Here's the cabinet. Also included: the safety glass (it's there, maybe hard to see). The metal trim around the safety glass. The pencil box. Some CRT shroud mounting ears. No knobs or anything else but what you see.

Any interest? If not, has to go to the curb soon!

Any interest? If not, has to go to the curb soon!

I'm thinking of getting it for a half decent back for my CTC-4. I'm going to be swamped until after the ETF meet this weekend though...

Let me know ASAP after the meet (like Monday if you can) - judgment day and large item trash pickup are coming soon. If I have to hold it for you a couple of weeks that's OK, but not if it's going to be trashed anyway - there are limits on how much they'll take each week.

Got you! I'm probably going to be in Kenosha on Sunday to get some stuff, if so I'll try to combine trips.

It's time for this to go! Trash goes out Wednesday night, so speak now or lose it!

What did you get in kenosha, tom? I live in that hell hole lol. Anyways, are you getting that cabinet? If not, I'd like to see if I can get it here somehow if it's still available. Wayne?

Tom has dibs, but if no response from him by tomorrow, it goes to first taker or trash.

I just want the back (and to prevent the rest from getting trashed). If mark wants the rest, he can have it....Just save the back for me.

Mark, was down there to buy some parts for my '78 Lincoln (got in an accident that messed up the bumper, it's mounts, and a turn signal).....Would have tried to arrange for the CTC-5 pickup at that time, but realized the spare parts I was getting in the deal (rims, trims, and glass) would take up all TV space.

I might be able to make a trip there today, but there are a couple of folks that have first dibs on my time that I need to check with.

By all means, get the whole thing if you can Tom. I don't really need it but just wanted to make sure it doesn't get trashed. :)

What did you get in kenosha, tom? I live in that hell hole lol. Anyways, are you getting that cabinet? If not, I'd like to see if I can get it here somehow if it's still available. Wayne?

So you're back in Kenosha! Maybe we'll hear from you a little more often. :banana:

By all means, get the whole thing if you can Tom. I don't really need it but just wanted to make sure it doesn't get trashed. :)

I'm in the same boat of not needing the whole thing (I don't think I have room), but I don't want it in the land fill. A friend just had me get a monochrome Motorola that he needed gone before the folks got home.....I don't have room for that either, and will probably part it out or sell it.

PM me or something with a definite evening to pick up pieces or the whole thing. If no definite arrangement, I cannot save it.

Helloooo... does anyone really want to save it (or parts of it), or is it just talk?
Going...going...

I'm out unfortunately. I now am trying to make space for a 1963 Admiral color combo. I don't know what to do with the damn thing.

Ive had a CTC4 director cabinet, no top or back cover, buried in my garage for a few years.

If I want a 21CT662 in need of these parts to show up, I just have to dump it :(

I've given it a reprieve for one week - so, just one more chance -- anyone?

Wayne,

You've been really patient w/that cabinet. Time to bite the bullet and put it on the curb.

-Steve D.

I'm a few hours off from ending the existence of a 21" Motorola monochrome console that my folks say has to go before I can get anything else big. Once it is gone I'll PM you and set up pickup of that CTC5.

I'm out unfortunately. I now am trying to make space for a 1963 Admiral color combo. I don't know what to do with the damn thing.

I have it now. If you change your mind and want it let me know.

Waking up this thread because a brain cell must have just fired - I just realized an interesting thing about my CTC-5.

It has a UHF tuner but was bought and used in Chicago, which (I believe) didn't have a UHF station until 1964.

It's model 21-CT-7855U. I believe "U" means the UHF is factory original. So was the UHF sold as a feature to prevent obsolescence? Or, were there any UHF stations receivable in Chicago this early?

A secondary question: does anyone know if these sets sometimes had a UHF tuner added at a later date? Do (any) VHF-only CTC-5's have a plugged hole for the UHF shaft?

