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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 24th Jul 2018, 5:21 am   #1
red-duck
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Default 1933 - Airzone 303

As you guys battle through a "hot" English summer, I though you might enjoy reading a story from a cool, wet & wintry Western Australia where the maximum temperatures struggle to reach 21C.

I spotted this radio on "Gumtree" in January this year. I'd always wanted a cathedral radio. Admittedly, it looked just a tad rough. It had spent the last 30 or more years on the floor of someones shed. The valves & valve cans were missing, the speaker cone was torn and the plywood base was rotten in places. But the asking price was low (for Australia) ~GBP20. So, wanting both a challenge and a cathedral radio I bought it. It then spent a couple of months in the loft whilst I practised my limited carpentry and learnt the art of shellac-ing restoring the cabinet of a 1934 battery console (Another potential success story).

I also did a bit of research on the set. This model was originally brought to market in early 1933 during the depression. Some of the earlier sets were sold with out escutcheon to save on costs. My set appears to be an early production number - serial # 238 and had both the scored wooden "escutcheon" and a metal escutcheon over the top of it.

The circuit is very basic using 2.5V american style 57, 58 & 59 valves as the "1st detector/oscillator", IF amp & audio output respectively. I spent several hours going through hundreds of valves stored in the loft ... everything but these. I was fortunate to have an 80 & UX80 though.
There was no point in buying the 3 valves until I'd checked that there were no other show stoppers.

A check with the multimeter revealed that the "unique to this model" 6000 Ohm tapped field coil was open circuit and so was the primary of the audio output transformer.

The mains transformer checked OK, IF transformers checked OK, Oscillator coils OK, Antenna coil -open circuit but fortunately an easily repaired bad connection in the can. All resistors were within tolerance - note they are all wire wound resistors - see photo. And as expected all the capacitors were leaky with the exception of the 3 silver mica ones.

What to do? Maybe forget about the electrical restoration and restore the cabinet. It will make a nice ornament. So I set restoring the cabinet and replacing the speaker cloth. I replaced the rotted plywood base and rear "arch". r and filled in the rotted areas behind the veneer at the base of the cabinet with epoxy builders "bog". Once the cabinet was mechanically sound, I stripped it back to the wood using meths and wire-wool which removed the original shellac finish. Cleaned up the trim on the front base and reglued/pinned it. Then applied several coats of shellac, waited a week, cut it back, more coats of shellac, waited a week then polished with wire-wool & beeswax. I was pleasantly surprised on how well the cabinet had come. Even the cleaning lady commented on how good it looked.

Spurred on by this success, I decided to revisit the electrical side. There were no HT voltages on the circuit diagram. But I read that the original sets ran a very high HT to squeeze every drop of gain out of the valves. Mmmmm. would it work with a lower HT voltage?
I had another 6" loudspeaker with an intact 7500 ohm field coil and output transformer . The additional resistance would probably result in an HT some 30-40v lower than original. Only one thing to do try it.
I installed the speaker, and a 2700 ohm 10w resistor to provide the HT2.

I went ahead and ordered the missing 57, 58 & 59 valves from the HRSA valve bank, replaced the wax capacitors & two electrolytics, replaced the rubber bushings and grommets and a new 3 core mains cable and gland.
All voltages on the mains transformer checked out and I ran it for several hours with only the rectifier & dial lamp in circuit. No issues.

The three valves arrived in the post. I fitted the 59 output valve initially - HT1 ~275V & HT2 (screen) ~245v. Still a respectable HT.

Then in went the 56 & 57..
The set came to life .. and rotating the tuner brought in several stations ( Yes, we still have AM radio down-under!). However, the set was pretty unstable. I didn't have the original screening cans - which were unique to this set and now as rare as rocking horse droppings. I did have some "goatskin" valve covers which fitted the 56 & 57. With these installed the instability was gone.

The set performs reasonably well and has good selectivity. There is no AGC in the set. The volume control (gain) has to be backed off considerably when tuning in the two most powerful AM stations ABC Radio Perth & ABC Radio National otherwise the sound is pretty distorted. The lower powered commercial and community stations sound fine.

I'm quite pleased with the results and I now finally have a working cathedral style radio.
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Old 24th Jul 2018, 12:13 pm   #2
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Default Re: 1933 - Airzone 303

Pics were missing with original post
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Old 24th Jul 2018, 12:14 pm   #3
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Default Re: 1933 - Airzone 303

Few more pics and circuit diagram
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Old 24th Jul 2018, 2:15 pm   #4
merlinmaxwell
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Default Re: 1933 - Airzone 303

Classic short superhet, wonderful cabinet restoration too.
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Old 24th Jul 2018, 5:15 pm   #5
PJL
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Default Re: 1933 - Airzone 303

A pentode used as a mixer/oscillator. Not seen that before.

Very nice restoration and doesn't look too big which is the common problem with some of these earlier sets.
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Old 24th Jul 2018, 5:20 pm   #6
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Default Re: 1933 - Airzone 303

Well done. Very satisfactory outcome. Interested to know what you used for the speaker cloth? Looks very much in keeping with the original material.

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Old 25th Jul 2018, 2:42 am   #7
red-duck
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Default Re: 1933 - Airzone 303

Thanks for your comments Gents.

Using the 57 rf pentode as an autodyne-mixer was quite common in the early 30's before the more wide spread appearance of the "octode" & "pentagrid" converter valves.

If you are interested this link explains some of the nuances and background behind the circuitry of this set.
http://messui.polygonal-moogle.com/valves/SC199711.pdf

There is a trove of articles on this site ( http://messui.polygonal-moogle.com/valves/index.html ) on vintage radio (people, repairing, restoring, collecting etc) which originally appeared in Electronics Australia, Silicon Chip & other Australian magazines from the 1960s through to this century.

The speaker cloth is a reproduction of fabric which was used by Airzone. I purchased a couple of metres of it several years ago and I've forgotten the source now. But this style cloth was used by Airzone throughout the 30s and early 40s. I've attached a photo of a 1940 Airzone Theatre which I'm currently working on for which I originally purchased the fabric but got sidetracked. LOL. The other set I used the cloth on was a Battery console Kit radio from ~1935.
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Old 25th Jul 2018, 10:49 am   #8
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Default Re: 1933 - Airzone 303

Very nicely done Nigel!
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