Detrola 448 Radio/Phono (Updated)

I'm restoring a Detrola Radio/Phono, chassis number 448.

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I'd already pulled the changer and the chassis. The cabinet doesn't look too bad. Some missing veneer and flaking finish, but it looks pretty clean. I wonder if someone gave it the GoJo treatment before it went up for auction.

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The changer is rather stylish, in my opinion. The dual-rod tonearm looks pretty futuristic. The plastic is also interesting, with the swirled patterns. I wonder what kind of plastic it is? I don't want to use the wrong solvent when cleaning it.

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The radio itself is just an All-American 5 with the addition of a phono input. It has some interesting features. 

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First, it is permeability tuned.

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The inductance of the RF coil and the oscillator coil are varied by a powdered-iron slug that moves into the oscillator coil as you turn the dial. I don't know if the RF coil is missing a slug or if it is just tuned by the adjacent slug in the oscillator coil.

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The Solar waxed paper capacitors have to be replaced, along with the electrolytic. I'll have to check the value of the resistors, too, and replace any that are out of tolerance.

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The IF transformers are pretty interesting, too. Instead of extending vertically above the chassis, they are mounted horizontally beneath the ceramic body containing the IF trimmer capacitors.

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The radio has some interesting repair history. The 12SA7 has been relocated to the back of the chassis.

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I'll have to double-check, but I think the tube was relocated to make room for the record changer mechanism. That would point to the changer being replaced by a different model sometime after the radio/phono was manufactured.

On second look, I can see where some of the sheet metal of the changer has been cut away by the expedient of drilling a number of small holes along the cut line. I wonder who made the changer?

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The 12SQ7 detector/audio preamp tube has a sticker with "FAIR" written on it. I guess the tube didn't test into the "good" range on the tube tester. It's an original Detrola tube.

Update

I completed the electronic restoration.

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When I first checked it out, I missed the third coil in the inductive tuning assembly (the bottom one). The schematic only shows two coils in the tuner, the antenna coil and the oscillator coil.

Detrola 448 schematic.png

So now I see the two tuned coils with sliding cores. (The oscillator cathode choke is located under the chassis.)

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But what's that third coil at the top, that I first thought was the antenna coil? It doesn't have continuity, nor any apparent second terminal. It turns out to be the antenna coupling capacitor in the above schematic! 

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The dial cord connecting the tuning knob to the pulley was loose and broke when I tried to tighten it. I replaced it. Fortunately the dial cord for the tuning slugs was intact. I wouldn't relish having to get the tuning cores back in sync if it broke.

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The relocation of the pentagrid converter tube was a hack job. They cut off the old tube socket pins and jumpered them to the new socket.

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I relocated some of the components to make a neater job of it.

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I had to replace a number of Solar Sealdtite wax paper capacitors and a two-section electrolytic capacitor.

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One of the capacitors was across the AC line. I replaced it with the blue X-Y safety capacitor. I used 630 volt axial film caps for the rest. 

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I broke out my Ungar standard line iron with a #4039 heater with integral tip and soldered a terminal strip to the chassis. I then mounted a pair of axial electrolytic capacitors to replace the old dual-section filter capacitor.

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The radio-phono switch contacts were pretty cruddy ...

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... so I removed it and cleaned it up. Swabbing it with isopropyl alcohol was all it needed.

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I replaced the dial and pointer and powered it up on my isolation transformer. It worked, there were no problems with the component replacement.

I gave it an alignment, but the dial tracking was quite poor. Aligning it per the Rider instructions wouldn't let me tune below about 650 kHz. It only has a trimmer, so I compromised by setting the bottom end to 550 kHz and letting it top out at about 1600 kHz.

Is this poor tracking typical of these Detrola permeability-tuned sets, or do I have a problem that needs troubleshooting? If there's a fault, I'm at a loss to understand what might be.

Update:

After all that work, the cone surround popped loose from the basket.

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I glued it back, but the voice coil rubs on the field coil magnet assembly. The old cone had too many repairs, and the glue had shrunk a little over time so that the cone was no longer symmetrical. And I probably didn’t center it as well as needed. 

So, the poor Detrola 448 needs a new permanent magnet speaker. I tried just using a resistor between the first and second power supply filter condensers, but the hum was very bad.

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Some work with a ball-peen hammer and a cold chisel separated the field coil from the speaker basket. I hung the field coil off the back of the chassis.

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I replaced the speaker with a Philmore 4 inch speaker that I procured from our local electronics supplier, Electronics Plus.

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The old speaker had been mounted by the field coil. I improvised a new mounting using a couple of stand-offs and some long sheet metal screws.

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The output transformer was also mounted on the field coil via a pair of folded-over mounting tabs. The leads were not long enough for the new mounting location for the field coil, so I straightened the tabs, removed the output transformer, and epoxied it to the new speaker.

It actually sounds pretty good through the VM 800 changer.

© Steve Byan 2011-2016