Any information on a RCA CTC-39VA

[Phil R]

Jeffhs, there isn't much remaining on the analog spectrum in my area. The last time I checked, there was just one station with very poor reception, I couldn't even tell what it was or where it was from!

I do have a Channel Master converter box and a RF modulator to plug a DVD player. I don't watch TV but I still want mine to be working again!

And the chassis is a CTC-39VA, not 38.
BTW,
Can somebody tell me if RCA had a similar model with a pushbutton tuner available in the US and what chassis number was used? All the documents I have seen so far seem to feature sets with rotary knobs. I didn't find the 1973 brochure and I think the 1972 brochure I saw was just featuring the Solid State models.

[Electronic M]

Think of AGC as a sort of automatic RF system amplification factor adjustment. Basically a TV is a video monitor with a fancy radio attached. The video monitor portion expects the video signal along with the synch pulses and color information it contains to be one specific amplitude. The signal the monitor half gets from the video detector at the end of the receiver portion would naturally vary in amplitude in step with received signal strength if it were not for the AGC. AGC controls the receiver gain so that the signal out of the video detector is the same no matter what the signal strength at the antenna is. How it usually works is by measuring the level of the horizontal synch pulses and comparing them to a fixed reference, and generating a error voltage that is sent to the tuner and IF stages to adjust their gain so that the error goes away.

Look for a block diagram of a color TV set with explanation of the blocks, it will help you understand what is going on.

Edit: I don't think we sold tube sets with that style of tuner. It should not matter if the tuner section is right because most sam's did not explain how to change the dial light, or clean the contacts which are about the only service procedures someone without a LOT of experience and specialized RF/IF alignment equipment has any business doing to a tuner.

[Phil R]

Thanks Tom for the explanation! I'll look for that!

But can I just turn the AGC screw and see what it does to the picture without any test equipment connected?

[Phil R]

Tom, the tuner section seems OK but there are some burned lights in the pushbuttons (one on Channel "6" and on the "B" UHF channel which I won't use but...). I wish I also knew how to access them!

I just noticed that I've been following you for years on YouTube. You're partially responsible for having me posting here! I did get several emails from YouTube showing some of your TVs. I started to follow you because one of your videos featured a Frigidaire washing machine (I also collect old appliances, mostly GM Frigidaire, at least, I'm able to repair these!).
I know you sold that Frigidaire washer but I think there should be at least two other appliance collectors here!

[Jeffhs]

Originally Posted by Phil R

I don't have any testing equipment. (other than a multimeter!). I wish I had the schematics but I couldn't find that exact chassis model on SAMS photofact. Most CTC-39 chassis seem to have an "X" after the "39", mine has a "V" instead (at least, that's what's clearly stamped on the flyback cover).

Maybe I don't need the exact model but...

I'm really not familiar with TV sets. I haven't touched to the AGC, I have located the control at the back but I don't even know what it is for (I know it means "automatic gain control" but I don't know what that means!). I don't want to sound stupid but I'm really missing some basic understanding and I can't find the most basic information on internet! I have seen videos of people rejuvenating picture tubes, making adjustments, testing the high voltage but I was missing the more basic things... I wish there would be detailed explanations on how to service old TV sets on internet but I haven't found anything for people who also need to get the basic knowledge. The most complicated electronic job I have done so far was replacing 3 capacitors in two microwave ovens which had display problems... That fixed the problem but I went from instructions that I found online, not from my own diagnostic!

I don't even know what the service switch is for at the back of the TV! I noticed it has 3 positions but I didn't try anything that I don't understand by fear of causing other problems... I also have been electrocuted when playing with an old tube radio when I was a kid and I'm careful not to touch anything in TV set. I don't even know what has the potential to shock you! I know most users of this forum are experienced but I'm not! I'm fascinated by these TV sets since I was a kid but couldn't ever find somebody to explain me how they work in detail!

Yesterday, I went to the only place in town that repairs electronics. I went there years ago with my Pioneer SX 800 receiver and when the guy noticed it had tubes in, he told me he couldn't service it... I guess he knows how but he's not interested at all. I just asked about the TV and I got the same answer... Needless to say there's not a lot of action in this store, as people don't care about having newer stuff repaired anymore!

The letters AGC do, in fact, stand for "automatic gain control"; the control on the rear chassis apron of your TV sets the optimum operating point of the AGC system, which prevents wild variations in contrast, etc. under varying signal conditions. Setting the AGC threshold control too high will kill the picture; setting said control too low will result in a weak, washed-out picture. In most areas, the optimum setting is in the middle of the control's range. If the TV works, however, I'd leave the control alone.

