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 Post subject: Emerson 38
PostPosted: Jan Sun 28, 2018 6:40 pm 
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OK, here's the story. Bought this set at an auction, cabinet needed work but the set itself had supposedly been restored. So I finished the cabinet and was ready to install the set when I decided I might want to see if it really plays...it did, sort of. Large hum but if I got it on a strong station, the audio overcame most of that. So I thought i'd take a look under the hood. right off I could see that negative of both electrolytics went to ground. That doesn't match the schematic. also the field coil was gone and replaced by a permanent magnet (see pics) and there were 4 ceramic resistors wired in, 2 300's a 150 and a 135 = 885 ohms, the FC on the set was 2500 ohms. Nothing in that area seems to be as shown in the schematic. the weird thing is that is does play. I was about to start setting things right but I thought I would get some expert input before I quite possibly make things worse


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 Post subject: Re: Emerson 38
PostPosted: Jan Sun 28, 2018 6:55 pm 
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Hi

The speaker has been replaced with a PM type. In this radio you don't need a speaker field. It's just from B+ to ground.

There should be another 500 ohm choke. Maybe that's what the resistors replace? Without the choke you could have hum but former owner did use higher value electrolytic caps.

Draw out what the former owner did. You could replace the group of resistors with 1K @ 10 watts. That will reduce B+ a little and help filter voltage.

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 Post subject: Re: Emerson 38
PostPosted: Jan Sun 28, 2018 7:04 pm 
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ok Norm, let me work on this drawing for a minute thanks


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 Post subject: Re: Emerson 38
PostPosted: Jan Sun 28, 2018 7:18 pm 
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 Post subject: Re: Emerson 38
PostPosted: Jan Sun 28, 2018 8:04 pm 
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The two 300 ohm resistors in parallel are being used to drop filament voltage. In operation these will be really hot. They are being used to replace resistor marked 135 and maybe 20 in your schematic.

http://www.nostalgiaair.org/pagesbymode ... 004587.pdf

Other resistors are for lights. Does AC power come in on pin #2 or #5 of 25Z5?

Non of these resistors replace the 500 ohm choke shown between positive ends of electrolytic filter caps. Can you find this choke? If it's been removed need to add resistance between positive ends of filter caps. Without it there will be hum. See schematic where choke is located.

Once operating there are ways to reduce/eliminate heating of the 300 ohm resistors.

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 Post subject: Re: Emerson 38
PostPosted: Jan Sun 28, 2018 8:18 pm 
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Norm, I thought that 500 ohms was the audio transformer, I read 417 across it


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 Post subject: Re: Emerson 38
PostPosted: Jan Sun 28, 2018 8:20 pm 
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and looking again, it it but that's not what you were asking. So, no, I don't seem to have a 500 ohm choke anywhere


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 Post subject: Re: Emerson 38
PostPosted: Jan Sun 28, 2018 8:36 pm 
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power is coming in on 2 and 5 (they are jumped together)


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 Post subject: Re: Emerson 38
PostPosted: Jan Sun 28, 2018 8:41 pm 
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Where are two electrolytic cap positive leads connected? One should be on each side of the 500 ohm choke. Maybe there is a resistor in its place?

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 Post subject: Re: Emerson 38
PostPosted: Jan Sun 28, 2018 9:54 pm 
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one goes to a coil, the other goes to the #3 to the 25Z5, #2 on the 43 tube and #3 on 6D6 and one side of the audio transformer. both negative ends go to ground


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 Post subject: Re: Emerson 38
PostPosted: Jan Mon 29, 2018 12:25 am 
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What resistance do you have between two positive ends of the electrolytics? Schematic shows a 500 ohm choke. You may have a resistor between the two points?

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 Post subject: Re: Emerson 38
PostPosted: Jan Mon 29, 2018 12:54 am 
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Nothing obvious but I show 8900 ohms between the two positives


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 Post subject: Re: Emerson 38
PostPosted: Jan Mon 29, 2018 2:06 am 
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Can you find a resistor between the two points? Schematic shows the choke as being 500 ohms. If I was to use a resistor in place of the choke might be 1000 ohms but 8900 ohms is too high.

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 Post subject: Re: Emerson 38
PostPosted: Jan Mon 29, 2018 2:55 am 
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the wiring on the electrolytics doesn't match the schematic. I'll do some more digging and let you know what I find. Thanks Norm


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 Post subject: Re: Emerson 38
PostPosted: Feb Sun 11, 2018 9:03 pm 
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OK, Sorry for the delay. Life gets in the way. Here's what I have found. Some of the wiring on the positive side of the electrolytics was off, so I put that back to match the schematic. Now I have no sound at all and I attribute that to the missing choke (500 ohm) between the electrolytics. I don't have an actual choke, is there an acceptable resistor I could substitute?


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 Post subject: Re: Emerson 38
PostPosted: Feb Sun 11, 2018 11:20 pm 
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You need something in place of the 500 ohm choke but not in place of the field coil. Field coil is between B+ and B- so isn't needed.

I would use a 1000 ohm @ 10 watt resistor in place of the 500 ohm choke. Without it you won't have B+ power to your radio. May need higher values of electrolytic filter caps if there is hum.

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 Post subject: Re: Emerson 38
PostPosted: Feb Mon 12, 2018 12:27 pm 
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That did the trick, no hum, thanks!


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 Post subject: Re: Emerson 38
PostPosted: Feb Wed 14, 2018 2:57 am 
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Nice radio, and you got it cleaned up and playing. Congrats.

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 Post subject: Re: Emerson 38
PostPosted: Feb Mon 19, 2018 5:51 pm 
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The latest, final and somewhat humorous update.

After all the small issues, I still had a very weak pair of dial bulbs. As near As I could figure the Previous owner decided that if two like resistors in parallel reduced the resistance by half, then two in series must cancel each other out and since a 135 and 150 were used, the net difference would be 15 and that would be close to the 20 needed. It's just a guess but it's the best I could figure. SO I pulled the two in series and replaced them with a 20, turned the set on and promptly fried a dial bulb. The wiring to the resistors (seen on pics in earlier posts) had always bothered me so I did some investigation (Yes, I should have done that earlier but where's the fun in that?) Yep, wired wrong. So, that's fixed and she plays very nicely, thank you, and the dial bulbs light as they should! Something else I learned on this project. I was told that some speakers have not only a field coil but also a choke. because this speaker wasn't original, no choke. Anyway, thought I would pass that along.

Thanks for all the help.

Mitch


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