18 February 2017

Raytheon 2051 Thyratron vacuum test

The WikiPedia page about Thyratrons mentions that they contain some gas. Either neon, argon, mercury or xenon. This means that the tube can be tested in the same way I do with Nixies, using the high-voltage AC generator.

Now that I built a HV-AC tester out of a CCFL driver crcuit the test is fast. And the picture shows the result:


My Thyratron lights up, so it contains some gas. Which one, according to the color?!

Only the lower half of the glass lights up this way, which confirms something I've read: the gas tends to leave deposits on the glass. I am now holding the tube upside down to see if something changes.



15 February 2017

Tektronix 7A13 plugin ... fixed!

Contary with what I stated in the previous post, I tried to fix the 7A13 plugin. I couldn't leave a burned capacitor in there.

Messages on the TekScopes list suggested a simple test: measure with an ohmmeter the impedance across the tantalum capacitor: if it is zero, then there is a problem. And it measured zero.

So I cut off the original C158 capacitor and promptly replaced it with a modern 100 uF 25V electrolytic. Why cutting instead of desoldering? Because pads might not sustain several re-heating and, in any case, that component will not be reused. Even if the board is densely populated, I could operate without disassembling it, that could have been a nightmare.

With the new capacitor in place, the meter reading was still zero. So I located another couple of tantalums and one of them was dead short even if it looked brand new. Off it came, and a shiny 10uF 16V electrolytic got in the scene.

No more 0 readings across those capacitors. Time to a test in the 7603 chassis ... GO! The baby lives, again.

Left: C158. Right: C165.
This 7A13 has at least one more tantalum that I should replace before I forget where it is located. Then I should do the same replacement in my other plugins, just in case!

Out of curiosity, the burned capacitor measures open circuit and does not smell: I suspect it went up in flames long ago. The smaller one measures short circuit.


11 February 2017

Small change in HP 5082-7300 clock firmware

The picture doesn't show "3" is weaker.
I always feel awkward when I flash a new firmware in a clock. That was the case again with my 4x HP 5082-7300 bedside clock.

After more than six months of uninterrupted operation, the tens of minutes display started loosing brightness and the decimal point burned out. In order to prolong its life I changed the code to switch off the display between 9.00 and 21.00 unless a large change in the incoming light is detected. In that case the time is shown for few seconds, and then off again.

Moreover during night hours the display is PWM'ed to reduce brightness, and heat dissipation is decreased as well.

The updated code is on github.

06 February 2017

Tektronix 7A13 module on fire (almost)

I spotted it in a drawer, so I tried to plug a 7A13 input module into my Tek oscilloscope chassis. In less than 10 seconds on the CRT a bright spot appeared and then all lights went faint. I shouldn't admit that I replugged the unit and tried a second time, and the experiment lasted just two seconds since all lights were faint since I pulled the switch (on this device you pull to power up and push to switch off). No smoke/smell was released.

Next I did two things:
  1. visual inspection of the 7A13 but nothing was obviously burnt (but I met many nice looking components)
  2. searched on TekScopes Yahoo group for similar failures
The search revealed that tantalum capacitors do fail abruptly, and they were used to decouple power lines inside the instrument.

Back to the book then! The user and service manual is a work of art itself, so it was a pleasant experience to go through it. Looking at parts list and their position I finally spotted a polarised capacitor (C518) hidden under wires and behind the module structure:

C518 is the burnt blob in the centre.


I really doubt it originally looked "burnt brown", with a shade of orange towards its pins. So, this is the starting point to fix this instrument.

I will not go much further, since I don't really need this module and it already had a broken relay when I acquired it (debugging and broken relay posts from 10 years ago). Needless to say, this piece of history is the result of 1960's and 1970's engineering and I am glad to have had the chance to see and touch it. Oh, is has a digital display too!


28 January 2017

Arduino Nixie voltage booster: 12 V to 200 V

Nixie PSU kits are cheap and robust, but sometimes they can be "too much" for a project: too big, too costly, too powerful . It is the case of single tube circuits, where the current requirement is pretty low and board/case space too.

The Net is full of projects that use the 555 in place of specialised IC's: a PWM signal controlling a MOSFET it's (almost) all that you need. Since my circuits are usually based on Arduino, why not use its embedded PWM generator? Again, there is at least one fully working Arduino code and diagram on the Net, and that's where I started (thanks to Ian of nixieclock.biz).

While I could get clean HVDC, it was too high: even more than 350 V! But but but it would drop to about 200V when current was drawn. It behaved like a far-from-ideal voltage generator with a high internal resistance.

All of this could be fixed with careful run-time trimming of PWM parameters in software after a thorough (software) calibration, but still the firmware would lack the real-time response provided by specialised circuits. So I went a different way.

I used a Zener diode to keep the voltage at 180 V. When the Nixie lights up it "pulls" the current its way, effectively cutting out the Zener. The trick here is to calculate the drop resistor Rz before the Zener in a way that it will load the power supply slightly less then the Nixie. When displaying a digit this Rz becomes part of the anode current limiting resistor you need to add anyway. Since the Nixie sustain voltage is less than the striking/Zener voltage, you need Ranode > Rz with Ranode = Rx + Rz. Just do the math in such a way that the anode current is a few hundreds of microA higher than the "stand-by" current.

Yes, I know I am wasting power into the Zener, but this way I keep control over the maximum voltage if something fails upwards in the PWM chain. Moreover, since I fully control the firmware, I can save power by turning totally off the voltage booster when nothing is to be displayed. Last but not least, as in the original circuit, a resistive voltage divider lets me read the output value though one of Arduino ADCs.

I will share the diagram in a second post on this topic.

12 January 2017

From Eagle to KiCad

For Xmas 2016 I gave myself a little time to test KiCad as a replacement of Eagle.

Even if I used it for very few hours, I could not get the right feeling with Eagle interface and most of the boards in my wishlist would not fit the free version limit.

So I jumped in the Hack-A-Day tutorial series on "Creating a PCB in Everything" and tried KiCad. I love their old-school keyboard shortcuts. I love the way the interface responds when drawing a schematic diagram. I could easily (cough... I had to) design my own symbols for Nixie and VFD and their respective footprint.

So far it has been a positive and productive experience with ZM1332/5870S and IV-6 drawn in KiCad, both symbol and footprint.

I need to define a couple more tube display components and then I'll work on my first PCB to send to a fab house.

Meanwhile I keep up-to-date my own KiCad libraries on github.

09 January 2017

Nixie Tube Ciapapuer - 3 - final firmware

Lately I have pushed to github a new version of the Nixie Tube Ciapapuer firmware.

The main change lies in the way the random number generator is initialised, that now relies on an external library called Entropy. For simplicity I added the library in the github area too. It relies on small speed differences of oscillators within the Arduino microcontroller.

A second change is that the depoison routine (or slot-machine effect) can go either backwards or forward, and it's all chosen randomly.

That's it.