Name plate and tags

Well the remote control for our transmitter is officially finished. Today as a last detail I built a couple of name tags for the cables and a bigger plate to mount on the front panel.

They are not only for aesthetic purposes but also to keep things where they should be, as sometimes they drift apart with strangers.

I cut some pieces from a scrap lamp and used the alphabet punch on them. They certainly look better with a small touch of permanent marker.

I like how they lay on the cables. All of this gives me an old time vibe feeling.

Metal name tags installed on audio and rf cables

Sunday worklog

Today I started to build the rope guides for the lightning fixture. I have some nice ground rods from a textile machine that are perfect for this.

Rope guide with rollers

On the evening I went to Futura and cleaned up a bit the stage, moving most of it to one place:

The Museum Corner at Futura

Then I started to install real cable trays instead of our improvised version with halves of water pipes. I need to cut a small section with a special bend to accommodate the opening on the wall.

Cable trays

I emptied most of the rack cabinet and installed the patchbay I built earlier. I had to drill and tap another set of holes as the power outlet interfered with the movement.

I also finished that small table:

Scope

We have this old scope for the students. It’s been unused for a while as it behaved erratically and then stopped working completely.

After setting the trigger to a more or less sane value I had something on the screen but the controls where flaky. A heavy dose of our deoxit equivalent and twisting it sprang to life. I adjusted the dc offset (drifts a bit while warming up) and matched the channel gains as much as I could and called it a day.

Fixing a Hitachi V212 oscilloscope

Sunday work log

Today I worked mostly on that remote control for our transmitter and on a simple pulse conditioner for a clock distribution system on our university.

We have a central clock reference used to synchronize experiments among various buildings. It’s not always spot on frequency but most of the time what matters is that its phase is continuous. There’s a network of underground cables that take that signal wherever is needed. Some of them are good despite their age (the youngest being around 10 years old) but others are crumbling and drowned.

On the seismograph building there’s a clock slaved to that signal and sometimes it looses track of it and lags as many as half an hour. Just as an experiment we decided to use the PPS signal out of a small uBlox GPS. A friend made a small module with the gps and an arduino to initialize it and display some ancillary status and I built a simple level shifter that makes a ~12V pulse out of that 3v logic output.

I also worked on the housing for the remote control. I’m using a weather proof junction box. After a bit of fumbling I had all the holes to mount the boards, connectors, indicators and cable glands done.

Then I finished a couple of details on the boards and hooked up everything for a trial run. The buttons and rf sense worked fine. But when I tried the dtmf audio input I got data on the digit outputs but nothing on the interrupt pin.

At first I thought that the module was bad (I accidentally fed it with 12V for a brief moment) and I swapped another with the same result.

I moved both of them on the breadboard and they worked fine. I inserted a bare wire on the socket and every time I touched GND with it I got an interrupt. But when installing the module that pin was stuck low. Which is quite odd.

I probed with the adjacent pins and all appeared open as they should. But there’s an unconnected pad on the other side, and there was a dead short to that. I used a scribing tool to clean up but the short persisted. That other pad, while not connected to anything on my board, was part of the dtmf module and when tied to the interrupt output it loaded it to the point of not being useful anymore.

When etching the board I used some fine steel wool and a strand perhaps stuck on the top side between the socket pins. After unseating the module, arduino and other connectors I charged a cap (around 3300uF) and applied it between the pair of shorted pins. Surely enough there was a spark.

And from then on everything worked as intended.

DTMF Remote control and sequencer

Many years ago I built a remote control for our transmitter. It was pretty simple, just a dtmf decoder rescued from an old answering machine.

It served us well for a long time, however it was lost when we moved to a new location.

For the last couple of weeks I spent a while polishing the firmware and making new boards in KiCad. The original was very crude, just a simple on/off panel and delayed power sequence.

This one adds an RF sense and automatic restart, so if one of our amplifiers bails out on a power dropout we don’t have to do anything about it.

Yesterday I etched the boards. I ran out of toner and had to ask a friend to print the transfers. They came horrible but work fine nevertheless.

It’s amazing that nowadays buying an arduino and a preassembled dtmf decoder is cheaper than the single chips (and not counting the time to layout a more complex board).

All the design files are here: https://github.com/fm-futura/dtmf-remote-sequencer

DTMF remote

Dumpster heaven

If there’s a heaven I think I visited it today.

After running a couple of errands early in the morning I headed to Lisandro Olmos on the outside of the City to buy some scrap metal for my welding classes.

A couple of members of the group said good things about Grúas Mársico so I went there, it was also a good opportunity to travel a bit on this part, as I don’t know almost anything about it.

