A look at a Bush DAC90A valve radio

Bush DAC90A Valve Radio

Growing up in the age when TVs and radios used valves I picked up an old Bush DAC90A valve radio to remind myself what old valve electronics was like.

I also wanted to see how easy it would be to restore one or convert it into something like a Bluetooth speaker or FM radio.

Bush DAC90A Valve Radio

In terms of restoring it there’s no microprocessors running custom software and obsolete chips that you get with modern audio equipment so other than the valves and transformers it uses off the shelf parts.

There’s lots of these old radios around so spare parts shouldn’t be a problem. The DAC90A is a simple design that uses five valves, one of which is a rectifier. It’s classed as a ‘superhet’ (superheterodyne) receiver.

The valve heaters are driven from the mains via a big dropper resistor. Better valve designs drive the heaters from a 6.3v winding on a transformer.

Bush DAC90A valve radio-mains-dropper-resistor
Mains dropper resistor (the large green cylinder on the right)

In this design the valve heaters are connected in series, so if one heater goes open circuit none of the other valve heaters will get any power. That’s like Christmas tree lights before the more reliable LED ones. One bulb blows and the whole lot go out!

If you don’t know what a valve heater is it’s literally a heating element inside the valve that’s used to heat a metal plate called a cathode. The transfer of electrons from the cathode to the anode in a valve only happens when the cathode is heated.

The heaters glow red when hot so valves take a few seconds before they start working. A valve TV or radio isn’t ‘instant on’ like modern day equipment.

Chinese audio equipment with valves that glow blue are lit by a blue LED!

I’ve put a video of the ‘taking it apart for a look’ process on my YouTube channel shown further down this page. The video might be followed up with another when I finish the conversion or restoration.

The Bush DAC90A was released in 1950 which by today’s date of 2024 makes it 74 years old! An earlier version didn’t have the ‘A’ designation.

Bush DAC90A valve radio underside components
Underside components on a tag strip

I got mine fairly cheap with a ‘buy it now’ price on eBay so I wasn’t expecting much. Most audio equipment I’ve brought on eBay sold as spares and repairs would be better described as butchered junk.

On this radio the variable capacitor tuner doesn’t rotate which is bit of a show stopper although I think it’s a rusted up pulley wheel rather than the capacitor itself. The volume control is really tight to turn and the power switch on the back of the volume control isn’t switching.

The speaker works but is very quiet so isn’t much use.

There’s a lot of rust on the chassis and some horrible looking capacitors so it’s probably been stored somewhere damp like a shed or unheated cellar. Damp is a killer for electronics and metal parts.

Bush DAC90A circuit board view

Someone has also poked a hole in the speaker grill damaging both the grill and the speaker cone, presumably when packing it as it wasn’t on the sellers photos. To round it off, the Bakelite has a small crack and broken bit at the back which is fortunately out of view.

This is where you hit the problem with restoring these things. All the above is repairable or replaceable, but if I spent twenty quid on parts (including glue and polish etc) and a lot of hours getting it working it wouldn’t be worth more than about £50 as that’s how much one that works costs.

If you don’t care about the money then getting an old one working would be a great challenge and a really good way to learn about old radios. At least with a Bakelite set you don’t have to worry about treating it for woodworm!

I bought it expecting problems and this was a good experiment to see what sort of issues to expect should I get another one.

The video below is on my YouTube channel and takes a look around the DAC90A internals.

I think this one is a bit too far gone to bother restoring it so I will just convert it to a simple FM radio by connect its somewhat knackered speaker up to an FM radio PCB driving a small class D 3w amplifier powered by a USB power supply.

I could hook the FM radio board up to the valve amplifier part of the original radio but for the following reason it’s not a good idea.

The chassis of the Bush DAC90A is connected to one of the mains input terminals via a 75 ohm dropper resistor so the chassis (and audio ground) is either connected to mains live or neutral depending on how the plug is wired.

This radio is very dangerous to work on. Don’t go near the inside of it unless you really know what you’re doing.

MW and LW which is what this Bush radio receives is prone to all sorts of interference from modern electronics and FM isn’t, so an FM modification will sound better.

A really simple modification would be to connect the speaker to something like a jack plug socket fitted to the back panel. Then get a cheap transistor radio with a headphone socket. You can then easily connect the transistor radio to the speaker in the DAC90A (or any other valve radio) via a cable.

I could turn it into a Bluetooth speaker like others do but I’d rather keep the DAC90A as a radio as that’s what it originally was.

Related (ish) article:

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