Sunday, May 12, 2013

Preparing to clean the septic system

Today was a day of rest, but tomorrow...

We need to clean the electrodes in accordance with the manufacturer's instructions:

     Using a plastic bucket combine one pint (.5 liter) of muriatic acid with two gallons
     (7.6 liters) of fresh water....

Something in the back of my head said, "it's not that simple." Thanks to McCallie for my chemistry lessons.

So quite a while later after googling about, I hope this makes it better for others:

We were told:
Here lies Gillian, still and placid;
Who added water to the acid.
Clever Jane did as she oughter;
Added acid to the water.
And the reason for doing this is not simply to do with splashing, per se: If you add water to acid, the first drops of water will react completely and exothermically with the acid, very likely boiling it. If, on the other hand you add the acid to the water, the acid reacts completely with the water, and dissipates. 

Results to be reported tomorrow...

Saturday, May 11, 2013

Boat Audio Nirvana: FM Transmitter

The boat's audio system is the Denon Stereo Cassette Receiver DCR-530RD.

It comprises an FM/AM radio, tape cassette drive, and offboard CD changer.

As a 1997 device it is from the dark ages. All my music now resides on the laptop or Internet feeds, such as Spotify.

So I needed a way to get audio from the laptop to the stereo system.

Easy solution:

For £20 a little Belkin TuneCast II FM Transmitter plugs in to the headset jack of the laptop and streams whatever audio to the FM stereo receiver.

About as good as it gets.

Some tweaking:

• The device) runs on AAA batteries, so we need to keep a supply. It comes with a cigarette lighter plug and can run on a 12V supply, but we don't have one of those handy near the laptop.

• Initially the signal was poor. There is a lot of discussion of this on various  forums. Basically, you need an antenna. The power cord serves as one option, but is a non-starter here, since we don't have a 12V power socket near the laptop. An audio extension cord for the audio cable works somewhat, but that is a lot of wire wandering about. So what to do?

• Well, rummaging around in the electrics bin I found a two-foot stretch of bare braiding. Stuffed the end into the 12VDC power receptable, and presto, a nice clean signal. But that is sort of a hack.

So went googling. There are several good YouTube presentations:

     ° Tracker
     ° Crash

But both refer to an older version with a "black box" that controls the signal strength. My unit is Model #F8V3080eaBLKP. The videos show how to open the unit correctly, but when it came to finding the black box it is now completely gone!

There are just some residual pinholes where it used to be.

But all is not lost: In its place there is a pad of some six test points in a rectangular grid:

I energized the unit as shown in the first video and started touching things with a stretch of 20 gauge wire. On touching the second pad in the first row the signal jumped up. I checked out the rest, but this seems to be the best.

So with some very delicate soldering, tacked a stretch of 20 gauge to that pad:
Then put the unit back together, and all is well.
• Next, adjust power, frequency levels. This may be witchcraft, but the engineer in me feels better to be operating in the middle of the operating ranges for volume and frequency.

So I set the frequency at 92.5 MHz, the PulseAudio Volume Control (PAVC) for Spotify at 90%, and the Denon volume level at 20.

Sounds great. YMMV.

• Next: Antenna length.

This takes me back to my amateur radio days. Under well established principles:

we have the wavelength of an electromagnetic ("radio") wave in meters as

where λ is the wavelength in meters, f is the frequency in Hertz, and ν is the speed in meters per second, typically taken as the speed of light:

      299,792,458 metres per second

which is very, very close to 300 million meters per second.

The convenient fact here is that if the frequency is in Megahertz (millions of cycles per second) and the speed of light is in millions of meters per second we can get rid of the millions and seconds, so it becomes simply

      wavelength (in meters) = 300 / Frequency in MHz

which for a nominal frequency of 90 MHz (90*10^6 Hz) gives a wavelength of about 3.33 meters.

This is consistent with the standard band data:

      HF 14 MHz      20m

      HF 28 MHz      10m

      VHF 150MHz     2m

So anyhow, setting out a metre or so of ordinary 20 gauge wire gives us a half dipole antenna.

That works much better than the braid. :-)


There is some farkling about required:

• Experiment with different frequencies. I started out mid range at 92.5 MHz, but for the length that I had cut the reception was noisy. Moving down to 80.1 MHz solved that.

• Experiment with various volume levels. A level of 20 worked for me.

• Experiment with antenna placement. Laying it out flat on the table worked for me.


So here is what you do:

1. Buy the transmitter somewhere.

2. Open up the case and solder one meter of wire to the second pad in the first row.

3. Close up the case.

4. Turn on the unit, to say 88.1 MHz.

5. Turn on and tune the FM receiver to the frequency you have chosen.

6. Enjoy.