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Hints, Tips and Solutions (Do NOT post requests for help here) If you have any useful general hints and tips for vintage technology repair and restoration, please share them here. PLEASE DO NOT POST REQUESTS FOR HELP HERE! |
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7th Mar 2005, 11:02 pm | #1 |
Retired Dormant Member
Join Date: Apr 2003
Location: Salt Spring Island, BC, Canada
Posts: 368
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Improving a 1932 Ultra "Blue Fox".
Having restored my 1932 Ultra "Blue Fox" (see success stories), I was anxious to try it out, bearing in mind that it was only a two valve model with reaction as the only volume control. I connected an aerial and switched on. The result even with reaction at zero was a cacophony of many stations at full volume. I live in a city with about 20 stations within 5 miles.
I had to reduce the aerial input signal and the best solution turned out to be a 365 pf variable capacitor in the aerial lead. Now the loudest station could be reduced to a whisper but selectivity was aweful because of the simple reaction circuit. When I tried to advance it almost to the point of oscillation (maximum selectivity), the radio would plop into a loud howl, not to be confused with a heterodyne whistle. This was "Fringe Howl". When the detector goes into RF oscillation, the anode current dips because of the increase in leaky grid bias. This current change causes a downward inductive voltage kick in the interstage transformer primary which stops the oscillation because of the reduced anode potential. When this inductive voltage pulse ends. the detector resumes oscillation. This repeats at an audio rate hence the howl. The solution was to end the current in the primary by adding a 51K resistor in the anode circuit and parallel feeding the interstage transformer through a 1 ufd capacitor and earth connection. The reaction was now silky smooth and between stations would glide into oscillation with only a hiss. However if the reaction control was greatly advanced, the oscillating detector would start squegging with an earsplitting howl. This particular defect was cured by adding a 1N4148 diode across the reaction feed back coil. The RF oscillation was now limited in amplitude, with the further advantage that the RF energy fed into the aerial and radiated was greatly reduced, no doubt to the relief of neighbours listening to the same stations. These modifications were duly documented and placed in my file folder on this radio so that a future owner could restore the radio to its original condition. Disclaimer- In the course of these mods, no vintage parts were removed or holes drilled. John. Last edited by Paul Stenning; 8th Mar 2005 at 8:03 pm. |
5th Jul 2005, 8:22 pm | #2 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Improving a 1932 Ultra "Blue Fox".
Hello GHG,
Very interested to read about your ULTRA 'BLUE FOX'. I called into my local rubbish dump around 10 years ago and one of the lads said 'I've put an old radio in the shed for you John, I've taken the back off and its got valves in....' I discovered in the shed an old cardboard box containing small balls of crushed paper surrounding the BLUE FOX . It is in fantastic condition, original and complete. In the workshop I ran it up on the variac and discovered the AC/PEN glowing blue due to being 'gassy'. A replacement was sorted from the bowels of my valve store and the set then worked as new! The performance is incredible delivering full output from the AC/PEN on several stations here in rural West Sussex. I use about 10ft of aerial wire tacked along the ceiling beam. The set is used from time to time and is kept in my living room. I often wonder who owned this set from new. It must have been quite a prized item in its day and a credit to Teddy Rozen the owner of ULTRA Radio who built it. John |
24th Jul 2005, 2:05 am | #3 |
Retired Dormant Member
Join Date: Apr 2003
Location: Salt Spring Island, BC, Canada
Posts: 368
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Re: Improving a 1932 Ultra "Blue Fox".
John, I also was positively amazed at the performance from a basic two valve radio. I can tune my multiple local stations with very little interference on one from another even though the frequency spacing gets down to some 50 kcs between them.
John. |
24th Jul 2005, 12:13 pm | #4 |
Dekatron
Join Date: Jun 2005
Location: Warnham, West Sussex. 10 miles south of DORKING.
Posts: 9,145
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Re: Improving a 1932 Ultra "Blue Fox".
Hello John and thanks for your reply. The 'BLUE FOX' is indeed incredible performance wise. As a young lad I built many sets of this type from scrap components but never reached anything like the results of the ULTRA. The output valve AC/PEN was the first MAZDA indirectly heated output valve and gave a very good account of itself that would hold good even today. The detector is transformer coupled to the output valve and I think the transformer and aerial coil must have been built to a very high standard. Its great to know you have one of these receivers, I don't think there can be many around. Regards JOHN.
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25th Jul 2005, 10:36 am | #5 | |
Octode
Join Date: Jun 2004
Location: Hampton Vale, Peterborough, UK.
Posts: 1,698
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Re: Improving a 1932 Ultra "Blue Fox".
Quote:
-Tony |
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