As I entered the warehouse I became speechless. There are shelves that extend up to the roof with parts from all kind of machines neatly organized on one side. On the other pieces of metal plate and tubing. Some big planers, milling machines and spot welders among them. And on the background, the tallest pile of industrial waste I ever seen in person. All of this on sale by weight save for a couple of weird stuff.

The shop was run by a very nice lady and her brother. This morning I only bought some iron but the next time I come around I’ll bring a small truck to pick some very , very interesting stuff.

Fixing an inverter welder

Today while working on the rack closet the welder died on me for no apparent reason. But, after power off it made a couple of noises and the fan ran for a moment.

For being a Chinese machine it actually looks quite good and neat on the inside. The boards have conformal coating and just a tad of flux residue on some pads. The only thing out of place was a blob of solder on a header instead of a jumper.

I made a couple of measurements, the high voltage section was fine but the control supply was totally dead, except for a brief moment when turning off the main switch. Traced back from that board until finding a small block with a forward converter (kinda odd, I expected a flyback). The only component with bad readings was the recovery diode. I swapped in an UF4007 and at the next flick it was alive again.

Of all the things that could go wrong with a welder on novice hands this was not expected.

Distribution Panel III

Now it’s time to cut the umbilical cord and properly link to the main feed.

I made a hole in the wall to house a small box. I also added another pair of breakers with an rcd to rewire the adjacent room (for other reasons it’s best to keep it apart from the radio panel).

As we couldn’t disconnect the aerial fuse at the power pole we worked on live circuits.

Fixing a microwave oven with a broken keypad

This is by far one of the most productive things I did this week outside of work (at least the one I can write about here).

A couple of months ago my ex gave it to me, it started with intermittent display issues and one day it stopped completely. I picked it up and stored it.

The other weekend I was in a bit of cleaning frenzy and I remembered that it was using valuable space on the shack doing nothing so I set to see if it had any hope of working again. Otherwise I’d take the transformer and dish motor, the magnetron would go to a friend and the rest sold as scrap.

This is the second time I fix a microwave oven and I’m amazed at the amount of grease and acid stench that accumulates inside them.

I bridged the safety interlock pads on the control board and powered it with an isolation transformer. It kinda turned on but was not responsive and only some digits were dimly lit. It was also very sticky.

After that I cleaned it using lukewarm water, detergent and a toothbrush, a scoop with a hair drier and then another bath with alcohol.

Now it works!

The keypad is a mess, besides being sticky and stenchy too the conductive traces were broken, like dissolved, on the connector side. For some models there are still replacements on the market but they aren’t cheap and also what’s the fun on that?

I peeled away the layers, traced it and make a replacement using tact switches. The decal will be glued on top of that. It works fine, there’s less waste (but I’m short of a spot welder) and off it goes to Radio Futura.

Distribution Panel II

Now it looks a bit more decent than before. It’s still hooked to the main feed using a 20 amp plug but at least doesn’t scare people.

Continue reading

Distribution Panel

We are about to make a lot of radical improvements at Futura.

One of them involves redoing the electrical installation. I chose to use separate breakers for lights and sockets per room, both downstream of an rcd. The upfront cost is bigger but the benefits later in flexibility are enormous.

Some of the materials have not arrived yet but to speed up things I built it on a piece of wood with DIN rails. I laced the cables instead of using zip ties just for the sake of it, I like the way it looks with a vintage air.

A statistical insight

I’ve been working during the weekends on an instrumentation frontend to precisely measure the resistance of an RTD sensor using a ratiometric approach.

After building it and waiting a prudential time to let it warm I saved an hour of samples (3600) and fired Octave.

The mean and standard deviation looked ok and while a plot showed a bit of noise it was well within reasonable limits.

Just for the sake of it I did a histogram and, oh the horror:

This is clearly not OK. It should be more like a Gaussian (the real formula is quite daunting but still retains symmetry) and that looks a lot like a bimodal distribution. Changing the number of bins does not help.

The ADC I used does not have a reference input so I make two differential reads and then take the quotient (I know… but it was the only one available when started).

Perhaps the input multiplexer is at fault? (the unused channels are grounded, so I discarded that as a cause). I repeated the experiment but this time doing a full run on each channel instead of switching them and this is the result:

Well, both are skewed so there’s something else going on.

Scoping at the inputs shows what seems to be AM at around 70MHz even without power applied (that’s on the tv broadcast band here) and it kind of makes sense because I didn’t use a shield. Head bangs on the desk.

Anyways, using a quick digital filter makes everything look nicer but I’ll still have to shield this:

The transient at the beginning is not going to be an issue, as in real life I don’t expect such a step change (from 0 to ~3k) and in any case the antialias filter will get rid of it.

On a second thought, those chunks skewed up are really interesting and I should spotted that as a failure symptom earlier.