The Geloso projects

Click on the thumbnail images to see a larger image. Use the back button to return to this page.

When you have read this page visit the excellent Geloso web site by Tony, I0JX

Ham radio equipment photographs, schematic diagrams, dial scales and technical bullettins. Many of these are in both Italian and English

At the Adelaide Hills Amateur Radio Society Saturday breakfast and auction on the 30th of June I picked up an interesting project after some spirited bidding by fellows of my own age. It was a CW transmitter made during the late 50's early 60's and it used a Geloso 4-104 V.F.O. Many Hams of those days lusted after these and the companion receiver front end. It also came with the Geloso tank coil, tuning capacitors and the original magazine article it was built from. This was a reprint from "The Short Wave Magazine" August 1958. There was also an advertisment flyer from R.H. Cunningham. The PA stage was built according to the circuit in magazine article and used an 807 valve. There was no power supply or modulator.

Geloso 4.104-S VFO module

The transmitter as it was built all those years ago was very large so I have taken it apart and will build a new version probably with power supply and modulator. I have also recently picked up a Heathkit SB-10 Single Side Band Adapter and am intending to use this with the "new" Geloso 4-104 based rig. The V.F.O. covers 80, 40, 20, 15, 11 and the 10M bands and uses a 6CL6 as the oscillator and a 5763 as the buffer driver. An OA2 is required for voltage stabalisation.

Geloso Receiver Front End Module

One a recent Wednesday evening, chatting on a local 2 Metre net, I happened to be telling the folks about the VFO find and was offered the companion receiver front end. Now, I have lusted after one of these since reading about them in a 1965 edition of "Radio Television and Hobbies" magazine. The project had been resold at the AHARS auctions a couple of times and along the way it lost the dial scale and the dial scale cover. luckily Barry (VK5BW) remembered he has seen a dial cover like that in a junk box in his shed and sure enough, I now have the dial cover and the dial scale to a general coverage version of the receiver. Now, that bring up another question, somewhere out there in South Australia there was a general coverage version of this front end. The version used in the earlier "Radio Television and Hobbies" receiver. It also had lost two crystals that were supplied as part of the second mixer/oscillator circuit. As it has turned out the originals would have been no use to me.

I was given the paperwork for the Geloso G.209-R receiver and a copy of an article from "Amateur Radio" November 1959 describing how to use the module as a converter. There was also the flyers from R.H. Cunningham. As you can see from the photos, not much progress had been made on the project by either the original constructor or the subsequent owners.

Digging in my memory bank I remembered the original artical in "Radio Television and Hobbies" that had me lusting after one of these modules and so I started to search in my various archives and eventually found "The 1965 Amateur Band Ten" published in the May 1965 edition. This was an Amateur band version of a general coverage receiver that ran over three editions, January, February and March 1965. This earlier receiver used a general coverage version of the Geloso front end module (2615-B).

After reading the articles and studying the original Geloso receiver this module was used in, I decided there would have been an image problem if the receiver had been finished as it was so I did a lot more research and found how very clever Geloso had been with their design. They used two second converter oscillators, one on 5.067 MHz, the other on 4.133 MHz. This puzzled me for some time until I had digested more of the information on the Geloso G.209-R receiver. They did this so the oscillator would run either above or below the first IF frequency so upper or lower side band could be selected without re adjusting the BFO frequency.

Why cannot I use the two crystals that came with the front end assembly ?. The answer is in the IF frequency. The original receiver used an IF frequency of 467 KHz and I will have to use an IF of 455 KHz, the usual frequency used here DownUnder. I will have to have two new crystals made, one on 5.05 MHz, the other on 4.145 MHz. 455 KHz above and below the first IF frequency of 4.6 MHz. "Radio Television and Hobbies" did not like this scheme and used a tuning coil for the second mixer oscillator and provided a fine tuning feature that I see as not at all neccessary. The original Geloso receiver had a 3.5 MHz calibration oscillator using a crystal but "Radio Television and Hobbies" did not make provision for this feature. They did however strongly suggested using a mechanical filter. In the articles on the full coverage version also provided details of a filter using several IF coils in series. I have a Collins 455 mechical filter I am intending to use in my version.

Looking at the metal work as it was built, I can see the original constructor intended to add some features not offered in The 1965 Amateur Band Ten. The metal work has thirteen valve sockets fitted and so far I have worked out the differences were probably using 6AL5 valves in place of germainium diodes in the detector/AGC and noise limiter circuits and a calibration oscillator. I have some thin sheet brass to make a shield box around the second mixer oscillators after reading the various articles and having a close look at the Geloso receiver articles.

Back to the shack equipment pages
Back to the Ham Radio pages
Back to the Domestic Radio pages
Back to opening page

Written by and Copyright, Phil. Storr © Last updated 17th Spetember 2013