Ivory Bush DAC90A

[m didier]

Hello. I have got an ivory Bush DAC90A with no stress marks or cracks. I have just restored it, and want to put it in every day use. Is there any way I can protect it from getting stress marks in the future.

Martin

[Alf]

I've never had an ivory DAC90A but I've heard that it's the heat from the dropper which is a primary cause of cracks so I suppose some form shielding around the dropper allowing for air flow.

[Sideband]

Originally Posted by martin

Is there any way I can protect it from getting stress marks in the future.

Heat from the dropper and output valve are the worst area's for causing stress fractures. One suggestion to reduce heat stress (although this may not appeal to a 'purist' restorer) is to replace the resistive dropper with a capacitive dropper. Full details are given on the main website. It will certainly help a lot but the radio will not then be 'original'. The original dropper can be left in place if there is room, but disconnected.

Rich.

[mickjjo]

I would suggest making a heat shield from thin aluminium to deflect the heat towards the back cover, this could be fixed under the original nuts on the dial support brackets, so no modifications to the set would be needed. Another thought would be to mount a small quiet computer type fan, powered by a "wall wart", to the back cover to extract the hot air. . .

Regards, Mick.

[m didier]

thankyou for your replies . the heat shield seams the easiest idea for me



martin

[Steve_Bell]

My ivory DAC90A, alas, has already got stress cracks above the hot components but the previous owner has actually fitted such a heat shield as described. This is thin steel sheet in my case, measuring 130x50mm. See pics for how it is mounted, easily removed if you sell it in the future or to maintain authenticity.

[Ed_Dinning]

Hi Gents, as an alternative run from a 240/200 V auto transformer or external deopper and put the sets dropper on a lower tapping. Bit messy but it will reduce the heat.

Ed

[Aerodyne]

It might be worth pointing out that Urea Formaldehyde can spontaneously 'stress - crack' due to internal stress created in the moulding process. I suspect there's an element of luck involved. I have seen Urea cabinets with such cracks in places where heat could not have been the primary cause.

-Tony

[SA-Draadloos]

Not a lot of help for the "home" Bush models but some export models I've seen also have a tapping for 110V (not sure off hand if the dac90a equivalent has one). ANyway I've found that sets can run much cooler on this tapping when powered through a suitable autotransformer (of course ).

Isn't there a way to modify the circuit to disconect the dropper resistor and use a suitable external transformer - which unfortunately would make the set truly non-portable and dependent on one power supply - simplest then to go for a capacitance-reactance type "Dropper".

I own an ivory American set (Pilot) wherein it is a Standard AA5 octal - the set is so tiny and the output and rectifier valves touch bases - runs failry hot but the bakelite has never warped or cracked - I think this is because there is no dropper in the set and I run it off an autotransformer. It seems almost to be chance or luck if the bakelite warps or cracks.

James

[mjizycky]

Originally Posted by SA-Draadloos

Not a lot of help for the "home" Bush models but some export models I've seen also have a tapping for 110V (not sure off hand if the dac90a equivalent has one).

Certainly does, James: of a random sample I just took of several export Bush service manuals all the Bush sets have provision for 110V operation, the EU3A included.

Whilst this is of little help to owners of a 90A, a viable alternative for the "90" is to obtain the auto-transformer from a scrap octal-based chassis (e.g. AC81, AC2) and fit that in place of the dropper - they're quite small. Considering that a service manual exists for an AC90 (and no, that isn't a typo) it's almost a legitimate conversion... but you still won't reduce the heat from the CL33/CY31!

[mjizycky]

I've just taken a look at the manual for an AC81 - you wouldn't get that transformer in a DAC90. Oops An AC2's would fit though, or that from a SUG91.