AHA - there was early UHF in Milwaukee (although I haven't found the details of HAAT and ERP):
https://emke.uwm.edu/entry/television/
Now I wonder if the original owners also had a big fringe area antenna for this set, or did the UHF capability go unused until 1964.

Just talking off the top of my head, from memory, I’m sure Milwaukee’s Chanel 18 was on prior to 1964, I think the 50’s. I can’t imagine WFLD 32 came after Ch. 18. In another thread, I just posted that Congress mandated UHF tuners be included in sets starting 1965. My CTC-15E has a space for a UHF tuner. Have no idea about the CTC-5.

Just checked, ch 18 first air date was 1953. Ch 32 first air date was 1966 according to Wiki.

Just talking off the top of my head, from memory, I’m sure Milwaukee’s Chanel 18 was on prior to 1964, I think the 50’s. I can’t imagine WFLD 32 came after Ch. 18. In another thread, I just posted that Congress mandated UHF tuners be included in sets starting 1965. My CTC-15E has a space for a UHF tuner. Have no idea about the CTC-5.
The first UHF channel in Milwaukee was channel 25, a CBS affiliate in 1953.
The second one was channel 19, an independent that showed, IIRC Dumont programming. The educational UHF channels came out in the 1960's.
Originally they proposed one more UHF channel 31, which never materialized.
I have a late 1953 model RCA with the big Roto-matic tuner which came from the factory with 19,25 and 31 strips. Those tuners were very good.
Later on, the channel locations were changed to channel 18 and 24. :scratch2:

I have 2 CTC-5 sets same model except one doesn't have UHF.... The non-UHF set has no provisions for adding a tuner on the cabinet... I think the metal tuner/control subchassis is different too.

The first UHF channel in Milwaukee was channel 25, a CBS affiliate in 1953.
The second one was channel 19, an independent that showed, IIRC Dumont programming. The educational UHF channels came out in the 1960's.
Originally they proposed one more UHF channel 31, which never materialized.
I have a late 1953 model RCA with the big Roto-matic tuner which came from the factory with 19,25 and 31 strips. Those tuners were very good.
Later on, the channel locations were changed to channel 18 and 24. :scratch2:

I grew in Milwaukee from the 40’s to 1979. Never saw a channel 25. When my parents bought our first b/w set in 1954, CBS was channel 12 WISN. It’s possible that CBS operated on channel 25 prior to 1954.

I think you meant channel 18 independent. Edit: There was a channel 19 which went dark in 1958. Never saw it. Possibly because the rabbit ears antenna could not pick it up or it was a low power station. It was WOKY, which I remember was only on the radio. Picked it up on my crystal set. Channel 18 was received with strong reception.

Thanks to all. From the page I posted, the first UHF in Milwaukee broadcast from an FM radio tower on a downtown hotel (that is still there at 509 W Wisconsin Ave.). It's hard to imagine it was high enough or powerful enough for reliable reception in Chicago when the CTC-5 was purchased.

My Jan 54 Radio Electronics lists in Illinois:
WTVP 17 Decatur
WEEK 43 & WTVH 19 Peoria
WTVO 39 Rockford
Any of those close enough to Chicago?

jr

My Jan 54 Radio Electronics lists in Illinois:
WTVP 17 Decatur
WEEK 43 & WTVH 19 Peoria
WTVO 39 Rockford
Any of those close enough to Chicago?

jr

Rockford is close enough. It was (is?) common for bars and a few homes to have an antenna capable of picking up Rockford to avoid sports blackouts, don't know if there would have been anything desirable on the Rockford station in 1954-1957 or not.

About 70-75 miles to Rockford from the approx location of the set in Chicago; 125 mi to Peoria; 155 to Decatur.

Perhaps another possibility, from the same list: WSBT 34 South Bend. :scratch2:

jr

I have an ad for the exact model of CTC-5 I have. It features Mrs. Robert Calvert of 222 Nelson Road, Scarsdale, NY, touting how beautiful and easy to tune the pictures are, and that they have not needed a service call in a year.

