Zenith PRO 841x/851x

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      >> Layout and Setup Tips 
      >> Focus Block Replacement 
      >> CRT Tube Replacement 
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The most common failure on the Zenith PRO 851 chassis is the focus block. The original focus block was under-designed, and would fail over time. While the vast majority of these sets were used in sports bars and the focus block would commonly fail within 2 years of the set being installed, there are many of these units on the surplus market and that are being pulled out of seldom used home theaters that still have the original blocks in them. The new style focus block is a 2 piece, and was not prone to the failure of the original one piece design.

The failure of the original block would vary in symptom. The most common symptom would be that the set would turn on, then would turn off, or would run for a few minutes and then shut down. This is due to the focus resistors in the block drifting, and the HV shutdown would turn the set off. Other symptoms would include shifting brightness and/or focus without set shutdown. In very rare cases, the original focus block would fail suddenly and would spot burn or short the tubes out. MY rule of thumb is to change out these old style focus blocks whether they are failing or not. The new style focus blocks never fail again, and will last the life of the set.

Let’s look at the original focus block:

The original focus block is a one piece unit, and is located in the right back corner of the set as looking at the set floor mounted. The 3 focus and three screen controls (master brightness) can be adjusted by removing the back cover of the projector.

Let’s identify the wiring, partially shown above:

4- ground screws, terminated in round screw lugs. These can be bolted anywhere to the metal chassis, Zenith usually puts two ground lugs under each screw, located beside each other.

Three gray G2 wires with single pin connectors that go to each CRT video board

Three red focus wires that connect to the red focus wire coming from each CRT video board.

One long gray and black wire, ending in a 2 pin connector that plugs into the bottom of the deflection board.

One gray or green wire that goes to the small focus modulation PC board next to the focus block. (note that only the PRO 851 vintage of the chassis (post 1993) used this focus modulation wire, the older PRO 840’s did not have this wire. The new style focus block may be used in the PRO 840 by simply leaving this wire unconnected and taped up).

One red wire that connects to the focus wire coming from the HV board.

Four receptacles on the block that accept the main HV feed wire and the three red HV wires going to each CRT.