I've been blessed!

[old_tv_nut]

Changing the HV regulator had no effect.

I have determined that the delay line output (which feeds the high side of the brightness pot) doesn't drift, but the negative voltage for the low end of the pot as well as the wiper voltage does.

Now to do a sanity check on how much the drift is at the wiper vs the low end. The wiper should be a voltage-divided version of the negative supply drift. If it's larger, there's some variable load on the wiper.

If the drift is coming from drifting of the the negative bias generated by the horizontal output, it's going to be a problem to get at because all those components are under the chassis

[Penthode]

Is the HV not steady? I would expect the raster size to change with changing the brightness setting.

There are various clues to rule out the culprit. A major clue is the time the process begins afterswitch on and how log it takes to complete the issue.

My rule of thumb is Vacuum tubes heat the fastest and show complete final operation point is about three minutes. This suggests we can rule out the 12BY7. Next resistor heat from 3 to 10 minutes. Lastly capacitor complete heating from about 10 to 30 minutes. That is unless the capacitor is close to a vacuum tube or resistor. I have seen old paper and even film capacitors begin to increase leaking immediately after power up in a thermal runawy effect.

That said, the problem occurring over a 10 minute period suggests the bias voltages around the 12BY7 are driffting. This could include the plate supply, the screen supply as well as the grid bias.

You have determined the 12BY7 plate voltage is rising which suggests either the screen voltage is dropping, the cathode voltage is increasing or the control grid bias is dropping. I think careful monitoring and tabling the voltages on a timeline from cold start up to 15 minutes on will reveal what is going on.

[dtvmcdonald]

One thing to try, which I have used successfully, is to measure the volatge at the bottom of the brightness control when its normal. Then let the set rest a while and connect an
external regulated power supply set to that value to it (the bottom) justas the set is turned on.
See what happens to brightness, and measure other voltages as they change when it warms up.

[old_tv_nut]

Originally Posted by dtvmcdonald

One thing to try, which I have used successfully, is to measure the volatge at the bottom of the brightness control when its normal. Then let the set rest a while and connect an
external regulated power supply set to that value to it (the bottom) justas the set is turned on.
See what happens to brightness, and measure other voltages as they change when it warms up.

Thanks. Have that in the back of my mind.
Taking a day off to clear my head a bit.

[old_tv_nut]

I calculated what the brightness control range should be from the schematic:
-8 to + 8 volts. The measured tap voltage for normal brightness is about 4.5 volts, not zero as you'd guess, nor -0.5 as listed on the schematic. I don't know at this point if this higher required voltage is due to rewiring the video output cathode circuit to match the CTC-7; could be. Anyway, when first turned on there's plenty of range for too bright to too dark.

When first turned on (bright picture) the voltage is -1.4 volts at the low end of the pot, which gradually drifts lower, darkening the picture. Eventually it's not quite possible to get normal brightness.

It looks like I could just ground the lower end to make it zero volts and no power supply will be necessary to see if that stops the drift. If that does stop the drift, it could be a long term "temporary" fix until I can pull the chassis to work underneath.

[old_tv_nut]

I have a box of clip leads somewhere, but it turns out I haven't used them in a long time and can't find them! I know I put them in a good place! I have some coming from Amazon tomorrow.

[old_tv_nut]

I'm confuseder and confuseder. Not only is there some drift with the bottom of the brightness pot grounded, but either way, the pot center tap does not want to be zero for proper picture, but something higher. The pot range is not -8 to +8 v as calculated in normal configuration, going from a volt or two negative to 11 or so positive.

Out of desperation, I swapped out the AGC tube - no difference. Also went back to the video board with the freeze spray, still nothing seems to be sensitive to cold.

[Alex KL-1]

Sorry if the following was said or asked before, but: the screens voltage also change with brightness change? Also, the potmeter itself perhaps is drifting or the circuit that fusnishes the negative voltage (is derived from...?)

[Electronic M]

If the pot is feeding a grid, have you checked if the tube has excessive grid leakage or tried subbing the tube?

[Penthode]

If the voltages are off on the the brightness pot, there are only a few components to check.

I am suspicious of the 11 volts positive. I would measure the DC voltage on the input of the delay line for if the 2ufd electrolytic is leaky, (C402).

The only other source of a higher than normal DC are the 1st Video Amp cathode resistors R 402 and R403.Try measuring the voltage at the junction of the two resistors and the cathode of the 6AW8 first video amp cathode.

Problem behavior question. I understand the problem is that the CRT is bright when first powered and gets darker over a period of 10 minutes or so? If that is the case, when power cycling, how long must the set be off to when it is repowered the return of the from cold high brightness problem is observed?

[old_coot88]

Then there's the old quick'n'dirty 7 seconds off test for borderline weak tubes. Although not for that specific purpose, the test might provide some clues.
With the set warmed up and in the fault mode, turn it off for 7 seconds, then back on. Does the fault remain once the tubes stabilize, or... ?

[old_tv_nut]

Thanks all for the suggestions.

The drift is quicker at first and is mostly done by 5 minutes. Just a few minutes off brings the brightness back, at least partially. I have swapped the tubes in the video section with no change in behavior.

Once the brightness has drifted to dark, off for 7 seconds is not enough to make it too bright again.

I'll check the voltage R402, R403 as suggested. The 2 uF is a good suggestion. It was replaced during the original recap, and the set hasn't had a lot of hours since, but maybe...

[old_tv_nut]

When first turned on, the cathode is 27 V, not the 22V the schematic says. R402/403 is ~25 V. The voltages are affected somewhat by line voltage but are never lower than the schematic. I just ordered a replacement for the 'lytic.

[Penthode]

Then there is the low end of the brightness pot. Assuming the bias to the horizontal output is staying relatively stable, the is resistor and capacitor coupling to the brightness pot..

[old_tv_nut]

I replaced C402 (2 uF electrolytic) and the brightness drift is still there. It seems like the problem must be coming from the negative bias supplied by the horizontal board, which I can't get to without pulling the chassis.

I discovered that C402 was installed backwards all these years since I recapped the set! You would think that would make it the obvious failure, but not so. With the somewhat cramped access and my tremors, the new install looks messy, but is solid.