The three-position service switch is used to collapse your TV picture to a horizontal line across the width of the screen. This is used for checking beam convergence inside your TV's picture tube, or CRT, although for proper convergence adjustment an electronic test instrument known as a dot-bar generator must be used. I'd leave the switch set at "normal" unless there is a good reason for changing it.

The third position is likely "raster", which removes the video input signal from the picture tube; you will see nothing but a white screen with the service switch in this position. I'm not sure what the "raster" position of the service switch is actually used for, although I wouldn't be surprised if some TV technicians in the '60s-'70s were sent out on home service calls for "no picture", only to discover the service switch was set, accidentally or otherwise, to the "raster" position. I bet many nuisance service calls were made as well on sets that showed just the bright horizontal line across the width of the screen (or a blank white screen), when the only "problem" was that the switch had been set to the service or raster position, respectively, perhaps accidentally or even by inquisitive children.

As long as your TV works, there is no need to change the positions or settings of any controls along the back of the chassis; in fact, incorrect settings of some controls can damage or destroy parts of the set. Today's flat screen TVs, and the last CRT TVs made before digital, are designed so that the service adjustments are absolutely inaccessible to the average person; in fact, with flat screens, the service adjustments are set using a special "service" menu which is accessible only to technicians. (My RCA CTC185 CRT TV has such a service menu; I've owned the set 15 years and have never seen it.) The keypress sequence used for accessing these menus is also proprietary information, for good reason: to keep unauthorized persons from fiddling with the service menus and possibly, even likely, causing severe damage to the circuitry.

Finally, electric shock is dangerous and can be deadly. You were wise to stay away from the innards of radios and TVs after your experience with being shocked, although saying you were "electrocuted" isn't quite accurate; after all, if you were in fact electrocuted by high voltage from anything, be it a once-common 5-tube table radio or TV, you wouldn't be here today. The high voltage on a television picture tube is just as deadly if you don't know what you are doing. TV CRTs (picture tubes) have what is called a second or "ultor" anode, to which a well-insulated cable coming from the TV's high-voltage cage is connected. The voltage on the second anode is measured in kilovolts (thousands of volts) and ranges from 15,000 for a black-and-white CRT to over 20,000 for a color tube. This voltage has the potential to stop your heart (at worst) or, at best (!), it will throw you across a room. The warning on the back covers of all TVs, from the very first ones to today's sets, says it all: "DANGER. HIGH VOLTAGE. REFER SERVICING TO QUALIFIED PERSONNEL." Some sets of early '50s vintage had even more strongly worded warnings: "DANGER. HIGH VOLTAGE. DO NOT REMOVE THIS BACK UNDER ANY CIRCUMSTANCES."

This warning did not, however, deter some people from replacing bad tubes in their TVs; in fact, from the '50s until about the end of the '70s, self-service tube testers could be found in drug stores and the like. One of the most sophisticated such testers I can remember was the Hickok "Cardmatic" tester, from the late '50s-early 60s. A drugstore in the next town east of where I grew up had one of these tube testers; it used punched cards to set the various operating parameters of the tube being tested. This tester was made for non-technical people; all one had to do was find the type number of the tube he or she wanted to test find the card bearing that number (for example, 6AW8), insert said card in a card reader on the front panel of the tube tester, insert the tube, and push one button. The condition of the tube then registered on a large analog D'Arsonval meter on the front panel. Tube testing didn't get any easier than that.

[Electronic M]

Yes it is normally adjusted by eye without test equipment in most sets. If AGC is too low contrast will fade to washed out white, and possible loss of synch. If it is too high you will get loss of synch, negative video contrast too high even with contrast control at minimum.

IIRC the adjustment procedure is usually adjust until high and synch starts to be lost then back down until normal synch and reasonable picture contrast is achievable with the contrast control.

If you can't get synch suspect the synch separator. I don't own any RCA based chassis that new so this advice is from what I've read: RCA chassis with more than two 6GH8 tubes tend to eat those tubes regularly due to bad circuit design*. If any of your problems are in sections that use one of those tubes try swapping the tube first.

*I've read stories here of how something like every month or three service men who had contracts with hotels equipped with RCA sets would literally go around with a shopping cart full of new 6GH8s and change them in every set in the building...Because the sets needed those tubes replaced that often.

EDIT: I can't believe jeffhs slipped that wall of text in under me so fast.

