Bush DAC90A Vintage Broadcast Radio

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Bush DAC90A     Bush TR82 transistor portable    

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The Bush DAC90A is the successor to the DAC90. It used more modern valves and an updated circuit when compared to its popular predecessor.

The DAC90A is easily recognised by its unique styling and bakelite case.

Covering the medium and long wavebands it was targeted at the lower end of the market, making it an accessible and popular radio for all.

Bush DAC90A background

The DAC90A radio was produced by the Bush Radio company which was formed to produce radios in 1936.

After the Second World War, Bush produced the DAC90. This radio as launched in 1946 and became a success. It was aimed at the lower end of the market, making it a good radio for the mass market.

As a result of developments with valve technology and the success of the DAC90, the DAC90A was developed.

Launched in February 1950, the DAC90A used the new B8A series Rimlock valves which had a small "nub" on the side of the glass envelope to engage with a slot in the socket to ensure the correct orientation.

Apart from the valves themselves, further improvements were made to the circuit to improve the overall performance and reliability.

When the DAC90A was introduced in 1950, it cost £12 1s 8d for the walnut or brown version, and £12 16s 9d for the cream version. These prices had purchase tax which was an additional UK tax added to these prices.

Although black and white versions are talked about in some areas, it does not appear that any versions of these colours were made. Some may have been painted these colours, but they were not part of the main production output. In any case white bakelite cannot be made as it turns out a cream colour as a result of the material used.

The DAC90A was very successful and continued production until around 1959 when transistorised radios started to become available.

DAC90A specifications

The performance of the DAC90A was quite reasonable considering its date of manufacture and the fact that it was a lower cost radio intended to appeal to as many people as possible.

**   note that the coverage is not given as a frequency in Hz, but as the wavelength in metres. This was common at the time for domestic radios.

DAC90A circuit description

The DAC90A was a five valve superhet radio where four valves were used for the radio receiver itself and one valve was used as the input supply half wave rectifier.

There are several elements to the overall DAC90A circuit description.

•   RF elements:   The signal is picked up by an internal frame aerial or antenna. This is located to the right hand side of the cabinet when looking from the front. The frame antenna is effectively an inductor and this is tuned by a two section variable capacitor with the other section being used for the local oscillator. There are also two section of the frame coil and only part is used for the medium wave, the remainder being shorted out. In this way it becomes resonant on both bands using the same variable capacitor.

•   1st mixer and local oscillator:   V1 is a triode hexode UCH41 is used as the basis for the first mixer and local oscillator.

The signal from the frame antenna is applied to the control grid of the hexode section of the valve that acts as an RF amplifier and mixer.

The triode section of V1 is used as the local oscillator which has a tuned transformer with one winding connected from grid to ground and the other winding in the anode. This provides the feedback required for oscillation.

The signal resulting from the mixer is passed to the next stage through an IF transformer placed in the anode circuit of the hexode section of V1.

•   IF stage:   The signal from the mixer is passed to a single stage IF amplifier. This is formed around V2 which is a UF 41 pentode. Additional selectivity is provided by the second IF transformer which is located in the anode circuit of the amplifier.

•   Detector and first audio amplifier:   Once amplified by the IF amplifier, the signal passed to V3, a double diode triode UBC41. This acts as a detector for the RF signal and it also provides an AGC voltage to apply to the grids of V1 and V2. This reduces the effect of signal variations on the output audio.

The triode section of this valve also provides the first stage of audio amplification, bring the level up from the relatively low levels after detection / demodulation.

•   Audio output:   The amplified audio is passed to V4, a UL41 output pentode. This brings the level of the audio up to a sufficient level to drive the internal loudspeaker.

•   Power supply:   The radio accepts both AC and DC because at this time, not all households had access to AC mains supplies. The use of DC gave another option for powering the set.

The incoming power was rectified by a half wave rectifier and smoothed using a Pi section consisting of two capacitors and a resistor.

Where lower voltages were required in the circuit a large dropper resistor was used to reduce the voltage, but this gave off considerable heat and the resistor was accordingly quite large.

Points to note

The DAC90A is a well respected radio and most of these vintage radios perform very well. However it is worth remembering that they can be over 70 years old now, and at this sort of age these radios can start to fail.

The radios have some old wax capacitors in them, and now these will always need changing as they will already be working poorly and could completely fail. When renovating one of these radios, it is always worth replacing these capacitors.

Often the main smoothing capacitor may be OK, but it is worth taking a careful look at it. Look for signs of bulges in the end cap as this can be a sure sign of an impending disaster. It is also worth looking at the leakage current to see whether this is acceptable. The smoothing capacitors are a dual electrolytic type, and although they may appear to be a specialised component, they are still available at specialist suppliers.

One of the main points to note is that the audio coupling capacitor can fail. If this happens it can destroy the output valve, the UL41 and it could also take the output transformer with it. When restoring a DAC90A, this capacitor should be replaced as a priority.

The Bush DAC90A is a very popular vintage radio. It is often used in movies where depictions of the 1950s UK are required. Its brown (or cream) case, the round speaker and the slanted dial panel as well as the overall styling are very reminiscent of the period.

The radio itself was simple and straightforward, built to a cost and accordingly with a minimum number of components and stages.

The radio was very popular at the time and now many DAC90A radios can be found at collectors fairs and on the Internet.

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