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Old 14th Apr 2013, 12:41 pm   #1
gary_crutchley
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Default Bush DAC90A. Dial lamps.

Hi folks,

I have a DAC90A, the serial number is 73/25342, not sure whether that is an early or late model, although I suspect late from the number? There is nothing in the case to indicate date of manufacture.

The radio works perfectly, but I have a couple of questions please.

The service sheet that I have (published in April 1950), says that the scale lamps should be 3.5V, 0.15A which is what I appear to have in mine and of course the scale is very dim indeed. However, reading these forums I have found that other lamp variants are discussed. Can someone tell me please, is 3.5V, 0.15A the correct rating and was the scale really this dim when the radios were new?

My second question is regarding the Bakelite, can someone suggest what polish to use please?

Many thanks in advance.

Best regards,

Gary
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Old 14th Apr 2013, 3:02 pm   #2
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Default Re: Bush DAC90A. Dial lamps.

Hi Gary

Re the polish – try searching ebay for Greygate No 5 Polish – it’s specially made for Bakelite and brings it up a treat.

There have been quite a few threads about the dial lamps – try doing a search which should bring up quite a few results.
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Old 14th Apr 2013, 3:08 pm   #3
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Default Re: Bush DAC90A. Dial lamps.

Polish... loads on ebay http://www.ebay.co.uk/sch/i.html?_tr...at=0&_from=R40

Dial lamps, yes they are dim, you can use a couple of easily got 6V 60mA ones in parallel instead, still as dim but easier to get (bicycle shops, rear light).
 
Old 14th Apr 2013, 3:39 pm   #4
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Default Re: Bush DAC90A. Dial lamps.

Hi Gary, it may be worh while checking the dial lamps shunt, this should be 75 Ohm, I feel that the manufactures dial lamp voltage and current rating will be correct and if the lamps are as dim as you say, try repacing the shunt as this may have fallen in value and allowing far too much current to be bypassed.

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Old 14th Apr 2013, 10:24 pm   #5
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Default Re: Bush DAC90A. Dial lamps.

Cheers Ken, I will do.
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Old 15th Apr 2013, 8:56 am   #6
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Default Re: Bush DAC90A. Dial lamps.

The manufacturer's service sheet gives the value of the resistor as 250R. However every DAC90A I've seen has had a 75R resistor fitted. The Trader Sheet says 75R and I believe these were compiled by reverse engineering sets.

The question is, is the manufacturer's sheet wrong or was the wrong resistor fitted in production?
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Old 15th Apr 2013, 9:22 am   #7
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Default Re: Bush DAC90A. Dial lamps.

My understanding is that Bush changed the value shortly after production started as if the user switched the set off and on again quickly it'd blow the lamps.

- Joe
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Old 15th Apr 2013, 10:04 am   #8
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Default Re: Bush DAC90A. Dial lamps.

When I repaired a DAC 90 some years ago, it was covered in the first volume of the annual red books that my local library used to keep. AFAIR it said that, in the later production both the resistor and the type of lamp were changed. I don't have a copy myself, so I can't be more specific but I think the later lamps were of a higher voltage.
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Old 15th Apr 2013, 10:23 pm   #9
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Default Re: Bush DAC90A. Dial lamps.

Hi all,

Thanks for the information, it is useful and fascinating.

I posted the serial number above, does anyone know whether this is an early or later model. I thought it was late judging by the number, but as it has the earlier lamp type, is it an early one?
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Old 15th Apr 2013, 10:38 pm   #10
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Default Re: Bush DAC90A. Dial lamps.

A photo would help as the knobs changed over the years.

- Joe
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Old 15th Apr 2013, 11:06 pm   #11
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Default Re: Bush DAC90A. Dial lamps.

Vol1 of the red books only lists DAC90A. It says nothing about dial light changes from/to 3.5V 0.15A, only mods to the lamp shunt as described and an increase in the rectifier surge limiter resistance.
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Old 16th Apr 2013, 7:27 am   #12
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Default Re: Bush DAC90A. Dial lamps.

A good way of telling how old the set is, is to look at the dropper resistor, early sets had a concrete looking one, the later (and more common) ones had a green one fitted.
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Old 16th Apr 2013, 1:42 pm   #13
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Default Re: Bush DAC90A. Dial lamps.

