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Old 9th Jul 2018, 7:34 pm   #1
coopzone
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Default Bush DAC90 mains dropper revisited

I know this hot topic crops up occasionally, but just thought i'd put my pennyworth in!

I recently acquired a DAC90, the mains dropper assembly was somewhat dilapidated - so i rebuilt it. But i did not like the idea of the asbestos lining the dropper housing. Yes I know white asbestos is not so bad if you don't disturb it etc, but NO asbestos is even better!

Anyway I went looking around for something to use and found small square heat-proof bunsen burner laboratory pads. Not too expensive (£3.00 x 2). They are made to withstand high temperature and don't have any asbestos in them. Perfect! They are even easy to cut and drill etc.

Just thought I'd mention them for anyone else interested.

Attached are photos of the completed mains dropper rebuild and the pad(s) as they arrived, I only used one pad for the dropper.

I may try and cut the other and drill out slots etc to replace the corner of the back panel for the radio, since as - ever the original had burned away over the years.
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Old 9th Jul 2018, 8:58 pm   #2
egerton
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Default Re: Bush DAC90 mains dropper revisited

Looks neat and seems a nice idea. Only thing that came to mind - make sure that air can circulate around the dropper in the enclosure as I think convection plays a part in cooling the dropper.
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Old 9th Jul 2018, 9:13 pm   #3
Graham G3ZVT
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Default Re: Bush DAC90 mains dropper revisited

Nice job. I don't suppose you measured before and after temperatures of the cabinet above the dropper?

Mine reads 45 degC without any insulation after about 1 hour, I wish I had thought about taking a reading with my IR thermometer while the asbestos was still in place.

I have a suspicion that the brown bakelite might not be too bothered, but the white and cream urea formaldehyde cabinets may well be affected and, so my theory goes, that is why the designers put in the asbestos.

Does anyone else think that is plausible?
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Old 10th Jul 2018, 8:16 pm   #4
coopzone
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Default Re: Bush DAC90 mains dropper revisited

sorry rambo1152, I did not measure the temperature with the old asbestos in place, I removed it whilst repairing the rest of the dropper, so it was not working before the new setup.

Just measured the case temp above the dropper it's around 38c after 2 hours use. The temperature above the vents at the top of the dropper is around the 58c mark (it's difficult to get an accurate reading with my cooking thermometer !)

I believe the thermal properties of the new insulation are like for like with asbestos (it was sold as a direct equivalent to it).

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Old 13th Jul 2018, 5:27 pm   #5
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Default Re: Bush DAC90 mains dropper revisited

As I mentioned in the first post, I've have a preliminary attempt at replacing part of the damaged back panel with the same heat resistant pad. Photos attached. It's currently using temporary clips to hold it in place and it's not painted! I'm going to try it like this for a day or two to measure temperatures etc.

I may still yet decide to replace the resistor with a capacitor dropper, it gets very warm!

coop
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Old 13th Jul 2018, 10:52 pm   #6
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Default Re: Bush DAC90 mains dropper revisited

There is another way to reduce the dropper dissipation without going the capacitor route- use a 1N4007 in the chain and a resistor in parallel with the main section of the dropper. This will reduce the dissipation in that part of the dropper from around 25W to about 13W.

Connect the 1N4007 in series with the heater chain somewhere away from the dropper and other high temperature parts and add a 680R 9W resistor (eg Welwyn W23) in parallel with the main dropper section. A 10nF 400V capacitor across the 1N4007 may be needed to shut up its reverse recovery RF noise, or you could use a suitable fast recovery diode instead.

The diode reduces the power to the heater chain by 50%, the parallel resistor brings it back up to full power but since the rms current is the same the total resistor dissipation is now less due to the lower resistance of the parallel combination.
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