COLORADO POWDER

  - A "messy" transmitter-receiver built on an old Colorado TV chassis.

Please apologize me because I did not knew what I've being doing..... I had only 17 years old....it was '96-97... I miss those dull days....We had not Internet, and I had never seen it...It was hard to get info .... I just didn't frittered and wasted the ours in an off hand way...(just to plagiarize Pink Floyd on "Time")

As all the other rigs I made at that time, all components were found at the junk, and I REALLY mean JUNK .....

This rig is so confuse that I could not explain how it still working. I used this rig to work on the "pirate band" around 7.600MHz, a lot of good friends I made, maybe they don't remember me, because it is about 5 years without operating at that band... Hope to climb up to the ceiling of the building to place my antennas and get back to the "aether" soon....

There were 2 versions of it, the old version had a lot of bugs, I had no experience at all!

The latter modified version, made some years after was really much better, but it is not so good as it should be.

OLD VERSION:

TX

The transmitter at that time had 4 tubes, as follows:

Oscillator (At ~ 3.8Mhz) and doubler: EF183 (or 6EJ7)

RF power class C amplifier (Self biased): EL84 (or 6BQ5)

Audio pre amplifier: ECC83 (or 12AX7)

Audio power amplifier: EL84 (or 6BQ5) in A class operation.

I have no schematics of that TX version....

The oscillator is of the Hartley type, with inductive cathode fold back. The RF PA used the basic PI-network to match a 1/4 wave dipole with 50-ohm coax feed line. The B+ was abut 400V and the mA reading of the output about 45mA.... being optimistic, something close to 12W was fed to the TX line....

The modulation scheme was done with just a centre tapped reactance (the primary of a  push-pull amplifier), well, since the transformer itself (a Willkason) was done to work with the EL84, the impedance matching was really good. I had always good reports from the fellows of the pirate band, regarding the fidelity and the intelligibility of my modulation, but the modulation depth was not so good. Indeed, I recall (I still having the notes I wrote on my notepad at those days - a real notepad, by the way )  that one of the first things I did after buying my VOLTIX 420C-5 oscilloscope was measure the modulation depth. Since the measurement is just the relation of the RF sign itself with the modulated audio, I had not to worry about the BAD response of the VOLTIX 420C-5, the proportion is what matters.

I had not audio generator and the 400Hz sine wave came from the Multi-EQ, also home made. The maximum modulation depth I could achieve was about 27% ... too bad for good AM with low power rig....

RX

The receiver was trivial:

6BA6 - RF front end, with separate tuning variable capacitor

6BE6 - Mixer and oscillator - note that the TX and RX had different oscillators....

6EH7 - 1st, 2nd and 3rd I.F.s, running at 455kHz.

OA90 - Germanium diode used as envelope detector.

16A8 - Triode used as preamplifier and Pentode as AF output.

The coverage of the receiver was from 5 to 9 MHz, and I used to listen to some Short-Wave stations, just to save the SX-43 of being heavy-used.....

The days was passing and the +B dropping resistor of the RX section running really HOT, I have no Idea of what would came to be.... I blew up the power transformer.....

Well, after placing another one it never got to his prior shape, and I had to move to another city and never turn it on again at that shack.

NEW VERSION

These mods were done around 2002-2003. I had a little more experience, and the Internet was available for me..... too much information for just one mind only.... at those days in '97 I have no idea of what to do because I have no options to choose.... when doing these mods in 2002 I have no idea of which option should I use amongst many others that now I had.... (BAMA was in it's early days....unbelievable plenty of info...!!!!)

TX

The new TX is really different from the first one, a new power transformer and a new real modulation transformer to replace the old push-pull reactor gave new gas to this rig.

Now the modulator is using a 6BL8, instead of the 12AX7. The pentode section is preamplifier and the triode section is a phase shifter, to feed 2 EL84's in push-pull.

The RF PA still with the same 6BQ5 used prior.... now running 420V at 50mA .... the plate now runs a little red.... maybe 15W of RF power input went to the antenna.....

But, the main difference is that I did not had to turn up the modulation control..... at the medium I could reach 80% modulation easily.... it gave new possibilities. The next mod is to replace the 6BQ5 by a 6BQ6GTB/6CU6GT at the RF PA....maybe 30W can be achieved, running at 100mA....

RX

The main nightmare I had when making this RX was to get good gain without loose stability....once the IFs were tuned, I always had to decrease the gain (increasing some of the 3 cathode resistors of each IF tube) in a manner to avoid oscillations. Since the  RX had just a simple AGC, the good point of operation meant decrease gain down to a certain point....the compromise was less sensitivity, but the operation was smooth...

The revelation of what to do came after seeing the schematic of the Drake R-4 receiver. It uses a 12AV6 as an envelope detection and amplifier that feeds with proportional -C voltage to all controlled stages.... with different time constants for fast acting and slow acting discharge AGC.... I had simulated the effect on the EWB and I wanted to see it working on my RX. I built it almost as it is on the Drake schematic....I used a 6BL8 triode section as the AGC amplifier and the pentode section as another audio preamplifier.... Guess what??? It worked beautifully...I could increase the If gain...it never went to the unstable state....and the sensitivity now is better than my SX-43....the 2 speed attack-discharge time constants are better for AM (using slow attack) and for AM-DSB-SC (using fast attack). Some guys uses the AM-DSB-SC for AM operation, (when they run their Kenwood or other newer TRX's).

By the way, the name Colorado Power is because of the chassis used, (of a Colorado TV), and the Powder is because it could blow up at any time, for sure........)

Front view of the "messy" cabinet.

 

Upper View.....

 

 

Inner view of the chassis.... a real hand made assembling....messy....

dirty..... wonderful....

 

 

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