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Old 16th Nov 2003, 12:26 am   #1
JHGibson
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Default Ultra "Blue Fox"

I found a 1932 Ultra "Blue Fox" AC radio at the NVCF, its cabinet was perfect and it had all the correct knobs. The price was a little stiff but the label did say "In Working Condition" so I bought it.

The "Blue Fox" is a MW/LW two valve plus rectifier set having a AC2/HL triode grid leak detector with reaction, transformer coupled to a AC/PEN output pentode.

I always analyze my radios on the test bench before putting them into operation. The power transformer is the most expensive component so in order to test it, the HV secondary was disconnected and tested for continuity and leakage to earth. That winding was OK. Next I insured the transformer for a premium of about 50p by installing a half amp fuse in the primary circuit, tucked neatly under the chassis. Now the set was powered up just running the filaments for about an hour. After unplugging, the transformer was felt and found to be barely warm to the touch and so considered to be in good condition. The HV secondary was then reconnected.

Next all other components were checked with a volt/ohm meter. The resistors had drifted a little high but not enough for concern. The paper caps were all leaky and were replaced by installing modern small ones inside the bodies of the old ones. The extra space was filled up with casting resin tinted black to ressemble the original pitch. The intervalve and output transformers and the loudspeaker voice coil were checked for the correct resistance and the valves were run through a tester.

The final results of the bench examination were that the 0.0002 solid dialectric reaction capacitor was broken beyond repair, (the foil-like moving vanes had torn away from the control shaft), the output transformer had burnt out, the LS voice coil was open circuit and the rectifier valve had an intermittent internal conection.

The reaction condenser was replaced with a 0.0001 microfarad unit which was the only one on hand and as it turned out, it provided smooth control on all frequencies.

The output transformer was easily replaced with one from my bits box, but the loudspeaker was a problem. Often a open voice coil is due to a dry soldered joint developing at the rivets on the cone where the fine voice coil wire is soldered to the ultra flexible connecting wires. I re-tinned those joints with plenty of flux but this time I was not lucky. The speaker would have to be replaced.

An original speaker was out of the question. I did have a modern one of the correct diameter but there was a problem with the Ultra speaker having non-symetrical mounting holes. This was got around by clamping the speakers face to face with the transformer mounting plates spaced 180 degrees. Holes were then drilled through the rim from the Ultra side, using the Ultra speaker as a template, taking great care that the steel chips fell away from the magnets. Clinging chips were mopped up with a piece of modeling clay. The new speaker was mounted in place.

There was still a problem. The modern speaker with its little round ceramic magnet looked woefully out of period. I was in a DIY shop later and noticed that the end caps for 3 inch plastic drain pipes were shaped like LS bells. One was bought and sanded to break the glaze and remove the manufacturer's markings and spray painted black. It was glued over the magnet with RTV silicone adhesive. The speaker now looked quite vintage.

Something had to be done about the defective rectifier valve. A UU2 is rather rare and expensive when found, so the old one was "rebuilt". For details, see my note "Rebuilding a Rectifier Valve" in General Hints - https://www.vintage-radio.net/forum/...read.php?t=397.

The last repair needed was to replace the torn gauze covering the ventilation holes in the plywood back. A good substitute was brown bridal veil found in a fabrics shop. It wa stretched in place and held down with masking tape. Then a bead of wood glue was run around the perimeter. After the glue set, the excess veil was cut off with a razor blade. The cloth was tightened by steaming it from the spout of a boiling kettle.

The Ultra was now finished and when plugged in, it played well, and even the rectifier filament lit up.

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Old 2nd Feb 2004, 8:46 pm   #2
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Default Re: Ultra "Blue Fox"

Quote:
Originally Posted by JHGibson
The paper caps were all leaky and were replaced by installing modern small ones inside the bodies of the old ones. The extra space was filled up with casting resin tinted black to ressemble the original pitch.
What exactly is Casting Resin and where can it be obtained from?

Thanks
Graham

Last edited by Paul Stenning; 27th Dec 2004 at 2:01 pm. Reason: Fix link or code for vBulletin
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Old 3rd Feb 2004, 5:18 pm   #3
JHGibson
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Default Re: Ultra "Blue Fox"

On my tin of casting resin it says Polyester Fibreglass Resin. I got it from a Hobbies and Crafts store together with black, and burnt umber dyes, the burnt umber colour closely resembles the wax used to seal the original caps, or you can use the black dye to make the resin look like pitch. I was worried the first time that the black might be carbon and make the resin conductive but this was not the case. I'm sure that the same resin can be bought from boat repair and car repair stores. This resin is nice and thin and flows instantly into all the corners.

I have also used Araldite for the same job. I also found that I had to wrap a few turns of wool around the small internal replacement cap to fix it in position inside the old cap's sleeve and soak the wool with a few drops of five minute cure time epoxy. Now I could hold the cap vertically in a vise and add the casting resin without it running out the bottom! After that had set, the cap was inverted and the other side sealed. Rather a labour of love but several caps can be done at the same time.

John.

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Old 3rd Feb 2004, 7:43 pm   #4
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Default Re: Ultra "Blue Fox"

Thanks.

I'll try my local Hobby Shop. Melting the pitch out of a TCC Brick Capacitor was a filthy job which burnt the paint off it and melted the solder holding the bottom in. I have no intention of refilling it with the pitch. As you say it's a Labour of Love.

Graham.

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Old 4th Feb 2004, 3:46 am   #5
JHGibson
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Default Re: Ultra "Blue Fox"

Graham, I found through several casting jobs that it is important to use the minimum recomended drops of hardener. Too much and the temperature of the mix rises to the point where it is best to whip the whole thing into the freezer section of your refrigerator.

John.

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Old 13th Jul 2005, 6:11 am   #6
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Default Re: Ultra "Blue Fox"

Polyester resin is very good stuff but if You intent to use it to seal something big - 2 cubic centimeters or more - there will be this issue of internnal cracks as the resin shrinks as it cures. For jobs which require constant volume (no shrinking) polyurethane is a better choise.

What a great job with the radio.
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