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!