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.