Thursday, September 22, 2011

Bogen T725 transformer

I have found a little gem of a transformer for my radio projects: The Bogen T725 audio transformer.  I basically gives you the ability to match 8 ohm loads to a high(er) impedance source from about 150 ohms to about 40k ohms.  Specs below courtesy of where you can find lots of ideas on how to use these devices.
Bogen T725 Schematic
I was able to locate a number of these little dudes online ( and picked them up for future projects.  In the table below, all values in the primary are relative to the black tap (blk).

     Color      Resistance     Inductance       XL @ 300 hz      Rounded Value
   White  (WH)  1424.3 ohms      24     H       45.239k  ohms      40k    ohms 
   Gray   (GRY)  886.4 ohms      12.04  H       22.694k  ohms      20k    ohms
   Violet (VIO)  516.5 ohms       6.06  H       11.423k  ohms      10k    ohms   
   Blue   (BLU)  260.1 ohms       3.04  H        5.730k  ohms       5k    ohms  
   Green  (GRN)   81.8 ohms       1.565 H        2.950k  ohms       2.5k  ohms    
   Yellow (YEL)   56   ohms         787 mH       1.483k  ohms       1.2k  ohms
   Orange (OR)    38.2 ohms         398 mH         750.2 ohms         600 ohms
   Red    (RED)   26   ohms         197 mH         371.3 ohms         300 ohms
   Brown  (BRN)   18.2 ohms          98 mH         184.7 ohms         150 ohms

   Pink to Pink    0.5 ohms        5.23 mH          9.86 ohms           8 ohms 

I plan to use one of these to get the B+ voltage from my National SW-3 off of the headphones and to provide an impedance match to a more typical 8 ohm speaker or headphone.

I also found a nice transformer P-T31 at Antique Electronic Supply that does 5k to 8 ohm impedance.  These were about twice the price and no taps on the primary.

Friday, September 16, 2011

Playing around with APRS tonight

I picked up my old TNC2 packet controller and dusted it off today.  After finding an old Hayes modem cable I was able to wire this dude up to my laptop.  About a million years ago, I registered a version of UIView32 which is a nice little APRS mapping package.

Of course there were no maps of the Western Washington area available for the software, but there is a nice feature where you can drag/drop a map bitmap onto the software and then define the lat/lon details for either the upper/left, lower/right corners of the map or by clicking on two points on the map and defining the lat/lon for those points.  I took a screen shot of a google map and defined two points.  Not totally precise, but close enough for giggles.  Here is a peek of the map after it has had a little while to listen on 144.39 Mhz to the APRS traffic.

I have an old Alinco dual-band rig that I inherited from my dad (W7QJC) R.I.P. that I might press into service for packet radio playing around.  Gotta get the transmit portion working next.  Just need to set some jumpers to match the microphone wiring and plug it in.

Wednesday, September 14, 2011

ArisSat1 heard this morning

This morning I had some fun with my Arrow antenna and an old Yaesu 2 metre hand-held transceiver.  I generally track a hand-ful of Ham Radio satellites, one of which was recently launched from the International Space Station (ISS).

This morning about 10:20 Pacific Time, we had a nearly overhead pass of the satellite giving me about 10 minutes of time to to track the bird and listen to its beacons.

At the appropriate hour (and minute) I was the image of tin-foil-hat-geekdom standing out in my yard with a pair of headphones on listening to a handheld radio whilst pointing a very strange contraption (the Arrow Antenna) skyward.  I live on some acreage, so I don't have neighbors withing spitting distance, but nevertheless, it was a bit of a "moment" for me wondering if anyone watching would think the old man has taken leave of his senses...

But, the sights being whatever they were, I was having fun...  I had a great copy on the bird, but didn't happen to plan ahead sufficiently to enable me to record the pass.  Even with the tall trees around, I had no problem copying the pass and sent off for my reception report giving two of the (english) passwords copied during the pass as proof of reception.

