Author Topic: Emerson model 522  (Read 1003 times)

electra225

  • Hero Member
  • *****
  • Posts: 3333
    • View Profile
Emerson model 522
« on: May 27, 2019, 02:05:16 PM »
My grandpa bought this radio for me in 1955.  He paid $5 for it from a man he worked with.  It is nothing special, except that I have had it for so long.  A little bakelite cabinet AA5 that was, until recently, a robust performer and was all original inside.  I fired it up after I got home like I try to do all my radios.  The output of the little Emerson was weak, not much volume.  So I tested the tubes, all Emerson-branded, RCA supplied.  They tested good.  I recapped the chassis completely, including filter caps.  I checked all the resistors.  I have good voltages on the tubes.  Both audio sections are amplifying the signal.  I still have weak audio out of the speaker.  The sensitivity seems good.  I used my signal tracer to verify that both sections of audio are amplifying the signal.  The signal is loud at the plate of the 50L6, yet weak at the speaker.  The output transformer measures 200 ohms on the primary and around an ohm on the secondary.  I have tried a different speaker.  It appears that the secondary of the output transformer is the problem.  I suppose it is technically possible that there could be a shorted turn or turns in the secondary.  I don't remember running in to a shorted secondary on an output transformer before.  I'd think that if the primary was a problem, B+ would be affected.  Am I on the right track, or am I overlooking something?

The schematic for this thing is in the 1946 Beitmans, pages 37 and 38.  There is a revision on the chassis for some models.  This revision has no center tap on the OP transformer.  Mine does not have the center tap.   
I don't need Google.  My wife says she knows everything.

TC Chris

  • Hero Member
  • *****
  • Posts: 2118
    • View Profile
Re: Emerson model 522
« Reply #1 on: May 27, 2019, 04:19:29 PM »
Weren't most of those little speakers 3.2 ohms impedance? And if the OPT matches that, its DC resistance would be pretty low.  Maybe substitute in another little transformer & speaker?  Seems to me it would be pretty hard to fry the secondary on an OPT.

Chris Campbell

electra225

  • Hero Member
  • *****
  • Posts: 3333
    • View Profile
Re: Emerson model 522
« Reply #2 on: May 27, 2019, 04:36:37 PM »
I fixed it!   :) :)

The problem was 100% self-inflicted.   ::) :-[

I have an Emerson 518 that has a little different bakelite cabinet, but it has the same chassis.  Turns out it is the early version with the center-tapped transformer.  I pulled the chassis on it and put it on the bench next to the other one to compare.  First off, the output transformer checked exactly like the other one.  Second, the audio was louder at the plate than it was at the speaker.  This radio has had the filter caps changed, but it's otherwise original.  I powered up both chassis and started comparing what I was seeing.  I poked and prodded with my plastic stick in the chassis I was having trouble with when the volume suddenly popped up.  I poked some more, and it went back down.  Prodded some more and it came back up.  I used my lighted magnifying glass and found the problem.  When I recapped the chassis, the replacement caps were smaller in size than the originals.  I had to remove the original spaghetti tubing and use longer new pieces.  I must have come to the end of the piece I was using, because on the coupling cap between the 12SQ7 and the 50L6, the spaghetti tubing was about a quarter of an inch short.  When I stuffed the filter caps back into the crowded chassis, the wire on the coupling cap came in contact with the frame of the output transformer, shorting out the signal.  Why it was not totally dead, but was just weak, I have no explanation for.  I cut the cap out, replaced it with a long enough piece of spaghetti tubing and all is well.  I still can't explain why the audio at the output tube plate is stronger than it is at the speaker.
I don't need Google.  My wife says she knows everything.

TC Chris

  • Hero Member
  • *****
  • Posts: 2118
    • View Profile
Re: Emerson model 522
« Reply #3 on: May 27, 2019, 10:09:26 PM »
Louder at the plate?  Depends upon what you're listening with, right?  I have an old high-impedance headphone that I use with a capacitor in series as a signal probe.  That might be louder at the plate than at the OPT because of impedance differences.  Last week I posted a photo of my original RCA suitcase phono with a massive one-tube amplifier.  As a kid I decided to hook it up to the 1938 Zenith's 12" speaker.  The output was disappointing.  I reasoned, "Hey, there's a transformer between the amp and speaker--maybe if I run the speaker directly from the plate it will have more oomph."  As I recall, the hypothesis was disproved.  The poor 25L6 must have been befuddled by just a few ohms of load.

Chris Campbell

electra225

  • Hero Member
  • *****
  • Posts: 3333
    • View Profile
Re: Emerson model 522
« Reply #4 on: May 27, 2019, 10:44:07 PM »
I was using my signal tracer.  I started at the hot end of the volume control and moved toward the speaker.  I probed the hot end of the volume control, then the plate of the first audio tube, then the grid of the audio output tube, then the plate of the audio output tube, then on to the connections on the speaker from the OP transformer.  The signal got progressively stronger at each subsequent location until I got to the speaker, where it was noticeably weaker.  I expected it to be at least as strong as it was at the plate of the audio output tube.  The signal was the same strength on both radio chassis.  I'm at a loss to explain just why it was like that.  The only reason I can think of offhand is that I had the gain up too high on the signal tracer.  Or that the impedence on the speaker terminals were different enough from the impedence on the tube plate to affect the signal.  I'm going to experiment on something else to see if I get the same results. 
I don't need Google.  My wife says she knows everything.