I looked up the street view of that address, and I'm sad to report that no TV antenna is visible.

[QUOTE=old_tv_nut;3222568]I have an ad for the exact model of CTC-5 I have. It features Mrs. Robert Calvert of 222 Nelson Road, Scarsdale, NY, touting how beautiful and easy to tune the pictures are, and that they have not needed a service call in a year.

Wayne,

I have a CTC-5 Deluxe chassis model. It has a 21FBP22A/21GUP22 replacement tube. Fortunately it has an RCA metal to glass tube mounting conversion kit. This allows the installation of the new tube to the existing mounting receptacles w/o using the 21AX plastic shield. The original yoke was retained.These kits are extremely rare and I have never seen another. Sorry I didn't post these photos earlier.

I’m getting the bug for resuming my Television DX addiction. I live in the Phoenix NW valley. Las Vegas, LA and San Diego are easy day trips. Thinking about a 35 foot tower with a Televes Boss Mix on a rotor. Heard good things about the antenna. 90 miles is the theoretical limit but I’ve seen exceptions under certain atmospheric conditions. This antenna is supposed to be exceptional for long range. At the same time it removes the noise and won’t overload on strong signals.

Edit: My elevation is 1200 feet.

I’m getting the bug for resuming my Television DX addiction. I live in the Phoenix NW valley. Las Vegas, LA and San Diego are easy day trips. Thinking about a 35 foot tower with a Televes Boss Mix on a rotor. Heard good things about the antenna. 90 miles is the theoretical limit but I’ve seen exceptions under certain atmospheric conditions. This antenna is supposed to be exceptional for long range. At the same time it removes the noise and won’t overload on strong signals.

Edit: My elevation is 1200 feet.

You do realize the specs on that antenna are fudged? The unamplified gain should be measured with respect to a tuned dipole, not a 1-meter dipole.

Also, adding the amplifier gain to the antenna gain and calling it the amplified antenna gain is bogus. The amplifier gain is useful to compensate downlead loss. The apparent 15 dB gain of their amp (reading their graph) is more than enough, but the important amplifier spec is noise figure, which they don't give. If their amplifier noise figure is worse than your TV tuner noise figure, it can actually be a detriment.

Another important amplifier spec is overload level. At least they have an option to shut it off if it suffers overload from local signals.

"Removing the noise" is an odd sounding claim - there is no way to "remove noise" from a signal - but this seems to refer to the amplifier shielding, which will help keep unwanted strong local signals that the antenna is not directly aimed at from getting into the system.

You do realize the specs on that antenna are fudged? The unamplified gain should be measured with respect to a tuned dipole, not a 1-meter dipole.

Also, adding the amplifier gain to the antenna gain and calling it the amplified antenna gain is bogus. The amplifier gain is useful to compensate downlead loss. The apparent 15 dB gain of their amp (reading their graph) is more than enough, but the important amplifier spec is noise figure, which they don't give. If their amplifier noise figure is worse than your TV tuner noise figure, it can actually be a detriment.

Another important amplifier spec is overload level. At least they have an option to shut it off if it suffers overload from local signals.

"Removing the noise" is an odd sounding claim - there is no way to "remove noise" from a signal - but this seems to refer to the amplifier shielding, which will help keep unwanted strong local signals that the antenna is not directly aimed at from getting into the system.

The build quality looks amazing. https://youtu.be/4jFfVEN-gFM

The antenna in the video is a smaller version, but a good representation of the one I’m looking at. Do you have any recommendations for ultra long range reception?

I have a small roof mounted antenna and one day, I rescanned, got lucky and a bunch of stations locked in from outside Phoenix valley. Tucson and a bunch of others I didn’t identify. So with a tower, rotor and a really good antenna, I’m hopeful of reaching further.

Edit: current antenna has no rotor pointed at the Phoenix transmitter towers.

