Procedure for setting high voltage - Videokarma.org TV - Video

Tubejunke · #1 06-18-2014, 10:15 PM

I am wondering if you set the brightness control to mid position or some other position when setting the high voltage on an early color set. I set mine with the brightness turned pretty much down, down to a darkened raster that is. Everything works OK, but I don't have quite as bright of a raster as I did before.

Seems like I have read of a specific protocol for doing this. Any advice is greatly appreciated. Thanks!

andy · #2 06-19-2014, 12:58 AM

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marty59 · #3 06-19-2014, 08:06 AM

Not sure if you run your sets through a variac or a bucking transformer but the increased line voltages will affect your adjustments.

+1 on Andy's point. High voltage should maintain within a few percentage points between load/no load conditions unless there's a problem with regulation.

DaveWM · #4 06-19-2014, 10:27 AM

if the HV was higher than it should have been, then yes the brightness will be slightly lower if no other settings are changed (G2s).

I look for good regulation at correct voltage (generally about 23kv for typ 60's tube era 23v CRT).

I have found zeniths a bit more if a challenge with the pulse regulation vs shunt tube, but that may just be the varistor acting up. Doug H had a thread where he was running into this, think he was going to do some exp with MOVs vs NOS varistors.

when ever you are having problems with getting HV to work right I would suggest starting with new tubes (HO/REG/HV rec/Damper) before getting into any real diagnostics.

andy · #5 06-19-2014, 11:11 AM

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DaveWM · #6 06-19-2014, 11:34 AM

Quote:

Originally Posted by andy

It's often skipped, but it a good idea to check the HV on any set. It's surprising how often it reveals a problem.

that's a good point, If it does not regulate well that should be looked into. Blooming/convergence can be effected, so good to get that rock solid before trying to correct other issues. Some sets us boost for things like vert drive on osc, just another thing to consider.

Electronic M · #7 06-19-2014, 09:32 PM

I've got factory Zenith service lit for most of their tube color stuff so I can probably look up the chassis # and give you the factory procedure.

Tubejunke · #8 06-19-2014, 10:11 PM

Quote:

Originally Posted by Electronic M

I've got factory Zenith service lit for most of their tube color stuff so I can probably look up the chassis # and give you the factory procedure.

Thanks, it's a 25LC30 chassis. And I did set it to 25Kv with the brightness low, but not all the way down. Again, the raster just doesn't seem to be as bright with the control turned up as it was. Seems like it was a little high from the get go, but dead on 25K now.

A brand new rectifier went bad the other night. I had accidentally powered the set with no damper tube. Don't know if that would damage the rectifier or not, but I put the old one back in that the new one replaced and the HV came back. The new tubes I am getting are made by International Servicemaster. Not sure about the quality as I have never heard of them. A local shop still has 70s stock tubes believe it or not. Not worth a nickel though if they can't take the heat! LOL

DaveWM · #9 06-19-2014, 10:20 PM

stay away from international service master or what ever it what I have always been told. Can't see how a missing damper could cause any issue, no power thru the flyback. Not sure it would be a good thing for the HO tube, may stress out the screen grid with no plate voltage from the fly.

Tubejunke · #10 06-20-2014, 06:43 AM

Quote:

Originally Posted by DaveWM

stay away from international service master or what ever it what I have always been told.

I sort of expected to get that kind of response. I have been dealing with tubes for decades and have never even heard of this brand. It's not all that the guy carries, but it was all that he had of HV rectifier and HO tubes. At least he didn't charge me much @ about $12 for the two. The HOs aren't cheap at all, so if just that works for more than a day I will have done well..

So, I'll keep pluggiin along till I get some performance out of this old thing, and stability. I saw something the other night that puzzled me. Someone through the years had repaired a section of a mult section electrolytic. They simply jumped over the bad cap. Now I knew that you could do that as a temporary measure, but I would think that you would want to get the bad cap out of circuit as it is in parallel with the new cap and unless it is open there is still a path for current to flow, be it shorted or leaky. Am I right on this or completely off base?

DaveWM · #11 06-20-2014, 07:47 AM

it may be ok if its open, but being as old as they are now it is common practice to take them out of circuit for the reasons you noted. I presume back in the day if it was open the assumption was it would be fine, but if you told the tech that oh by the way I plan on using this set for the next 40 years AFTER the repair, I suspect they would take it out of the circuit (and prob call for the guys that ware the white coats to come get you).

kx250rider · #12 06-21-2014, 12:45 PM

"Zero beam current", is the standard (as said in various ways above)... Basically black screen, regardless of what you need to do to get it. And for RCA, I always set the HV to about 1KV below the specs. Another good rule is to measure it first (before making any repairs in the HV supply), then when you set it, keep it AT OR BELOW where it was before. My experience is that if it's been 500 volts (or so) low, then bringing it up can trigger a weak FBT to fail.

Charles

Electronic M · #13 06-22-2014, 05:14 PM

Don't have the manual for your chassis, but here is the data for the 24NC31/24MC-- series sets. It may be valid for yours.

Tubejunke · #14 06-23-2014, 10:55 PM

Wow! Thanks Tom. Those line voltage references can mean quite a difference just say from 110 to 120 Volts you have an 1.Kv difference. My service drop is providing anywhere between 113 and 118v, so I think I will give the old set a break and go with about 24Kv.

And I will hope that my last HV rectifier holds out. This is the tube that had me talking out the low level hiss that all but went away with the change to the new International which just went out on me. I'm starting to think that it's time to start getting some new caps in this chassis and moving on. I think there are two or three bumblebees still in there. I need to get all that and anything else paper out of there. Probably save myself a lot of headache.