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Emerson 25A restoration - Printable Version

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Emerson 25A restoration - klondike98 - 08-30-2018

Back in July's Phinds I noted that I'd picked up an Emerson 25A, schematic LINK.  

[Image: https://www.dropbox.com/s/v0ioedp4u8x80my/emerson25A_01.jpg?raw=1]

At the time I just posted a quick picture and it came up that it might have a resistive "curtain burner" power cord.  From the schematic it appears that R7 305ohms would be the power drop resistor and it would be in the chassis.  But after taking a look at the chassis more closely I can see remnants of the resistive element in the power cord (a bit of asbestos insulation still showing) so that R7 is simply the schematic representation of the resistive power cord element.

[Image: https://www.dropbox.com/s/7tvcs04pgx5659y/emerson25A_02.jpg?raw=1]

I've not replaced one of these before so am reading about capacitors as voltage dropping components.  There are a number of articles out there...more later.

The on/off/volume control was very dirty and I thought might have failed completely but after taking it apart and cleaning it up it seems to be work OK now.  I did find that the volume pot is only a 25K ohm pot rather than the 200k noted in the schematic.  It remains to be seen whether the schematic is in error or the pot is a replacement.  I do have a 250k on/off pot that I can substitute if needed.
[Image: https://www.dropbox.com/s/vphk5xynx07gucd/emerson25A_03.jpg?raw=1]

RE: Emerson 25A restoration - Radioroslyn - 08-30-2018

Such a nice numeric tube line up. I would think that 20K is a good value for the pot. At 200K all of the adjustment is going to be a small portion at one end.
Yeah you don't want to use a resistor to drop the line to the heater voltage. Has to drop abt 100v@ 300ma = 30w. That's the same heat from four 50L6's. Cap is the way to go. 6.8 mf should do it for you.

RE: Emerson 25A restoration - Paul Philco322 - 08-31-2018

That is a way cool cabinet, with real inlaid marquetry no doubt. Even if not the best player that will look very nice on the mantle.

Paul

RE: Emerson 25A restoration - klondike98 - 08-31-2018

I agree Paul...I like the styling of these little emersons.

Thx Terry, here's one article on the cap voltage drop approach. Its example is essentially the same heater voltage requirement as mine, 25.2 volts. http://www.northlandantiqueradioclub.com/article-linecord.shtml

But...I'm confused...36, 37, 38 and 39 tubes (a 38 is included in the article example) have DC heaters don't they?? Capacitive voltage drop circuit yields an AC voltage. Or can they run on either AC or DC since the radio schematic sure looks like the heaters would be AC if you used AC line voltage.

RE: Emerson 25A restoration - Eliot Ness - 08-31-2018

Nice set Bob!  I like those small depression era mantel sets and I have an Emerson similar to yours, but I think yours is a little fancier cabinet.  I also have a neat Fada of the same style I got from Kirk a while back.

Bob, the radio is AC/DC so using the capacitor to drop the AC voltage would be the same as using the resistive line cord on AC

RE: Emerson 25A restoration - morzh - 08-31-2018

That white stuff in the resistive element is asbestos. Run! One mile while holding your breath! Icon_lol

RE: Emerson 25A restoration - Radioroslyn - 08-31-2018

Hi Bob,
<But...I'm confused...
I didn't realize we are from the same state[Image: http://philcoradio.com/phorum/images/smilies/icon_eek.gif]

<36, 37, 38 and 39 tubes (a 38 is included in the article example) have DC heaters don't they??
No. They can be use with both ac or dc. The short answer is that a tube w/a cathode can use both a tube that doesn't have a cathode (aka directly heated) only use dc.

> Capacitive voltage drop circuit yields an AC voltage.
Correct as long as the input voltage to the cap is AC.

> Or can they run on either AC or DC since the radio schematic sure looks like the heaters would be AC if you used AC line voltage. 
Yes you can! BUT only a resistive dropped will drop the ac or dc voltage by the same amount. You could use a resistor and a diode as the dropper. Will work w/ ac applied to it but if you apply dc to it depending the polarity one way tube won't lite at all. The other way the diode will conduct full on w/o any drop. You can get abt a 40v drop across a diode.
A capacitive dropper w/dc on it I would think it won't conduct so the heaters would lite at all.

Hope this answers your questions.

RE: Emerson 25A restoration - klondike98 - 08-31-2018

Thanks guys, very helpful!!

RE: Emerson 25A restoration - klondike98 - 09-02-2018

Unfortunately the power supply choke is open on this set.  It has had some previous work done to it.

[Image: https://www.dropbox.com/s/wryjchvk05soox1/Emerson25A_04.jpg?raw=1]

I peeled back some of paper and found the connection to lug was bad.  

[Image: https://www.dropbox.com/s/cbzkb27c1ofyfxb/Emerson25A_05.jpg?raw=1]

Slpliced it and now have continuity but it is reading 400k ohms which seems too high for a choke.  Am looking for some additional info on this choke but so far have not found it.

RE: Emerson 25A restoration - klondike98 - 09-05-2018

Dang it...The choke fix did not work. I retested it today and got 1.2meg ohms and futzing with it a bit shot it up to 4meg...

I think my best bet would be to replace it but I have not found any info yet on specification.

Does anyone have any thoughts on the correct size this choke should be? Its connected to C9.

Scematic link http://www.nostalgiaair.org/pagesbymodel/564/M0004564.pdf

RE: Emerson 25A restoration - Radioroslyn - 09-06-2018

The specs for the choke aren't very critical. Physical size is one of the more important so it can be fitted to the chassis. The more inductance the better. Current wise 20ma or higher should be fine. If you have a spare tube type output transformer that fits you can use the primary as a choke and leave the secondary unused. Or shoot Gary a note w/the dimensions and he could scare up something suitable. Or you could replace it w/resistor (abt 1-2K 2w) and increase the value of the two filter caps to 47mfd@160vdc.

GL

RE: Emerson 25A restoration - klondike98 - 09-06-2018

Thanks Terry!!!
Your advice is always so helpful.
Icon_thumbup

RE: Emerson 25A restoration - klondike98 - 07-08-2019

I have finally gotten back to this little Emerson 25A.  I did get the capacitor-voltage drop components into place, replaced the choke with a new one and got it fired up successfully.  Unfortunately by the time I had it all back together and buttoned up I realized I had not taken any photos of the chassis...  Icon_e_sad  It plays OK although at relatively low volume.  I only have a short 10' antenna so longer would help some as well but I've played with this one enough so ... done for now.  I left the finish pretty much original.  It was dirty and the original clear lacquer had started to alligator a bit so I very lightly sanded, did a little restore-a-finish treatment, let that dry and then added a few coats of clear gloss lacquer.  I think it came out surprisingly well.  The grill cloth was replaced with the original grill cloth that I had taken out of the Emerson 250AW that I had worked on previously.  

[Image: https://www.dropbox.com/s/18bs8n0tqcj394i/emerson25A_after.jpg?raw=1]

[Image: https://www.dropbox.com/s/xjmmpcjlubhtf9h/emerson25A_back.jpg?raw=1]

RE: Emerson 25A restoration - Eliot Ness - 07-08-2019

It looks very good Bob, nice save on the finish!

RE: Emerson 25A restoration - mikethedruid - 07-08-2019

Congratulations ! The finish looks great. Nice job! Icon_clap