Sorry, no recommendations to give, too hard to find companies that don't give hyped specs. That antenna may do just fine, (I'm guessing the 20 dB gain in the chart may be real for at least one frequency) but there's no real hard comparison possible.

[QUOTE=old_tv_nut;3222568]I have an ad for the exact model of CTC-5 I have. It features Mrs. Robert Calvert of 222 Nelson Road, Scarsdale, NY, touting how beautiful and easy to tune the pictures are, and that they have not needed a service call in a year.

Wayne,

I have a CTC-5 Deluxe chassis model. It has a 21FBP22A/21GUP22 replacement tube. Fortunately it has an RCA metal to glass tube mounting conversion kit. This allows the installation of the new tube to the existing mounting receptacles w/o using the 21AX plastic shield. The original yoke was retained.These kits are extremely rare and I have never seen another. Sorry I didn't post these photos earlier.

That looks like the stock CRT back plate and mount ears out of a CTC9.... makes me wonder if the mounting holes on the 5 and 9 masks are the same and the mount kit was simply RCA grabbing parts off the 9 line.
I have a 5 and a 9 maybe I should do an experiment to confirm.:scratch2:

[QUOTE=Steve D.;3222580]

That looks like the stock CRT back plate and mount ears out of a CTC9.... makes me wonder if the mounting holes on the 5 and 9 masks are the same and the mount kit was simply RCA grabbing parts off the 9 line.
I have a 5 and a 9 maybe I should do an experiment to confirm.:scratch2:

Tom,

I'd be very curious to see the results of your comparison. The CTC-9 mounting plate does look similar. Difficult to see in the SAMS photo.

-Steve D.

Reopening this "I've been blessed" CTC-5 thread to cry about the brightness control drift getting worse and worse. Sarts out with brightness way high and over 5-10 minutes warmup drops down to the point where the maximum control setting is not quite high enough.

Right now, I don't know what's drifting, cathodes, G1's, G2's?
I replaced the luminance output back when I first saw it, and didn't get any improvement.

The thing is, as I have gotten older, it's become difficult or impossible for me to pull the chassis to work on.

I'm thinking I will open the back and measure what I can, voltages and resistor values, to see if I can determine what it is that's doing the drifting, and hope it's something easy to get to.

Don't know if I will get to it before the holidays.

Wish me luck!

If anyone has a good guess, let me know!

A brightness drift? I have a few questions.
I recall the old rule of thumb to pinpoint drift how long does it take to occur from switch on? If the drift is within the first few minutes after powering up, I would suspect a vacuum tube. If over 10 to 20 minutes a resistor and after 20 minutes a capacitor. Is there a reliable timeline?

Looking at the schematic, there is limited DC coupling to the input of the 12BY7. So the drift we can be 80% certain is the from the 12BY7 through to the 21AXP22A and it's surrounding components.

You did not report blooming so we can assume it is a bias change.

My first focus would be measuring the voltages and checking was changes from start up to running when the screen goes dark.

So first three questions:

How long a period does the darkening occur?

Have the capacitors been replaced around the 12BY7?

Has the contrast control 12BY7 cathode modification been made?

Futher thoughts.

I would think it less likely the problem is with the voltages to the 12AX22A screen or control grids unless you saw a hue change as the picker got darker.

The gradual dimming will be most likely due to the plate voltage of the 12BY7 rising gradually. Depending upon how long it takes, it will be the 12BY7 heading to cut off. It may be the control grid potental changing. Chief suspect would be R407 180K drifting upward.

The other chief suspect to me would be the screen resistor R410 22K drifting upwards lowering the 12BY7 screen voltage.

...So first three questions:

How long a period does the darkening occur?

Have the capacitors been replaced around the 12BY7?

Has the contrast control 12BY7 cathode modification been made?

Thanks.

The darkening continues over 5 to 10 minutes, but fast enough that you can see a change in under 30 seconds to a minute.

