06 February 2015

Cheap assorted resistors kits: a *real* bargain?

Not long ago I bought a pack of assorted resistors: 56 values at 1% 1/4W, 20 pieces each. The item name was something like "1120Pz 1/4W 1% Metal Film Assorted Resistors kit Set 56 Values (1 ohm~10M ohm)".

The whole batch would be a good companion for building my BITX20, but I lost 15 minutes looking for the 120kR strip. I gave up and luckily I found one 120kR in my junk box, but what?

I went back to the item description and cross checked what I actually bought:

1R, 2.2R, 4.7R, 5.6R, 7.5R, 8.2R, 10R, 15R, 22R, 27R, 33R, 39R, 47R, 56R, 68R, 75R, 82R, 100R, 120R, 150R, 180R, 220R, 270R, 330R, 390R, 470R, 510R, 680R, 820R, 1K, 1.5K, 2.2K, 3K, 3.9K, 4.7K, 5.6K, 6,8K, 7.5K, 8.2K, 10K, 15K, 22K, 33K, 39K , 47K, 56K, 68K, 75K, 82K, 1M, 1.5M, 2M, 3.3M, 4.7M, 5.6M, 10M.
Look at the "hole" in the series: the whole 100k to 820k batch is missing!!

Sure enough those values can be combined by summing other resistors, but it is very inconvenient!

My mistake. I did not read all the values before shelling out 4 USD@2014. Probably other sets for sale do include the 100kR serie. Pay attention, so you will not end up with a lifelong supply of sub-10R and above-1MR resistors.

04 February 2015

BITX - first changes for my yet-to-be-built v3

Since the BITX v3 PCB arrived earlier than expected, I was not ready to build it. I dug out a decade old bag of components containing 10 MHz XTALs, diodes and a lot of BC547B: they all had been collected for my original BITX construction, that has never begun. So, instead of buying the parts listed in Sunil's documentation, I started checking if I could still use those "old" components from the original BITX design.

Well, it turns out that BC547 is equivalent to 2N3904 and the pinout is just the other way around (EBC vs CBE). Moreover my batch of BC547B has an higher hfe than 3904 found in the NPN drawer. So, there goes the first change. It implies that for each transistor I will need to double check where E-B-C should actually go, also considering that the soldermask contains errors for some Qn so I cannot simply put it in reverse.

Obviously I do not have a 2SC2570A. The original BITX project uses a more common NPN like 2222, but later discussions call for BF199 that happen to lay in another NPN drawer of my parts box. Need to check the pinout as well.

Third digression from the official v3 documentation will be in diode mixers. In 2011 I bought a pair of ADE-1 mixers and, curiously enough, they are used in BITX v3B and v3C. There is even a wiring diagram to use ADE-1 mixers in a v3 board: this means three transformers less to wind. I am not afraid of winding transformers, but that means fewer sources of possible trouble.

03 February 2015

BITX - first steps, jumpers

The first step in building a BITX v3 (on PCB) is to install jumpers. You can see that I got better in the process of cutting to proper length since the first couple of jumpers came up quite wavy:

I could not stop soldering and you can see the beginning of the BFO area being populated on the left-hand side of the picture.

22 January 2015

BITX, I'm in

Ten years six months after joining the BITX transceiver project mailing list I decided to actually build one. A decade ago I had even matched the XTALs for the IF filter, but other interests hijacked my time.

Ten years later there are (several) PCB designs available, and I could not resist a sale price of 10 USD@2015 for BITX version 3. I don't mind if it is not the latest version with frequency counter, DDS or other amenities. I want a QRP SSB RTX I can carry around without fear of breaking it but, most of all, I am after the fun of building it.

The PCB set is on its way from India in a Registered letter. India Post tracking system seems a bit slow with database updates since 14 days after shipping there is still no mention of my parcel. Meanwhile I study the documentation and gather components.

13 January 2015

A variable PSU with LM2596 adjustable DC-DC step-down regulator?

Now that I have a lab PSU with an adjustable regulator inside, how about bringing the voltage control to the front panel?

The voltage regulator module is based on LM2596-ADJ and sells for ~3 USD@2015. Output voltage is controlled via a 10 turns 10kohm trimpot.

The datasheet says that the adjusting resistor must be connected as close as possible to the LM2596 (and away from the inductor). Moreover it is completely floating off GND, so any modification is out of question.

It is better to look for a different step-down adjustable circuit, or use a long shaft to bring the control to the outside. Nevertheless, multiturn trimpots are not meant for daily usage, but rather for a one-time set-and-forget application.

12 January 2015

(27 y.o.) PSU update

The aforementioned LED strip in the shack needed a 12V power supply. Since I had wanted to turn one of my fixed PSU into variable (or multi-output), I jumped in and applied the mod.

The chosen unit is my "rms K132" linear PSU:

It contained a 7812 regulator produced in mid 1987. Since it never needed a repair, the unit should be ~27 years old! That's when I started on 27 MHz Citizen Band. I remember the price was a linear function of amperage: 2A = 20'000 (good old) Lire; 5A = 50'000 Lire. By the way, 20000 L@1988 ~= 20 €@2015.

The idea: replace the 2A 7812 linear regulator with an adjustable switchng board of equal continous current capacity. I used a board based on LM2596-ADJ and it fits comfortably given the large PCB and the amount of free volume inside the PSU.

One more change was to replace the red LED with a yellow one, as a warning sign that the PSU is NOT 12V anymore.

For the LED strip I tuned it down to 10V, and I probably go even lower. The FT817 receiver does not complain of this switching PSU since there is no added noise vs my usual RTX PSU.

DISCLAIMER. This operation was made inside an high-voltage AC unit. ALWAYS disconnect the power cable from the AC socket and dischage the large capacitor before touching anything inside it. I will not take responsibility for injuries or damage to anything/anyone if you decide to replicate or reproduce what I have shown above.

11 January 2015

LED strip, a little discovery

Last Friday I received my first roll of LED strip. 5 metres of warm white 5050 LEDs, out of which I need only 1.10 m. When preparing my strip for deployment I noticed that it is not made of single, long, uninterrupted material: it is a sum of 1 m long sections.

At 12 Vdc the strip is pretty bright and does produce a decent amount of heat. No wonder people complain these LEDs loose luminosity! Heatsink is required for a long LED life.

Since I do not need that much light I reduced the supply voltage to 10.0 V and the result is still too much for night work.at the laptop.