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Old 15th Aug 2023, 7:41 pm   #1
PJL
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Default Lumophon AC3

Another local auction win, must stop putting in low bids as I seem to win too often. A little dusty/dirty but looks in great condition.

It is an odd design with its arched wooden cover although that does stop plant pot stains. Serial number is 18945 and it was set for 250V AC. It has a set of 4 Triotron valves that I guess are original. Inside looks original too but first impressions is it seems to be engineered down to a price.

First step is to check continuity of the transformer/chokes.
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Old 16th Aug 2023, 10:54 am   #2
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Some pictures as found. The centre knob for tuning rotates one of the ganged tuning condensers. The front panel is not wood, just painted steel.
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Old 16th Aug 2023, 11:17 am   #3
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Default Re: Lumophon AC3

Looks like a nice original set. Early 1930's I guess.
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Old 16th Aug 2023, 9:13 pm   #4
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Default Re: Lumophon AC3

Transformer and heater continuity checks are good BUT I need to park this for a bit as I need to paint the outside of the house whilst the weather is good.
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Old 19th Aug 2023, 12:40 pm   #5
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Default Re: Lumophon AC3

Hello All,
I have one of these and it works surprisingly well.Mine also came from ebay a long time ago.
Mine had an o/c coil that i un-wound, found the break, repaired and re-wound and it worked a treat.
The case on mine cleaned up really well and with a bit of furniture polish, looks great.
Good luck with yours,
Alan.
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Old 19th Aug 2023, 9:27 pm   #6
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Default Re: Lumophon AC3

I saw your thread and picked up the schematic from radiomuseum. The case looks a bit bleached in the pictures but is in pretty god condition and the chassis will improve with a clean.
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Old 31st Aug 2023, 12:28 pm   #7
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Default Re: Lumophon AC3

Started looking at this and all upward facing surfaces are covered in a layer of soot that I am laboriously cleaning off. The rough wire wound cathode resistor for V1 (tetrode) is broken in multiple places, it is marked as 1000 ohms so I have temporarily replaced it with a modern resistor (assuming the inductance is not relevant). By design It is not decoupled which might reduce the gain outside the tuned frequency of the capacitor coupled anode load.

Anyone know where I can get insulated resistance wire?
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Old 31st Aug 2023, 7:11 pm   #8
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Default Re: Lumophon AC3

Hi PJL, I have a range of sizes here of various types and materials. Let me know what you need and I'll see what I have that is suitable

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Old 31st Aug 2023, 8:16 pm   #9
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Thanks Ed, I will get the set going first then send you details.
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Old 31st Aug 2023, 8:26 pm   #10
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Default Re: Lumophon AC3

HT is shorted to earth in the capacitor block. The metal cased block leans like the tower of pisa and the capacitor at fault should not be connected to earth at all so I would guess a meltdown and short to the case. As the only service data I have is a circuit diagram for a German version I need to work out what each tag on the box does before attempting a rebuild.
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Old 31st Aug 2023, 10:01 pm   #11
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Default Re: Lumophon AC3

Lumophon (Bruckner & Stark, Nuremberg, Germany) was well known for fine radios and quite elegant cabinets. A great find, I wish I had saved one.

It is not surprising to find a shorted condenser block. Dismantling ist you may find some greasy or oily paper capacitors all soaked in wax, and a little brown or black area is the culprit of the short. My remedy has often been in just substituting all interior by new capacitors to keep the original box as if nothing has ever happened.

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Old 1st Sep 2023, 9:59 am   #12
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Default Re: Lumophon AC3

Thanks Joe, I have found all the capacitors I need to rebuild the block. I have checked other critical components, resistors and transformers, and they all seem OK. I am busy for the next few days but will post a picture of what I find inside!
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Old 3rd Sep 2023, 6:25 pm   #13
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Default Re: Lumophon AC3

I have the capacitor block apart now and it shows a short across one of the 2uF smoothing capacitors.

There are also several differences from the W30 circuit diagram.
1. The aerial tap switch has been replaced with a variable aerial coupling capacitor.
2. The output valve grid bias feed has had the 0.15M replaced with a link.
3. There is an extra 1uF capacitor in the can that I suspect decouples the 600 ohm resistor but difficult to prove as many of the connections are buried in bitumen.
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Old 3rd Sep 2023, 9:17 pm   #14
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Default Re: Lumophon AC3

Some pictures of the capacitor block. The box had split and needs soldering back together.
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Old 4th Sep 2023, 12:55 am   #15
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Default Re: Lumophon AC3

That looks significantly 'cleaner' than the Murphy block I had to boil for an hour for all the bits to come out.
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Old 4th Sep 2023, 11:22 am   #16
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Default Re: Lumophon AC3

The black stuff is not true bitumen it is a wax. Turns out the shorted capacitor is the 1uF not shown on the circuit diagram.
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Old 4th Sep 2023, 11:26 am   #17
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Default Re: Lumophon AC3

Quote:
Originally Posted by PJL View Post
There are also several differences from the W30 circuit diagram.
It was quite common to upgrade the circuitry as radios developed so you may well find differences between the radio and the published circuit. During the war radios tended to get severely hacked as they needed to make use of whatever valves were available.
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Old 4th Sep 2023, 1:55 pm   #18
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Default Re: Lumophon AC3

A capacitor block with positions and values of the contents marked?


How neat is that?
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Old 8th Sep 2023, 10:43 am   #19
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It's back together but is running with HT a bit high and 15mA output current so I need to sort out a speaker as that is more than I would like to put into a period horn.
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Old 14th Sep 2023, 11:40 am   #20
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I chose to add a connection to the output transformer primary of another set I am working on. It is not very sensitive but a proper aerial and earth would no doubt improve it, I am using a 6ft wire. It did seem to get better over the first 5 minutes of use presumably waking up the old valves. Health & Safety not too good as it has 250V DC on the speaker wire.
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