I gave the set had a total recap when I first got it.

Yes, the contrast modification was made. In the category of full-disclosure of dumbness I accidentally wired the contrast control backwards, so max contrast has been CCW ever since, but I think this can't have anything to do with the problem because it's capacitively coupled in the rewired configuration, and besides, it worked fine after reconfiguring.

Futher thoughts.

I would think it less likely the problem is with the voltages to the 12AX22A screen or control grids unless you saw a hue change as the picker got darker.

The gradual dimming will be most likely due to the plate voltage of the 12BY7 rising gradually. Depending upon how long it takes, it will be the 12BY7 heading to cut off. It may be the control grid potental changing. Chief suspect would be R407 180K drifting upward.

The other chief suspect to me would be the screen resistor R410 22K drifting upwards lowering the 12BY7 screen voltage.

Thanks.

I may not get to this until January, but will put these on the list of things to check first. Hope these are on the top of the boards.

Yes looks like a fun troubleshooting project!

I think for a start you at least won't have to pull the chassis. The prime suspect resistors I believe are both on the Video PCB.

A can of freeze spray with a stream aimed precisely at the resistors in question when the dimming occurs should instantly reveal if they are bad.

Freez spray to pinpoint drifty components was a game changer more times than I can count.

Freez spray to pinpoint drifty components was a game changer more times than I can count.

Thanks - I forgot!

Update: No joy yet.
I started chasing the brightness drift this afternoon The suspect resistors measure OK cold, and after the brightness has drifted, freeze spray shows no effect on any components on the video board.

Cathode voltages drift by 10-15 volts, and adjusting the brightness control for the same variation shows the same brightness variation as the drift.

I noted that the far end of the brightness control goes to a negative grid bias voltage developed by the horizontal output tube, but changing the HO tube and/or the H driver made no difference.

I backed up and measured the 385 V supply, and it is low (as low as 330) and varies with beam current. If I understand the HV regulator properly, it should keep a constant load on the HV with brightness changes and therefore the 385 supply should not vary drastically. So, the regulator may be wonky or just acting wonky because the B+ is low.

I think my next move is to replace the 5U4's in the power supply, but I don't have any at the moment.

Maybe tack four Si diodes, each with proper series resistance, across the 5U4s(?). Just a thought for a quick&dirty test. :o That'd also stiffen the B+ against sag.

[EDIT.] Interesting vid here on a CTC-5 resto. https://www.youtube.com/watch?v=3GPrCym5qXw

Update: No joy yet.
I started chasing the brightness drift this afternoon The suspect resistors measure OK cold, and after the brightness has drifted, freeze spray shows no effect on any components on the video board.

Cathode voltages drift by 10-15 volts, and adjusting the brightness control for the same variation shows the same brightness variation as the drift.

I noted that the far end of the brightness control goes to a negative grid bias voltage developed by the horizontal output tube, but changing the HO tube and/or the H driver made no difference.

I backed up and measured the 385 V supply, and it is low (as low as 330) and varies with beam current. If I understand the HV regulator properly, it should keep a constant load on the HV with brightness changes and therefore the 385 supply should not vary drastically. So, the regulator may be wonky or just acting wonky because the B+ is low.

I think my next move is to replace the 5U4's in the power supply, but I don't have any at the moment.
If the variance of the +385v supply is that great, something is wrong. On my CTC5, upon inspection before I even powered it up, the ground to the transformer center tap was not properly soldered from new! I never had the opportunity to find if it affected the B supply! But a drop to 330 volts can mean a few things eg bad 5U4 tubes, bad fuse holder of a leaky electrolytic. But if it varies with beam current that would suggest the series power supply resistance is too high: bad weak 5U4 tubes seem the most likely candidate.

The voltage sag for a 5U4 at 250mA is about 50volts per plate. The seiers resistor would be 50/0.25 = 200. Two plates in parallel son 1 100ohm 50W resistor per would work.

