|
So I went back to step 6 in the alignment procedure just to be certain the adjustments I made to the top and bottom cores with the different frequencies fed into the converter grid were accurate and that the VTVM was reading the minimizing/maximizing accurately i.e. checking my work that I had the trap coils tuned to the correct resonant frequencies and the IF stages tuned properly. A couple of days ago, I posted the issue with the VTVM going off scale was tied to the signal strength going into the converter and with careful adjustment of the signal output I was able to tune the traps and stages accurately without the VTVM requiring a voltage range setting higher than 5 volts DC. When rechecking tonight, I encountered the same issue. I could not get the VTVM to read in the 5V scale when measuring across the video amp load resistor.
I believe I've found what is doing this but how and what is happening I don't know. The brightness control seems to be a problem. It is intermittently grounding out and sending full voltage through the yellow wire to the CRT. A bright horizontal line appears on the screen. Touching the shaft will cause the VTVM to jump way out of range. I don't know if this is what is causing the issue with measuring the voltage changes that happen when I'm trying to tune traps and minimizing at the R119 load resistor but when it happens, the VTVM pegs and goes off scale.
I don't see how sending full voltage to the CRT cathode could be causing the issue. The only possible link I see may be the fact the yellow wire from the control to the CRT cathode runs directly under the load resistor I'm measuring to adjust the frequencies in the various stages. This has been an intermittent issue since I started doing this series of adjustments and I guess, when I worked on it a few days ago and reported no problems, it wasn't happening. So at this point, I don't know if I even have the trap coils adjusted correctly. I don't know enough about what that voltage spike in proximity to a test point in this effort could do. I guess I could move the wire and see if it helps.
|