gives problems. Not my intention. the problem is that the "Manage Attachment" button on the advanced reply window is not appearing on my computer, so I can not upload the images as I did in the previous post.

Don't know the reason. I apologise, but then how can I upload the pictures?

Try enabling Javascript (or disabling whatever Javascript blocker you have installed) for this website.

[Miguel Lopez]

Well, it seems that is a problem with Firefox. I'm using Chrome this time and the button does appear. Let's go on.

The circuit for the X sweep generator is the same of the Version 1.0. It is the one of the Telequipment Serviscope Minor with some changes, but this time it is on PCB. You can see it in the pictures.

[dominicbeesley]

I like your PCBs Miguel! I'm guessing you're hand drawing them. Do you buy new PCBs or have you found a source for copper covered board?

[Miguel Lopez]

Yes, I draw PCB's by hand. There's no way to buy PCB here, but I'm the lucky guy who picked up a lot of PCB residual from a Cuban factory during the early 90s. I have a lot, and I mean a lot, of small pieces (some of them not so small) of PCB both one-sided and double-sided.

[Miguel Lopez]

With PSU, CRT, X sweep generator and Y amplifier on their respective places, there's nothing else to do but adjust both time and voltage ranges. On Version 1.0 I used point-to-point wiring and there was a lot of wires eager to get induced noise on them, so this time in order to reduce that mess, I used PCB too for the range selection.

The better way I found, it was to attach the PCB directly to the selector pins. (see the picture). The Y ranges (voltage) requires just one PCB as I just need to change resistors for the voltage divider. It is not frequency compensated cause I do not intend this scope to be a standard, maybe for a Version 3.0...........

The X range selector requires two PCBs as I need to change resistors and capacitors, so you can see in the picture the way I found to do this. Very simple, one PCB on each section of the selector, and the use of small caps to fit between the two PCBs.

[Miguel Lopez]

Here is the front panel. I used the same as in Version 1.0. The markings are made of a photograph, designed in Photoshop and then covered by a transparent plastic piece.

[Miguel Lopez]

This is how it looks like on its working place. On top of my bench.

[Al (astral highway)]

Once again, a phenomenal success, Miguel. Kudos to you, full of respect, particularly for your ingenuity, staying power and the way you overcome even some really tough problems with whatever you have to hand...

What is the bandwidth of your scope, I forgot? And what are you working on next?

Did you make some of the other equipment up there on your bench?

Cheers,

Al