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Broadcast transmitters come in all sizes, some are enormous, running hundreds of kilowatts, with a wide coverage area, and are expensive to build and maintain. However, in certain sparsely populated areas, where only a small town is to be served, rigs of only a few hundred Watts are used. The subject of this story is one such little transmitter, which had apparently been working for about sixty years in a small town in Central Nigeria. It all started when a Nigerian businessman showed up at the Harris offices with a briefcase full of money, and a request for an Engineer to travel to Zaria in Nigeria and perform a frequency change on a pair of 10 kW medium wave transmitters. I would normally only be working on TV transmitters, but since I was in the office at the time, and was familiar with West Africa, I was asked to do the job. The naivety of office bound management was clearly demonstrated when the job was accepted without reservation. The briefcase full of money took precedence over such mundane questions as the provenance of the two transmitters which could only have been moved from another site in the country and might very well have been stolen. Before leaving, I made my reservations known to the divisional manager, predicting that the transmitters would be in derelict condition, that there would be no suitable power service to run them, and there would probably be no aerial available on the site. In the event, all my predictions proved to be correct and it was impossible to do anything other than make a list of parts, which would be required to repair the two derelicts before anything else could be done. Nothing more need be said of that particular fiasco. On the site where the two transmitters were stored, I found an old 500 Watt medium wave Gates transmitter. It was the oldest working broadcast rig I had ever encountered, and having nothing better to do, I took some photographs. The old single cabinet transmitter was working in a small non air-conditioned roomlittered by years of dust and lizard droppings. Opening the rear door revealed the inner workings of the old beast. All things considered it was in remarkably good condition for its age, and clearly somebody had been lovingly dusting the components over the years, even if they had ignored the rest of the room. The aerial system was a simple Marconi inverted ‘L’ supported by two poles about forty feet high. An inexpensive but effective arrangement for the relatively small service area. The installation must have dated to the colonial era, when the BBC was providing planning assistance to the West African territories. The old relic has probably been working with minimum maintenance for 60 years, and if left alone will probably continue to operate for another 60.  Gates 500 GY MW broadcast transmitter
The Gates 500 GY was produced in Quincy Illinois in 1948, and is a classic American design of the time. The Programme Input Rack appears to have been stripped of its equipment over the years. These relatively inexpensive transmitters were sold all over the world, as well as being popular in the US domestic market for small town radio stations. Many of these radio stations, which operated on high power during the day, were required to reduce power at dusk to avoid interference with ‘Clear Channel’ stations. It was easier to keep a small transmitter for night time operation than to try and reduce power on a larger rig. Today they are sought after by Boat Anchor enthusiasts, and some have been converted for operation on top band AM.  Power supply components The power supply components are mounted on the floor of the cabinet. The windings of the transformers and chokes in the older Gates transmitters were inclined to short to the frame. In this example, the HT transformer appears to be standing on a wooden insulator.When isolated from ground, the transformer will work happily for years, but is a death trap for anybody who might come in contact with it when the transmitter is switched on. The original 8008 mercury vapor rectifiers have been replaced by silicon rectifier units at sometime in the past. The row of four valve bases for the old rectifiers, can still be seen on the left hand side of the picture below the oscillator chassis.  Master oscillator assembly
The master oscillator assembly is mounted on the left hand wall of the cabinet. The black plug-in crystal oven can be seen at one end of the chassis, and the output tuning assembly at the other. The oscillator valve is an 807, which begs the question of what the crystal current might have been. A socket can be seen between the 807 and the crystal oven, for an optional second crystal unit. The change-over switch can be seen on the side of the chassis.- Note the old style large Jones plug providing power to the unit.  Driver stage
The Driver stage, boasting an 813 is mounted above the master oscillator. This drives a pair of 833A Triode PA valves, which loaf along at half a kilowatt. The 250 Watt version of the transmitter is virtually identical except for only having one 833A final.  Modulator components The modulator components are mounted on the cabinet wall opposite the R/F section. The valve line-up consists of a 6V4 double triode, 6L6 push pull drivers , and a pair of 810 Triode modulator valves. The modulation transformer is on the floor of the cabinet. |
