1970s Zenith Console - Troubleshooting

[Jeffhs]

Originally Posted by ThePlague

Hello all,

I recently acquired this 1970s Zenith. From what I can gather, the chassis is 25DC56 or 25DC57.

The seller provided video proof of it working prior to being stored in the garage for ~year. There is a slight letterbox burn in on the tube. Hopefully this won't be an issue when playing content.

Currently, I plugged the unit in for the first time and upon turning it on I smelled ozone. It is currently non-responsive. There was no audio present either or else I would suspect the HV circuit.

Does anyone have any experience or item to check on this particular chassis before I begin replacing capacitors in the power supply section?

If the TV is not showing a picture after it is turned on, but the sound, pilot lights, etc. operate normally, the problem might be in the deflection circuits; however, since you say there is no sound, and the TV is otherwise dead, the problem may well be in the B+ supply, or perhaps a fuse has blown. If the latter, however, make absolutely certain the problem which caused the blown fuse is corrected before replacing the fuse, otherwise the set will just keep blowing fuses.

You mentioned capacitors in the power supply. These could also cause problems with the B+, as I mentioned; if any caps are shorted, they must be replaced immediately, or else the TV will blow fuses left and right. The TV may still work with leaky capacitors, but I would replace those caps immediately before doing anything else. If you see hum bars or other picture distortion, the problem is being caused by 60-Hz hum getting into the deflection circuits; again, the cure is to replace defective filters or shorted tubes in the deflection systems.


If there is a burn mark on the CRT screen, I cannot see it. The burn must be quite small, or else it is located in a corner of the screen where most people would not see it unless they were looking for it, and I zoomed in on the image to have a closer look at the set; I still could not see any evidence of a letterbox burn mark anywhere on the screen. These marks can appear on older TVs' CRTs if the horizontal or vertical sweep is not working properly; when the vertical sweep failed in CRT sets, and the set owner did not turn down the brightness control immediately, the line across the width of the tube could and often did burn a mark in that area--ruining the tube, of course.

Burn marks at the sides of the CRT screen can also occur if a 16:9 image is viewed on an older TV, since the picture will not fill the screen, as was mentioned. This type of screen damage can also occur in flat-screen HDTVs if a 4:3 image is viewed with the zoom control set incorrectly. However, if a 16:9 image is viewed with the zoom set for 4:3 aspect ratio, black bars will appear at either side of the screen; this can and likely will cause permanent burn marks (from the LEDs or backlight) if the picture is either viewed with excessive brightness or for an extended period of time.

I can only imagine how many flat-panel HDTVs were ruined because the set was either not shut off immediately, or the brightness control was not turned down at once when the sweep failed. IMHO, there should be a fail-safe system in HDTVs which will shut down the set immediately if the sweep fails for any reason. Such a protection system, after all, was built into all analog color TVs to guard against X-ray problems, if the horizontal sweep system produced excessive HV or something went wrong with the HV regulator tube or circuitry.