To test I would simply replace the two 5U4 tubes with a pair of of 2A 1500 PIV diodes.

If the variance of the +385v supply is that great, something is wrong. On my CTC5, upon inspection before I even powered it up, the ground to the transformer center tap was not properly soldered from new! I never had the opportunity to find if it affected the B supply! But a drop to 330 volts can mean a few things eg bad 5U4 tubes, bad fuse holder of a leaky electrolytic. But if it varies with beam current that would suggest the series power supply resistance is too high: bad weak 5U4 tubes seem the most likely candidate.

That's what I'm thinking - both the time that it takes to drift and the fact that it always comes back to the same starting condition when cold points to tubes.
I ordered a couple, should be here in a few days.

By the way, dragging it from the corner revealed that the convergence plug was not making good contact, and exercising it a bit seems to have restored it.

I see Tube Depot sells solid state replacements, but warns they will increase B+ by 10 to 20%, so they must not include resistors. I'm doing baby steps by replacing tubes first, then may consider solid state sub.

I sat down with the service info and reread the grayscale tracking procedure, partly in case it might be needed when the B+ is restored normal, but mainly to try to understand how it works.

The strange thing is that the CRT grid DC bias points in the chroma section are adjusted to correct highlight color, even though the controls are labeled "background" and really do affect the background more strongly than the highlights.

The instructions say to set the "background" controls to counter any tint in the highlights and then adjust the G2 controls to counter the resulting tint in the shadows.

After reading this carefully and looking at the electrical characteristics of the CRT, I realized that the CRT gun is actually a variable-mu (variable gain) tetrode, and when the G2 controls are adjusted after adjusting the grid bias, the CRT is operating at a different mu (gain), effectively changing highlight white balance.

I also note that the CTC-5 called for a 8200 Kelvin color temperature, not as blue as the 10,000 Kelvin that was spec'd for later chassis. In any case, I don't believe accurate color temperature was ever achieved by eyeball, as the eye adapts too readily to color temperature changes. No TV repairman had a colorimeter in that era.

I try to clolor balance the black and white image against a black and white television of the era. If you had a colorimeter you could do it more accurately but in those days, especially with color drift as the set warms, I find eyeballing it against a P4 phosphor television is the best you can do.

I try to clolor balance the black and white image against a black and white television of the era. If you had a colorimeter you could do it more accurately but in those days, especially with color drift as the set warms, I find eyeballing it against a P4 phosphor television is the best you can do.

I do have a colorimeter, but I usually balance by eye against natural light coming in the window. I haven't tried measuring the CTC-5 for a long time, but using my DSLR as a rough colorimeter, it measures somewhere around 7700 to 8000 Kelvin as set by eye. I think many/most P4 tubes from the early/mid 50s onward were far too blue as a color set reference. I seem to recall GE boasting how blue their P4 tubes were.

By the way, since I got some adjustable color temperature LED light panels, there's much less post processing needed to match the screen and room light in photos.

I do have a colorimeter, but I usually balance by eye against natural light coming in the window. I haven't tried measuring the CTC-5 for a long time, but using my DSLR as a rough colorimeter, it measures somewhere around 7700 to 8000 Kelvin as set by eye. I think many/most P4 tubes from the early/mid 50s onward were far too blue as a color set reference. I seem to recall GE boasting how blue their P4 tubes were.

I was using a late 1940's 10BP4 as reference. It is has a lower color temperature as it is much less blue.I agree as I have seen how mid to late 50s monochrome CRTs do have a higher color temperature.

I was using a late 1940's 10BP4 as reference. It is has a lower color temperature as it is much less blue.I agree as I have seen how mid to late 50s monochrome CRTs do have a higher color temperature.

That's great. I have definitely seen the difference in the older sets at the Early Television museum.

I have an I1 Pro3 spectrophotomter I use for calibrating my computer monitors, as the less expensive colorimeters I had in the past could not do a good job of matching different models of monitor.

