zenith HV problem

[drh4683]

got a problem with my zenith GA-5041 console from 68. Chassis 20Y1C50. HV is just under 30KV, way too high. The HV adjustment does nothing at all, but it tests ok. The flyback is getting hot. Is the problem a burnt up flyback? I know they say once a flyback gets hot or overly warm, they are junk. But can a component cause them to become defective? Picture is still very nice, but because I heard corona, I found out why. I called moyer to see if they have one. this is strange as Ive never had a defective flyback in a zenith.

[andy]

My book on 60's Zenith TVs has the following for your chassis with an inoperative HV control and excessive HV:

C142 ( 0.22uF)
open 220 ohm resistor across C142
defective HV reg tube (6HS5)

C142 is in the cathode circut of the 6HS5. If you need the HV section of the schematic, I can scan it for you.

I can't see how a bad flyback could cause excessive HV. Any shorted turnes would put excessive load on the HOT and result in low HV.

[drh4683]

Thanks for the info andy. I will get beneath the chassis later today and check/replace those components. Ive got a zenith service manaul w/ schematics, but they dont have a simple troubleshooting guide. I did check the cathode current and its fine @ 200ma. It is to be anywhere from 180-220ma. Guess this was considered a typical problem if it was listed as a trouble for the chassis. Will let you know what happens.
Thanks!-Doug

[andy]

Let me know what you find. This book has some troubleshooting and theory of operation information that may be useful.

[drh4683]

Hi Andy,

The 220ohm resistor and .22uf cap were ok. They tested fine. I started investigating the HV control circuit. The defective part was located, the voltage dependent resistor. I installed a new 63-9068 VDR and the voltage was now adjustable from 19KV up to 30KV. Balanced well at 25 KV. The light HV corona stopped, and the focus improved to the point of not having the control all the way over, but still was close to the end of its rotation. The original part number for the VDR is to be a 63-6485, but I put in a newer componet, 63-9068. There are no values for these, so I cannot be confident that this is ok to have in its place. Ive got many parts chassis and nos parts, but nothing would match the original. Even though they physically look different, the voltage adjusts correctly. Its in there temporarily with aligator leads. after removing the VDR, it simply split in half! These are almost as delicate as thermistors, which are first things I check when I find an old color set. Many times the heat in the therm causes the leads to separate. Anyway, So hopefully that flyback is going to be ok now. moyer has one just in case for $25.

[drh4683]

Ok, seems like my problem persists. However, HV is fine, seems to be stable. Flyback is still getting too hot. You know its too hot when the HV cage gets hot! It focuses well, but the focus starts to fade out worse just slightly as the set is on for a period of time. Im thinking my flyback is starting to go to toast. Any opinions? cathode current is at 200ma. What else is there to check?

[andy]

Who knows how long it was run with excessive HV which could have damaged the flyback, or other parts in the HV circuit. When it starts to go out of focus, does the focus, HV, or both change? Have you tried replacing the focus rectifier? I assume you've already adjusted the efficiency coil.