Emerson 639

7" screen, 1949
click on image to enlarge

Tabletop model 639 and portable model 600 are more than simply transformerless, they are true AC/DC sets.  The design is remarkably compact, squeezing ten octal and nine miniature tubes onto the small chassis along with with the 7JP4 electrostatic picture tube.

Tube Complement
RF amplifier 1  6AG5  
converter 1   6J6  
video IF amplifier 3  6AU6  
video det/AGC rect 1  6AL5  
video amplifier ½ 12AU7  
video output 1  25L6GT  
audio IF limiter 1 6SH7  
discriminator/AF amp 1  6S8GT  
audio output 1 25L6GT  
sync separator ½ 12AU7  
horizontal phase inv 1 12AU7  
horizontal phase det 1  6AL5  
horizontal oscillator 1 12SN7GT  
horizontal output 1  25L6GT  
vertical oscillator 1 12SN7GT  
vertical amplifier 1  6SL7GT  
HV oscillator 1  25L6GT  
HV rectifier 1   1B3GT  
Restoration notes

This is the first 7" electrostatic set I worked on.  Although I had acquired one at the ETF auction in 2012, another in much better shape turned up on Facebook in 2017 and that's the one I restored.  Except not exactly.  It was kind of working, but with just enough distortion to appear as if it was struggling with its aged original components.  I soon discovered that the set had been partially restored previously.  I attended to some critical items that that restorer did not, namely the replacement of the orginal electrolytic capacitors.  After swapping the selenium rectifer for a 1N4004, I then rectified the previous restorer's fatal error:  I removed the ceramic high voltage sweep couplers he had installed and put in 6KV film caps.  Ceramic capacitors are not suitable for that application due to the fact that they have a certain voltage dependency at their rated capacitance.  That is, when the deflection plates see huge voltage swings, the caps' capacitance varies nonlinearly, which ruins the linearity of the sweep.

There's hum in the video, but I couldn't figure out what was causing it.  None of the tube swapping I did identified a culprit for heater-cathode leakage and (of course) all electroltics are new.  It may be a lead dress issue, as I was forced to slightly redress some lines in order to fit all the electrolytic caps underneath.  For now I've decided that it's just not an issue major enough to delay progress with new projects. 

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