If you have not tried to copy ArisSat1 yet, it is very easy even with a handheld receiver and a modest vertical antenna.

The next round for me will be to try and catch some of the telemetry.  I will have to look at the satellite status page to see if the transponder is still functional, but a contact or two over the bird would be fun.

Tuesday, September 13, 2011

And now for something completely different...

Reading the blog, I came across today an 8080 emulator that is implemented (of all things) in JavaScript.  I can load it up in a web browser and load a working CP/M OS image and found myself soon playing Zork1.

Cracks me up to think about running an emulation of hardware that I wrote code on for a living, running a predecessor to MSDOS operating system and playing an adventure game all written in an emulated language embedded in my web browser...

Geeze, I am getting old...  :\

Powering up the SW-3 (again)

After discussing with my buddy Eldon, I have decided not to use the voltage divider approach to power the filaments.  They are all wired in parallel and the risk from one of them opening up and thereby taking out the entire set of tubes is too great, not to mention the royal pain in the tush finding a 1.15 ohm 15 watt resistor will be.  I decided to order a 2.5 volt 6 amp transformer from Mouser for $13 today and will use it to power the tubes.  The other option would have been to rewire them all in series and use a voltage divider to come up with 7.5 volts from a 12 volt source.  At least if a filament opens up we power everything down, plus the voltage divider would be easier to construct without having to locate 15 watt resistors.

I love a few select old pieces of equipment, but powering them can be a pain in the empennage.  I have a couple sources for the B+ 135 volt supply, so we should be good to go once the transformer arrives.

Meanwhile, I am on the lookout for an old National "doghouse" power supply for the SW-3.

Monday, September 12, 2011

Powering up the SW-3

A while back, I posted a note about having found an old 1930's National SW-3 receiver.  The rig came without a power supply.  I need to have 2.5 volts at about 3 amps to power the tube filaments.  I have a 6.3 volt supply at 4 amps, but need to bring this down to 2.5 volts at 3 amps.  So, I thought I would just use a voltage divider. 

o--------------------+-----------------o  +6.3V
                     R1       3.8 volts across R1
                     +-----------------o +2.5V
                     R2       Load = 3 amp @ 2.5 volt
o--------------------+-----------------o 0V

So, how to design a simple two resistor divider?  Assuming that there is no load current:

    Vout = Vin * R2 / (R1+R2)

The problem with this is that our load DOES draw current (about 3 amps) and therefore this will not work because the load can be considered another resistance in parallel with R2.

The 10 percent rule is a standard method for selecting R1 and R2 that takes into account the load and minimizes unnecessary power losses in the divider.

So, the first thing to do is select R2 so that I2 is 10 percent of the desired load current.  This resistance and current is called the bleeder resistance and bleeder current.  In this example, the bleeder current is:

  Ibleed = I2 = 0.1 * 3 A = 0.3 A = 300 mA

Using Ohm's law to calculate the bleeder resistance:

  Rbleed = R2 = 2.5V /0.3 A = 8.3333 ohm

Now, we need to select R1 so that the output is maintained at 2.5V.  To do this, we simply calculate the total current through the resistor and use Ohm's law:

  I1 = I2 + Iload = 0.3 A + 3 A = 3.3 A

  R1 = (6.3 V - 2.5 V) / 3.3 A = 1.1515 ohm

Now considering standard resistor tolerances and value, this would indicate the need for R1 = 8.1 ohm for a 5% resistor and R2 = 1.15 ohm.

In terms of power ratings:

   P1 = V1^2 / R1 = 3.8^2 / 1.15 = 12.55 W
   P2 = V2^2 / R2 = 2.5^2 / 8.1 = 0.77 W

Did I do this right?

I am not sure what happened, but the first time I did this exercise, I (somehow) came up with R1 = 7.5 ohm 3 watt and R2 = 5.6 ohm 5 watt...

More coffee...