This discussion prompted me to get out the Minolta spot colorimeter I got when Zenith closed down their lab in 2000. Its last calibration was in 1998. It is way off when measuring the two LCD computer monitors (Viewsonic with sRGB gamut and HP with wide gamut). These monitors match visually when calibrated to 5000 Kelvin with the I1, but measure 4000 to 4400 with the Minolta. So, I guess the Minolta suffers the same problems as other colorimeters even though it was an expensive top-of-the-line unit at the time.

I know the home-built colorimeters that Zenith used on the assembly line had to be recalibrated any time the phosphor formulation was changed. This was a long chain of calibrating a spectrophotometer (which used a photomultiplier tube at the time) using an NBS traceable incandescent lamp and then measuring a CRT with the new phosphors and calibrating the production line colorimeters to match those readings. I never talked to whoever was using the Minolta, so don't know if it required the same calibration procedure, but I suspect it did.

The standard incandescent bulbs I mentioned were calibrated with respect to standard bulbs maintained by the NBS, and originally traced back to the definitive standard consisting of a black-body cavity in a piece of platinum at a specified temperature

Today, of course, instruments use stable solid state silicon photodetectors. I believe (but don't quote me, have to research it) the base standard now consists of lasers whose output can be measured accurately thermally and then transferred to a standard photodiode and from there to secondary standards.

Here is an excellent PowerPoint Level 2 presentation by ISF. (Imagining Science Foundation) A certified calibrator will have completed Level 3 training.

https://visions4netjournal.com/wp-content/uploads/2018/02/ISF-Level-II-2017-33.3.pdf

I got the new 5U4GBs from Tube Depot (new from JJ Elctronics, made in the Slovak Republic). They may have increased th B+ a few volts (hard to tell because of the loading effects of beam current) but have not fixed the brightness drift. Cathode voltage still drifts up about 10 volts during this long warmup.

Brightness still starts high and drifts lower slowly, and returns to too high after the set is off a while. Still wondering what can have this characteristic. Still a possibility of a resistor under the chassis where I can't get to it with the freeze spray?

Don't know - more head scratching is in order.

I tried running with the line voltage 5 volts higher to bring up the B+ some, but this doesn't seem to make a big difference in the drift.

I'm not running totally out of brightness control range, but have to set it fairly high after the set is warm. Running through a slide show, I just see the usual effects of less than 100% DC coupling.

Had you swapped the HV regulator? (I can't remember, was it a 6BK4 that early?) Maybe a bit gassy? Stabs in the dark.

Had you swapped the HV regulator? (I can't remember, was it a 6BK4 that early?) Maybe a bit gassy? Stabs in the dark.

I'll try that tomorrow, but as you say it feels like a stab in the dark. I have already tried the horizontal oscillator and output, which seemed more likely because one end of the brightness control gets a negative voltage from the grid of the horizontal oscillator.

Unfortunately the horizontal circuit board is hidden behind the high voltage cage and practically impossible to get at, but I am going to measure voltages at the brightness pot tomorrow - slapping my forehead for not thinking of it before.

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

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.

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.

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.

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.

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! :stupid: I have some coming from Amazon tomorrow.

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.

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...?)

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

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?

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... ?

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...

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.

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..

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! :yikes: 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.

So the question remains with the voltages applied to the brightness pot, which end is causing the drift? I would be useful if we could see the voltages either end a the pot upon power on and after 5 minutes or so when the frifting has stopped.

The resistor at the lower end of the brightness pot is still suspect.

I managed to get the voltmeter on the junction of R601 (470K) and R131 (120 ohms), which should be the same DC as the HO grid, and I measure -26 volts, not -40.

The in circuit resistance of R602 is between 300k and 400k (don't recall the exact number) not surpirsing because of parallel paths.

Now suspecting R165, 1M HO grid resistor, which appears to be under the chassis.

I have to drop this now because I have somewhere to go, but will get back to it