This page is dedicated to all those
Grandads (and Grandmas) who learned radios in a rougher era.
This is the oldest BC-312 I have ever found. The most common seem to
be from 1943 and 1944. This type of receiver (or its line voltage companion,
the BC-342) can be seen in the lower center of the photo above.
A detailed close up of the name plate can be viewed
here.
Here is a high resolution line drawing of the orignal BC-312, scanned from an SCR-210 Manual (TM 11-272, February 23, 1942). (here)
In the interest of documenting the BC-312 series, as some have for the BC-348,
I have started a table of BC-312 and BC-342 that
I own as well as other photos of nameplates I have gotten from Ebay and others.
Eventually it will be great if the table would have all the units. According
to the documents I have (TM 11-850 manual supplement of 8 April 1954) the
following units were produced:
BC-312, -A, -C, -D, -E, -F, -G, -J, -L, -M, -N, -HX, -NX
BC-342, -A, -C, -D, -F, -J, -L, -M, -N
BC-314, -C, -D, -E, -F, -G
BC-344, -D
The BC-342 was modified for use in OA-65/MRC-2 and OA-65A/MRC-2 and ended
its life as an R-336.
These radios are often found at flea markets and ham fairs in fairly ragged
condition. Sometimes the dial calibration is so badly off that it is hard to
hunt the calibration points. To make it easy for myself, I slapped together
a small WWV spotter (2.5MHz harmonics), for quick testing. Click thumbnail
for schematic.
I took this a little bit further to the point of designing a small board. I
cleaned up the modulation by adding a nice hartley oscillator on the power
rails for a low-voltage CMOS gate. This gives nice harmonics out to past
50MHz. The board is here:
If you click on this photo you'll get a schematic of the board configured
for a 100kHz crystal calibrator, which emulates what a Heathkit
or Hallicrafters 100kc crystal calibrator would have done. I have lots of the
boards and I planning on making a parts kit. You can build your own by
using this manual:
(click here for PDF, MO=Mouser, DK=DigiKey).
It is set up to be a WWV spotter, or a 100kc crystal calibrator,
depending upon parts loading. The schematic shows the 100kHz loading.
Two AAA batteries mount to holders on the back side and supply enough power
to run for about 600 hours (with modulation off).
The kit parts are now here (March 25, 2008). If you wish to order a kit, send
an email to "novatech@eskimo.com" and mention the kit. The price is $17.95
each, with $2.00 1st class postage to the USA, for a total of $19.95.
You can purchase by sending the total payment to my Paypal account (please
mention "Crystal Calibrator" in your notes) or to the Novatech Instruments
address. Please do not call the sales office, which is for our "professional
products" only.
This reflects a new lower price due to volume purchases of the parts. (The Ebay
price is higher due to their fees).
Note: If you are in Washington State, please add $1.80 for sales tax to the
total.
If you want to build a complete "retro" crystal calibrator, using the
large style crystals of the past, go to the website of Brian Carling, AF4K,
at
http://www.af4k.com/crystals.htm. He has 100kHz HC13 and HC6 crystals
at reasonable prices.
Here's another board I just did. This is a simple 12-bit DDS board which
outputs a triangle wave and square in 1kHz steps from 1kHz to about 1.7MHz.
The concept was to build a small circuit, with crystal controlled accuracy,
that I could use to align old tube radios. Since radios are narrow band, I
decided to forego the sinewave ROM to make the circuit ultra simple, with
no custom parts. Everything is available from Mouser or Digi-Key.
Here is a small photo of the board and a Tektronix Scope screen shot of
the outputs at 256kHz. The board is being run from three AA alkaline batteries.
You can see the kit contents by clicking this link.
This kit is $27.95 when purchased direct from this website. Shipping will
be $2.50.
Shortly before and after WWII a modern farm radio design using a 90V "B" supply
and a 1.5V "A" supply became somewhat common. Most of these radios used a four
tube line up consisting of a 1A7 pentagrid converter, a 1N5 RF pentode, a
1H5 dual diode/triode and finally a power pentode (1A5, 1C5, 1Q5, etc.). These
radios were intended for areas without commercial electricity distribution;
hence, the farm radio name.
In an effort to play these radios in a manner similar to how they would operate
with batteries, the power supply in the photo below was designed.
This supply provides highly regulated B+ and A voltages for these farm radios.
Please see the manual
here for detailed information and
specifications. Note that this is a fully isolated, regulated, over-voltage
and over-current protected power supply.
Click
here.
Please note that this is for
"do-it-yourself" people, we don't supply the kit.
Note that lethal voltages are involved, so only use this power supply or your
homebrew version if you are familiar with high voltage safety precautions.
Check Ebay, seller "novatech-instr," to see if we are still selling these.
In 1939, the 1D8GT, a diode-triode-power pentode tube and the 3A8GT, an
RF-pentode, diode and audio triode tube, were introduced. These two tubes
replicated three of the four tubes used in the pre-war 1.5V farm radio. What
is missing is the equivalent of the 1A7GT (pentagrid converter).
Several manufacturers adopted the 1D8 and 3A8 tubes in various combinations
with other standard tubes.
For example, the Majestic 130 series used the 1A7, 1N5 and a 1D8. This allowed
them to build a transistor sized portable radio in 1939. This radio
was privately branded and sold in the Spiegel Catalog. Motorola (B-150 and
41H) and GE (HB412) went to a different configuration using the 3A8 in
combinations with traditional power output tubes (1Q5 and 1T5 respectively).
I decided that these would be a fun pair of tubes to build a simple radio.
I wanted a radio that would pick up local stations and play loud enough that
I could listen to a ball game or the news in my work area. I messed around
with several topologies: superhet, autodyne, homodyne, synchrodyne and TRF.
For local
stations, the TRF is the simplest and gives decent results. The synchrodyne
gave the best audio, but I cheated with an IC detector and PLL chip.
To get decent
selectivity on the TRF, I used a permeability tuned front end,
which (in the prototype)
was built from old slug cores salvaged from "hash chokes" and
hand-wound coils on ball-point pen bodies. (loaded Qs of about 100 at
1000kHz)
You can just as easily take apart an old car radio for the parts.
(I am building a two stage permeability tuner for production, which will
be nice for this kind of radio, or even a crystal radio set.)
The Tube Collector's Magazine (http:www.tubecollectors.org)
has recently published articles on the 12volt car tubes
(12volt plate and filament). I think they would make a nice version
of this radio. I might give it a try.
You can see the schematic
here.
Any set of battery tubes can be used in a similar fashion. If you look at
the 1U4 and 3V4 miniature battery tubes, you can get almost as much gain
as with the 4-tube equivalent. As built, I can get about 10 stations at
a comfortable listening level and reasonably low QRM.
It is not a DX set and doesn't have enough
selectivity for close-in stations, although it is good enough to separate
KJR (950kHz, sports) and KOMO (1000kHz, news) at my location.
I am gathering parts to be able to offer a kit for this, but feel free to
experiment with the design. I hope to have a kit ready by summer.
Look here for BC-229 stuff:
You can see the schematic here.
You can see how I parsed the manual and came up with a test bed
here and here.
The output is just two wires about 18 inches long, which I dangle close to the
antenna input. The modulation is pretty rough, but good enough to get a
distinctive tone on the radio. Since even a crummy junk box crystal is almost
always better than 0.01%, the calibration error is only 1kHz at 10MHz.
Farm Radio Power Supply
Home Brew Two Tube Farm Radio
(c) 2008 Grandad's Electronics, Seattle, Washington, USA
(a registered tradename of Novatech Instruments, Inc.)