Thanks for the clarification: it was around 15 years ago that I mended my friennd's DAC90, and the colllection of red books that our public library used to have are long gone.
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Old 16th Apr 2013, 10:59 pm   #14
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Default Re: Bush DAC90A. Dial lamps.

I've seen 150mA 3.5V bulbs suggested. Has anyone tried these, are they any brighter?
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Old 16th Apr 2013, 11:29 pm   #15
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Default Re: Bush DAC90A. Dial lamps.

Those are the standard bulbs for the set. 3.5V 150mA is same as 3.5V 0.15A.
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Old 17th Apr 2013, 10:29 pm   #16
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Default Re: Bush DAC90A. Dial lamps.

I have noticed that when I turn the set on, the lamps initially come on at maximum brightness, then they dim as the set warms, returning to full brightness (not that bright is exactly the right description!) when the set reaches operating temperature.

Has anyone seen this before? Can anyone explain it? Is it due to the current consumed as the valve heaters warm up?
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Old 17th Apr 2013, 11:59 pm   #17
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Default Re: Bush DAC90A. Dial lamps.

In post 2 of this old thread GMB gives a good explaination of flare-up as it applies to valve heaters. Dial lamps flare up for the same reasons.

I have run a DAC90A with two 6.3v 0.15A lamps with no shunt resistor for a good while now with no problems. I am using a capacitive dropper which gives a lower surge current at switch-on and the theory is discussed in the same thread linked above.

The lamps used give a much better illumination in spite of being under-run (0.1A) and the capacitive dropper doesn't allow the switch on surge to exceed the rating of the lamps. The only disadvantage is that if a lamp did fail the set would go dead due to no valve heaters.

Colin M
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Old 18th Apr 2013, 11:03 am   #18
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Default Re: Bush DAC90A. Dial lamps.

You may find that with this arrangement, a brief (1 or 2 sec) power interruption, or the switching "off" then quickly "on" of the set will blow the lamps. This is because the reservoir capacitor will have time to discharge and then recharge rapidly (through the lamps) when power is restored, as all of the valve cathodes will be hot.

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.

Leave the poor old design as it is, if lamps are scarce you can use 6V 0.1A cycle rear "dynamo" (alternator) lamps from a cycle shop with reasonable results, or two 6V 60mA lamps in parallel with a lower margin of safety. Bush were not fools...

Leon.
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Old 18th Apr 2013, 12:10 pm   #19
gary_crutchley
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Default Re: Bush DAC90A. Dial lamps.

Hi Leon,

Thanks for the comments, they are much appreciated. I'm certainly not planning to change anything at all. I was simply curious as to the way in which they operate, and their brightness.

I enjoy the originality of this set.

Gary
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Old 18th Apr 2013, 12:39 pm   #20
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Default Re: Bush DAC90A. Dial lamps.

Quote:
Originally Posted by ukcol View Post

The lamps used give a much better illumination in spite of being under-run (0.1A) and the capacitive dropper doesn't allow the switch on surge to exceed the rating of the lamps. The only disadvantage is that if a lamp did fail the set would go dead due to no valve heaters.

Colin M

I need to correct the above state as it implies I wired the lamps in series with the heaters only, I have in fact kept the lamps in series with the whole set (HT and heaters).

Quote:
Originally Posted by Leon Crampin View Post
You may find that with this arrangement, a brief (1 or 2 sec) power interruption, or the switching "off" then quickly "on" of the set will blow the lamps.
I had made this observation myself but the HT surge only produces a slight brightening of the lamps, not one likely to blow a lamp. I think though that if you habitually switched the set on and off like this the long term life of the lamps would be reduced.

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
Leon, I'm afraid I'm going to have to disagree with the above for the following reason. The lamps are in series with the neutral from the mains supply. If you had an open circuit at that point the whole set would be at mains potential including the valve heaters and the cathodes, so there would be zero volts between the valve heaters and their cathodes. It would perhaps be more desirable that the lamps were in the live side, providing the lamp holder insulation was up to the job.

It is true that the designers at Bush were not fools, but as a manufacturer they could not afford to have a set go dead as a result of lamp failure. That might not be true for a vintage radio enthusiast. Many of us have made improvements to the DAC90A (particularly in the audio stages) to improve the sets performance. Many others will want to keep the set completely original and that is fine too.

Colin M

Last edited by ukcol; 18th Apr 2013 at 12:46 pm.
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