[Phil R]

I've been watching 6GH8a tubes on eBay... It seems some sellers are selling them by the dozen (or almost!).

I'll probably buy a few!

[Electronic M]

Just to clear up Jeffhs fuzzy description of the service switch. The line position of the service switch was not often used for convergence, but rather gray scale adjustment. One would drop the brightness and color controls to min, enter the line mode of the service switch, turn the screen/G2 controls down, increase red until faint line appeared, increase green until line became yellow, increase blue until line turned white, then return service switch then brightness and color to normal positions...If whites looked tinted after that adjustment of color drive controls with a black and white picture would be the next step.

The blank raster position of the service switch could be handy for purity set up if you left your pattern generator at home. Purity with a blank raster can be achieved by turning the blue and green G2 controls down for a red raster, sliding the deflection yoke back until the screen looks tie dye, *centering the red dot using the purity rings, then sliding the yoke forward until the screen is completely red and readjusting the green and blue G2 controls. If you find that need to do a purity adjustment after you get the color demod working right a good short cut is to use a test pattern DVD with a fully red screen to do the purity without messing with the switch or the G2 controls. I find the G2s generally are more stable than purity, and I hate to mess with the G2 once I have them set just the way I prefer them.

*a degauss of the screen before this may be necessary.

[old_coot88]

There's no way he can hurt anything by turning the AGC control. Given the symptoms displayed in his vid, the first thing I would do is see if the AGC setting affects the pic. And go from there.

[Phil R]

I tried, it definitely changed something, but no real improvement... I ordered 5 6GH8A tubes just in case I'd need some in the future (and I think I might have a few used ones already but no way to test them!). and I hope to get some tools.

Another question, I can hear some static noise sometimes coming from the back of the set, not constantly but sometimes. I know most older CRT TV sets I have seen make this kind of noise sometimes and I haven't been able to locate it but I wouldn't be surprised if it came from the flyback transformer (when you're looking at the back, the noise definitely comes from the right side of the cabinet). With all the comments I read about some late 1960s RCA sets having problems there, should I have a look at something or it's normal?

[zeno]

Originally Posted by Phil R

I tried, it definitely changed something, but no real improvement... I ordered 5 6GH8A tubes just in case I'd need some in the future (and I think I might have a few used ones already but no way to test them!). and I hope to get some tools.

Another question, I can hear some static noise sometimes coming from the back of the set, not constantly but sometimes. I know most older CRT TV sets I have seen make this kind of noise sometimes and I haven't been able to locate it but I wouldn't be surprised if it came from the flyback transformer (when you're looking at the back, the noise definitely comes from the right side of the cabinet). With all the comments I read about some late 1960s RCA sets having problems there, should I have a look at something or it's normal?

I pulled the Sams on a CTC39 & its pretty much a CTC38.
Kinda surprising RCA didnt make a hybrid before going all SS.
A sleeve of 6GH8 is a good start. Start with the pair on the
forward board & see what she does. Those two sockets were trouble
also & we changed quite a few.
The noise is probably arcing & usually you get an ozone smell.
Can be coming from focus socket, HV rect socket or wires or
the plastic cup. Flybacks usually melt & short. Sometimes they
burn. Check it for cracks & wax dripping. A little wax is OK
but if its a big pile its a concern.

73 Zeno

[Phil R]

I thought mine was a hybrid set? I don't see many tubes in the tuner section:

https://www.flickr.com/photos/337230...7650024076848/

[Electronic M]

It is a good idea to discharge the HV before messing around inside the HV cage. There are a lot of good threads on it if you search Videokarma. The HV stores charge even with the set turned off so you can still get shocked by it with the set off. The good news is that unless you have heart problems the charge in the stored in the HV with the set off won't kill you....It will just feel like the worst static jolt ever, and possibly trigger your reflexes to pull away.

[Phil R]

All I need to do is to clip an alligator wire to a well insulated flat screwdriver and to the chassis, and touch the anode under the cup with the screwdriver? Right?

[Electronic M]

An HV probe is better. If you discharge through a resistance (such as an HV probe) there is less chance of bounce back. If you short HV to ground then it will temporarily drop the stored charge, but after awhile it will recover some of that charge....Which is often enough to be worthy of some foul language if you discharge that through your body (ask me how I know). If you use the screw driver and clip lead method leave the clip between the HV connector on the CRT and ground until you are through touching parts in the area of the HV.

A good way to fix HV arcing and corona is to take a damp cloth and clean every surface inside the HV cage. Be careful not to break the wires leaving the fly windings.