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Old 18th Apr 2013, 12:46 pm   #21
kalee20
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Default Re: Bush DAC90A. Dial lamps.

Quote:
Originally Posted by Leon Crampin View Post
It's also not really safe to use a DAC90A without a dial lamp shunt from the point of view of heater cathode insulation. The lamps are in the neutral side of the heater chain, meaning that an open circuit here will place the whole chain at almost full mains potential, whilst most of the the cathodes are at neutral. This operates the valves beyond their rated heater to cathode potential.
Yes it is, and no they're not!

If you switch on the radio without the lamps, then that set will still work because of the shunt resistor. This will carry a higher current of course, but no other problem.

If the shunt resistor burns out, then yes the heater chain will be broken. But so will the chassis connection to mains neutral. So everything will go out, but the heater-cathode insulation will not be stressed because the heater chain is taken direct to chassis, so heater-cathode potentials will drop to zero.
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Old 18th Apr 2013, 2:46 pm   #22
Leon Crampin
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Default Re: Bush DAC90A. Dial lamps.

Quite correct - my oversight. Thanks for pointing this out, I must work on these sets more often. The trouble is, once restored, they're very reliable.

Leon.
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Old 18th Apr 2013, 2:56 pm   #23
ukcol
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Default Re: Bush DAC90A. Dial lamps.

Also to be fair Leon, there is a good chance my inaccuracy in post 17 (that I corrected in post 20) may have planted in your mind the idea that the bulbs were in the heater chain.

Colin M
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Old 19th Apr 2013, 10:52 pm   #24
gary_crutchley
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Default Re: Bush DAC90A. Dial lamps.

Hi Glowing bits,

I checked the mains dropper, it's not green so I guess it's an earlier model.

Gary
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Old 20th Apr 2013, 3:13 am   #25
Glowing Bits!
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Default Re: Bush DAC90A. Dial lamps.

Gary, I'll get the set out the cupboard and have a look at the serial to see if it's anywhere near to yours.

It would appear that your set is an earlier example.
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Old 20th Apr 2013, 5:50 pm   #26
john.north
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Default Re: Bush DAC90A. Dial lamps.

Re Bush Dac90 dial lamps.

I use two 12v 100Ma lamps with 560R shunt. when fully warmed up scale is slightly brighter than stock. Just bought another set that has standard lamps and shunt resistor and the difference is noticeable.
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