Arena T 2400
Written by Bryce Ringwood   
The Arena T 2400 was a transistor stereophonic receiver/amplifier manufactured by Horsen (Hede Nielsen's Fabrikker - Denmark) in 1966. It is housed in a stylish flat wooden cabinet, that would not be out of place in today's surroundings.
 
General view of Arena T 2400 FM receiver
The Arena T 2400 on the bench
 
The receiver has only the VHF/FM waveband and (in theory) can receive mono or stereo transmissions. In the South Africa of the '60s, there were no stereo transmissions, so many suppliers would remove (or order without) the stereo option. Alas, this is the case with this set. (Stereo FM was introduced in South Africa in 1985). The set accepts input from phono (magnetic pick-up), or piezo and tape. CD had yet to be invented until 1982. The amplifier has a dual 12-Watt output into 8 Ohm speakers. It might work with 4 Ohm, but I would' chance it. At 5W output, distortion is given as 0.15% with intermodulation distortion at  0.2%.
 
The FM receiver seems to be tuned with varicap diodes. The push-button tuning on the top of the set alters the bias. It also means that you cant put other units and junk on top of it. There is also a scanning function and optional AFC.
 
The entire set operates from a stabilised power supply, which tells me that someone took great pride in the design.
 
On applying power, the set worked very briefly on one channel before breaking into oscillation and motorboating followed by smoke and silence. After replacing all the capacitors in both left and right amplifiers, as well as the burned out resistor in the left channel and a few transistors, one channel worked again. This time it was the turn of the right-hand channel to cause problems. Advancing the volume control a hairs breadth caused massive amplification - and more blown transistors. The problem with the amplifier was obviously a defective resistor, and since all the resistors are mounted end-on, it took some time to find the culprit. 
 
Inside the Arena T 2400.
Inside the set - Note the envelope and missing stereo modules
 
Another problem was that the transistors used are all "unknown" ITT types. In the end I gave up trying to find the originals and resorted to commonly available 2N2905/2N2219 types. The original output transistors were 40312, but the 2N3054 is a direct replacement. Transistor matching was carried out using a venerable Tektronix 575 transistor curve tracer. The batch of 2N3054 transistors to hand had current gains varying from 4 to 140 - there's some sort of moral there. I managed to match the driver transistors, but could only get about a 30% match on the power output stage. These are now quite expensive transistors, but still a lot cheaper than 40312s (supposing you can find them). I will talk about transistor curve tracers and Hi-Fi amplifiers in some later articles.
 
Amplifier section of the Arena T 2400
Close-Up of the amplifier section.
 
The design uses a pseudo-complimentary pair in the output stage (I will explain this in a later article). There is no current limiting, so a short on the speaker terminals will cause general failure of the amplifier. Also, the amplifier has massive negative feedback - the + supply to the earlier stages is from the output stage junction between the  upper and lower 40312 feeding the emitter of the first stage via a 1.5K resistor. If that resistor goes open circuit, everything becomes loud and hot. Some people don't like negative feedback in their amplifiers, as it is said to "deaden" the music.
 
The circuit board is made from SRBP (Synthetic Resin Bonded Paper) with thick tracks. The board is not roller-tinned and the flux wasn't removed after soldering, so that it now looks rather scruffy and the copper has a slight patina. The board coating was not effective. In spite of this, components were all replaced without any "lift" of the circuit pads. The board is rather difficult to work on, because of the upright resistors and lack of any identifying marks. With the vision of hindsight, it would have been better for me to photograph the boards and then trace the circuit, using "paintbrush" to mark the parts for identification. 
 
The circuit diagram is supplied with the set, in a small envelope under the lid. (Come on Tivoli Audio - I dare you to do the same). The way it is drawn is a little confusing. Note the presence of the stereo decoder modules in the circuit diagram  - they are missing from this set.
 
 
 
 
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