Electrostatic (ES) vs Electromagnetic (EM) Focus

CRT with electromagnetic focus

Most projectors made until about 1994 used an electrostatic focusing method. This consisted of one electronic focusing control as well as several magnets located around the neck of the picture tube for focusing fine tuning. This was a perfectly acceptable method of focusing, and standard TV sets and non HDTV rear projectors still use this method of focusing today. The ability to focus slowly degrades over time as a tube is being used. This is why a ES focusing CRT projector installed in a sports bar with many thousands of hours on it will show a defocused picture. Sports scores become hard to read, fine print is impossible to read, and you cannot correct this with internal focusing controls. Old regular television sets with the same technology suffer from this as well.

In about 1993-1994, Electromagnetic focusing was introduced. This consisted of the main electronic focusing as well as the magnets around the CRT’s, however an additional focus coil. was added around the neck of the picture tube. This allowed the electron beam to be focused more than with ES focus, and projectors with EM focus will allow specific areas of the screen to be focused independently. Found in all current mid to high end date grade projectors, and is desirable to have (but also adds to the price of a used projector). Perhaps most importantly, EM focusing sets maintain about 95% of the original focusing capability over the life of the tube. EM focusing tubes will still produce poor colors after the phosphor has significant wear on it, but the tubes retain the ability to focus over long periods of run time.

7” ES focus sets:

- Electrohome ECP series
- Barco 400, 500, 600, 700, 701, 708, Cine 7
- Sony D50
- Runco DTV 852
- Sony 10XX series, D 50
- Zenith PV 800, 810, 820, 830. PRO 840, 841, 851, 880, 895, 900, 900x
- Ampro 1500, 2000, 2600, 1100, 1200

8” ES focus sets:

- Sony 12XX series (sometime referred to as 7” sets)
- Barco 800, 801, 801s

9” ES focus sets:

- Barco 1000, 1001, 1101, 1500, 1600
- Ampro 4000

7” EM focus sets:

- Ampro 2300
- NEC PG series sets (actually 8” )
- NEC GP-3000, DP-1200 (older technology, not as sharp as the later PG series sets)

8” EM focus sets:

- Ampro 3300, 3600
- Barco 808, 1208, Cine 8
- Electrohome Marquee 8XXX series
- NEC PG Xtra, XG series
- Sony G 70

9” EM focus sets:

- Ampro 4200, 4300, 4600
- Electrohome Marquee 9XXX series
- Sony 1292, G-90
- NEC 10 PG
- Barco 1200, 1209, Cine 9
 

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