Bush Dac 90 Capacitive Dropper

[tim.54]

   
I`m in the process of getting aBush Dac 90 up and running . when i bought the set it had ,sometime in the past been fitted with a non standard dropper (see pic) whoever fitted it has had to remove the front rubber mtgs to enable it to go in the case , even so it still touches the top of the case ! im thinking about fitting a capacitive dropper ,has anyone here carried out this mod ? i`ve just about managed to work out the values of the parts required ! but what i need is some practical advice , where did you obtain the capacitor , how and where it was fitted ,
how it was wired plus pictures of the mod etc . would be very grateful for any advice .
regards, tim

[pwdrive]

Never done one myself, but you may or may not be aware that the capacitive dropper/heater load is a complex impedance.

Lawrence.

[Geaney]

Hi Tim,

I've not done this for a DAC90 but I've done it on my Radialva (French) set and it worked fine. I'm in Switzerland so probably can't advise re sourcing the caps - I got them from a local online electronics supplier.

I've posted various clips of the restore and the one that mentions the capacitive dropper is here:

http://www.youtube.com/watch?v=Y9h1qmsUOkQ

Best Regards, John

[Old Sparky]

I haven't made a capacitive dropper for that, or any UK AC/DC radio, but I did make one recently for a Canadian Mitchell radio, to allow that to be run from 230/240v. I used four class 'X' capacitors in parallel, totalling 4.47uf, which dropped the voltage to 112vac at the rectifier input. There is an article on the UKVRRRDF which goes into the rather complex maths. needed to work out the values required for capacitive droppers, and also explains the merits/demerits of several types of dropper. It's not in the forum as such, but in one of the sections on 'how to do' various things, so a search of the VRRRDF site will surely find it! N.B. The article in question is referred to by Geaney, and shown in the You Tube clips in post #3 above.

[AlanBeckett]

I hate to do this, but http://www.vintage-radio.com/repair-rest...calcs.html
There's a Speadsheet at the bottom of the Page and all sorts of other advice.
I've never done it myself as Pye almost always used a Transformer.
Being a DAC90 you could always take it down the Garden and give it a decent burial

Alan

[Joe]

[Aerodyne]

I've used the UKVRR spreadsheet of Paul Stenning's with success on a couple of sets but not on a DAC90. I used 'motor run' caps, probably from RS or Farnell.
Go for it, the method works well.
Tony

[Old Sparky]

I was going to use a motor run cap. in the Mitchell 1260, Tony, but there just wasn't room to fit one.

[AlanBeckett]

Haven't some people installed a small Autotransformer as an alternative?

Alan

[Aerodyne]

An autotransformer would be a good option, assuming there's the space for it, but finding a source for one could be a problem. As far as the caps go, motor-run quality types are in my opinion ideal but other good quality and well rated caps should serve OK. Motor-run capacitors do tend to be bulky. I fitted a capacitive dropper in a small Cameo table model receiver and also one in a Little Maestro to replace a line cord. Both very successful, both using motor-run caps.
Also perhaps worth considering is the use of a wattless dropper, i.e. a diode in series with the heater chain and backed up with a wire-wound ballast resistor. Advantages include less heat generation and a physically smaller ballast resistor when compared to a standard dropper. You need to pick a reliable diode, though!
A wattless dropper was used for the heaters of the Thorn 1400 monochrome TV series. It was a BY126 and regularly failed but on failure it caused field rolling, so rendered the set unwatchable until repaired. The probable moral might be: don't use a BY126...
Tony