View Full Version : very interesting zenith symptom


drh4683
05-13-2004, 09:39 PM
Some of you probably have noticed that I have been looking for a flyback for my zenith 16Z7C19 chassis console. I have had very strange high voltage problems with the set. The screen would sometimes snap and go into a blur then lose HV down to 8kv. If the set were shut off then immediately turned back on, the set would perform for a few more minutes. I put my cathode meter on the HOT and noticed everything was fine, at 200ma even when the set would snap and lose the 25kv. I put the scope on the windings of the flyback secondarys and noticed transient spikes in voltage. I assumed that the flyback was causing me trouble. When the set was on, it could be watched for a few minutes then what appeared to be defocused "flashing" speckles would appear on the screen. This had me even more perplexed. I couldn not hear or trace down any corona. These flashing speckled blurrs would speed up to a rate that would blur the entire screen and would two side shrink the raster. I then checked my voltages at the regulator and noticed my voltages were all a bit too high. I did the obvious tube swaps but that didnt do anything.
Another unique observation as to when the set was working OK, the brightness level was not equal vertically. As the brightness was advanced the screen would brighten up from bottom to top of the raster with just a slight trace of retrace lines at the top. since I could barely see the retrace lines, I readjusted the screen bias to make sure they were not over driven. They were fine, but still had retrace and HV problems. The retrace lines tipped me off to check out the 121-587 blanker transistor. I tested it, and had severe leakage. I pulled one out of a good parts chassis and guess what! That little transistor solved EVERY problem! The brightness control range was linear, the HV problem stopped! I figured this is impossible, I let the set cook out for about 2 hours with no signs of any defect. For the hell of it, I plugged in the bad transistor again and all the problems came back, replaced with the good transistor again and the problems stopped. All voltages fell back to specs.
All these problems caused by the vertical blanking transistor! Extreme transistor leakage is bascially a shorted transistor. Just a note, if you ever experience any of these problems on your zenith, go straight to the 121-587, its the troublemaker!

drh4683
05-13-2004, 10:01 PM
Here it is! This is my 3rd space command 600 set. Just to let you know, every one of my space command 600 sets have had very strange and some difficult to pin down troubles. I guess I have to earn the right to watch these space command sets by fixing them, as my non space command zeniths always work when I find them. Go figure. Oh yeah, I had an open relay coil for channel UP on the space command chassis on this set too. Luckily I saved a parts SC chassis that had the relay I needed. Always something with these space commander sets! Dougs 1st law of TV repair "if its near mint with hardly any signs of use, it will be plagued with problems."