History Preserved AWA Broadcast 500W Transmitter Model P5 Serial Number 1

AWA BTM-P5 No 1 nameplate. Photo: Donald Bainbridge

The AWA BTM-P5 was a 500 Watt broadcast transmitter manufactured in Australia in the 1960s. The P5, and its big brother the BTM-2J, were produced by Amalgamated Wireless (Australasia) Limited for more than a decade starting in mid-1961. Many of this model were used throughout Australia.

AWA BTM transmitter advert, 1964.

The unit featured in this story is BTM-P5 Serial Number 1, the first ever produced. This one has a green coloured rack frame but all subsequent production ones were grey.

A later model unit, now abandoned in Queensland

Unit S/No 1 went into service as the new 500 Watt commercial station 4LM in the Queensland mining town of Mount Isa in 1961. The station’s authorised power was listed as 500 Watts on 1370 Kc/s in Radio Television and Hobbies magazine of January 1962, and the mast height was listed as 180 feet in a local aviation Visual Flight Guide of the time. (Read more about 4LM in our feature 4LM Mount Isa)

4LM station logos

The transmitter was moved to start 4GC at Charters Towers when that station opened on 13 Decmeber 1976. The original authorised power was 100 Watts but it was subsequently upgraded to 500 Watts then to 1 kW. Charters Towers was a rich gold mining City hence the call sign 4GC for Gold City.

4GC logos

The transmitter subsequently moved to Innisfail. It was in service as the tertiary stand-by at 4KZ and was used one Christmas day when both of the 5 kW transmitters failed.

4KZ logo

It ran as 4KZ after cyclone Larry of 20 March 2006 when two of the three 121 metre masts fell down. It would still work into a temporary Aerial Tuning Unit for the remaining mast.

We understand that the transmitter also started 4AY Innisfail. 4AY Innisfail is a registered trading name not a licence call sign. The transmitter started 4AY at Innisfail in 1990. This should not to be confused with the earlier 4AY at Ayr Queensland. (Read more about the original 4AY Ayr in our feature 4AY – The Voice of the North)

The transmitter was last on air in about 2012-ish as 4AY Innisfail 873 kHz.

4AY logos

After its retirement, 4KZ gifted the transmitter to Donald Bainbridge who took on the task of restoring the unit to its original condition in his transmitter museum in Victoria.

Inside the transmitter before restoration

The transmitter has been restored back to original AWA standard as far possible by using original components salvaged from other dismantled AWA radio and TV transmitters. The original oil filled high voltage capacitors have been replaced by modern plastic insulated ones.

The transmitter in the process of being refurbished

The transmitter is now in good working order and sounds very good working into a dummy load made of light globes. It uses about 10 amps of mains power at 240 V single phase.

Donald with the transmitter. It weighs 660 pounds, 200 kg.

Have one of your own? Here’s the instruction manual:

Thanks to Donald Bainbridge, Al Kirton (4KZ), and Bruce Carty for the material used in this article.

Donald adds:

The transmitter is significant because it was the first of that model which was evidently produced for more than a decade starting in mid-1961. I know that many of this model were used throughout Australia.

I have seen one in commercial service in Mildura about 6 years ago, 2 in a museum at Currajong west of Sydney, 2 in a collector/radio amateur’s hands in Adelaide and one abandoned in Queensland in attached photo.

I know of a man that had one until a few years ago for site test purposes and another man who took 2 to the rubbish tip.

Other people have told me that they worked at 2 other radio stations that each had one in their studio building, one place 3KZ Melbourne as a back-up, and another at Katoomba under the stairway as the station transmitter. The ABC used many in remote rural areas.

There were earlier AWA 500 W transmitters. I went to Perth looking for one from the 1930’s that was stored in the helicopter hangar at former TV studios of TVW7. The site had recently been vacated and was secure in the hands of the demolition contractor. The hangar had not been yet unlocked, the day before ANZAC day centenary the demolition foreman took me into the hangar were we found lots of things including a clean area of the floor nearly 1 sq. metre down the back corner where evidently that transmitter had been until very very recently.

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  1. Donald, OTC used them all around Australia & New Guinea on their Coastal Radio Stations. They were named the CTH-P5J (500 watt MF & HF AM), the CLH-1L (1Kw HF AM & SSB) & the CTM-2K (2Kw MF) The letter C stood for Communications while B stood for Broadcast. I serviced them & can send photos. The P5J ran 4CX250B’s in the final, CLH-1L a 4CX1000 & the CTM-2 ran four 4CX1000. Driver tubes were the trusty 6146. We did modifications to them which I still have details of the reason for the mods.

    1. Hi Neil, I just found your interesting comment.
      I would be interested to see the photos and read about the modifications.

      I undid most of the modifications that I found to restore it to near original.
      Mine is slightly modified apart from non original replacements needed to keep the thing operational.
      One modification is in line with the AWA circuit for later serial numbers for 2 steps in the main ht upon ht application.
      The minor HT frans4mer had been replaced by some lower voltage output trans4mer at some stage. I think I made the rectifier feed into capacitor input filter rather than choke input. But the volts are still low so I by-passed the cathode resistor of the 6146 with a mica capacitor to restore the rf drive to specification. That also improved the overall rf efficiency.

  2. Thanks Neil. I would be interested to see a photo of the 500 & 2kw MF units. I am interested in the mods & the reasons especially if they could have benefit for reliability.
    I have a lot of fun playing with the BTM-P5 to reduce distortion (typically about 0.5% up to 90% ish modulation), improve power efficiency, checking AWA crystals, checking rf ammeters, playing with rf current transformers etc.

  3. I actually worked on This transmitter when I was the tech at 4LM Mt Isa from July 1978 to Dec 1979. It was our stand by in those days and the main transmitter was a STC 2.5 kw