Author Topic: Emerson model 522  (Read 10270 times)

electra225

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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

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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

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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

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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

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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.

electra225

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Re: Emerson model 522
« Reply #5 on: August 28, 2019, 08:03:35 PM »
I never really found anything wrong with this radio.  I replaced the speaker.  That seems to have increased the volume and it has better fidelity, if that is possible in a five-tube radio.  I may start doing this, at least as a test, when I experience low volume and find no good reason for it otherwise.  I read in an old radio training manual recently that speakers housed in cramped cabinets with a lot of heat can experience warping and damage from too much heat.  That manual recommended replacing the speaker in cases of low volume. 
I don't need Google.  My wife says she knows everything.

TC Chris

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Re: Emerson model 522
« Reply #6 on: August 28, 2019, 08:13:29 PM »
Maybe the voice coil was warped enough that it hardly moved at all?  When they're warped just enough to make contact with the pole piece, they sound all fuzzy and nasty.  I wonder what you'd find if you cut off the dust cap (if it has one) and tried paper shims between the pole piece and voice coil former, to see if there's a gap all the way around.

Chris Campbell

electra225

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Re: Emerson model 522
« Reply #7 on: August 28, 2019, 09:14:43 PM »
The speaker did not sound bad.  It just did not have the volume it should have.  I tinkered around a found a couple small faults in the radio, mentioned above.  Other than that, it seems to be working well.  The signal tracer was the determining factor in this.  Replacing the speaker was a last-ditch effort.  I have a couple other small radios that do not have the volume I feel they should have.  I may try replacing the speakers in those and observe any improvement.  I see a lot of drifted resistors in little radios with cramped cabinets.  Chassis heat takes a toll on everything in some of these little sets.
I don't need Google.  My wife says she knows everything.

ed from Baltimore

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Re: Emerson model 522
« Reply #8 on: August 28, 2019, 10:15:07 PM »
                  If the audio transformer is matching a 2500 ohm output tube plate to a 4 ohm speaker, that 2500 to 4 step-down impedance ratio would become a 50 to 2 voltage ratio (the square roots of 2500 and 4) which works out to one twenty-fifth of the AC voltage on the voice coil that was on the plate, (although the AC current would be 25 times higher).
              If your signal tracer is like the ones I have seen, the tracer probe is monitoring  only the voltage (and not the current) of wherever you have touched the probe and this voltage has so far been higher and higher as you progressed "left to right" along your radio schematic
             Your last measurement on the speaker winding is like moving a voltmeter probe from the 117 volt primary of a power transformer to the 5 volt rectifier filament winding. If the probe was somehow measuring the power transfer, instead of the voltage transfer, you would get only a slight loss of power from primary to secondary, the loss being due to the transformer not being a "powered" source, just an 4 terminal "passive" unpowered electronic component, like a resistor. Transformers usually have above 90 % efficienc0y from primary to secondary but that is, again, power transfer, not voltage transfer.
              You should hear a similar loss of sound level when moving your signal tracer probe from the IF amplifier plate to the detector winding on the secondary of the IF transformer. The second IF transformer usually has a step-down ratio from primary to secondary because the plate of the IF tube is a high impedance and the diode detector is a relatively lower impedance. It should show in DC resistance readings of the primary and secondary. The secondary winding has less turns of the coil, and maybe thicker wire too. The first IF transformer, from converter plate to IF tube grid, does not have this step-down feature, only the second IF can.

         BTW, is that a permanent magnet speaker or field coil speaker. The permanent magnets in really old speakers, pre-Alnico, may have lost some of their strength after a number of years, and with some jarring of the radio. 


electra225

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Re: Emerson model 522
« Reply #9 on: August 28, 2019, 10:56:36 PM »
It is and was a PM speaker(s).  The one in there was a replacement my stepdad put in probably 50 years ago.  I put a new Radio Shack speaker I had on hand.  It sure sounds better, I'll say that.  The speaker did not rattle or anything like that and it was in good condition physically.  I have an Emerson model 518 with an identical chassis that was considerably louder than this one. 
I don't need Google.  My wife says she knows everything.