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Old 27th Apr 2009, 3:24 pm   #1
ajs37
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Default Bush DAC90 - dropper heat insulation issues

Hi - I have just started to investigate a DAC90 (which I have had waiting for a couple of years!) - First issue is the mains dropper - it had been taken out of it's metal box - and the top connection wire is missing. I have scraped back the vitreous enamel and managed to put a couple of turns of 24swg tinned wire around it and solder the turns to the exposed wiring - it now reads 780ohm total, instead of 800ohm - I think that should be ok (I'm not sure how accurate my DMM is!) - what do you think? Will the soldered joint suffer in the heat?
Also - the heat insulation between the dropper and it's metal box is missing - I seem to recall that it was some sort of asbestos cloth - does anyone know what else I could use instead?

Once this is sorted out, I'll start to re-cap and re-wire the chassis.
Cheers (and thanks)
Andy
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Old 27th Apr 2009, 3:40 pm   #2
paulsherwin
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Default Re: Bush DAC90 - dropper heat insulation issues

I think you should give serious consideration to converting the radio to use a diode or capacitive dropper, despite the loss of originality. The dropper in a DAC90 runs very hot indeed and will almost certainly melt solder on the surface.

If you decide to stick with the resistive dropper, a piece of ceramic tile is probably your best bet as a heat shield, though it won't work as well as the original asbestos mat.

Paul
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Old 27th Apr 2009, 4:52 pm   #3
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Default Re: Bush DAC90 - dropper heat insulation issues

A piece of Plumber's Soldering Mat may provide the type of material needed.

Edward
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Old 27th Apr 2009, 5:14 pm   #4
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Default Re: Bush DAC90 - dropper heat insulation issues

I'm with Paul on this one. If the dropper is in that much of a state, it won't really repair reliably and ordinary solder will probably not last long. If it was mine I would remove it completely and fit a capacitive dropper or, as Paul says, a diode dropper and smaller resistor. At least this will enable you to get the radio working reliably and safely with the added bonus of much less heat. If you really want originality, there is a good chance that another dropper will turn up at some time and you can revert back to the original circuit then.


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Old 27th Apr 2009, 5:22 pm   #5
ajs37
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Default Re: Bush DAC90 - dropper heat insulation issues

thanks again chaps! I may well go for the capacitive dropper - plumbers mat - great - I'll bear that in mind for the future!
There are a couple of other issues with this radio - loudspeaker cone is completely destroyed (a DAC90A L/S should fit) and, of course, the scale lamp is missing - that's more serious as it runs in series with the heaters (6.2V 0.3A)

It's worth perservering with this radio, as it is in such good physical shape - it will take longer than I thought though!

thanks again
Andy
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Old 27th Apr 2009, 6:50 pm   #6
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Default Re: Bush DAC90 - dropper heat insulation issues

Persevere indeed Andy. I haven't met a DAC yet that could not be repaired. They are lovely sets.

As you are obviously not going for originality the loudspeaker from a DAC90A will fit and work fine but there are subtle differences.

Maybe I have just been lucky but I haven't met a DAC90 with a duff dropper. What I have found is that the heat can damage the paxolin strip it is wired to. The grey DAC90A dropper is another matter but the green one is extremely reliable.

Dave
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Old 27th Apr 2009, 8:24 pm   #7
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Default Re: Bush DAC90 - dropper heat insulation issues

Quote:
Originally Posted by David Tilley View Post
Maybe I have just been lucky but I haven't met a DAC90 with a duff dropper. What I have found is that the heat can damage the paxolin strip it is wired to. The grey DAC90A dropper is another matter but the green one is extremely reliable.

Dave
I have to say that I've never come across a mains dropper in such good condition (apart from missing the wire at the top!)
Cheers
Andy
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Old 30th Apr 2009, 7:27 pm   #8
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Default Re: Bush DAC90 - dropper heat insulation issues

Hi. Couldn't you obtain an autotransformer (like the size of those used in the Bush AC91)? It might not be original anymore, but at least it will run cooler. Wasn't there someone offering autotransformers on this site some time back ?
Regards
Mike
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Old 6th May 2009, 9:13 pm   #9
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Default Re: Bush DAC90 - dropper heat insulation issues

Hi

How thick is the insulation ??

Richard
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