1948 7" Teletone Problems

Hello everybody- This my first post so I hope everything goes well. Thanks in avance for any help. I am working on a 1948 Teletone 7" electrostatic deflection set. I have completely recapped the set and brought it as far as you see in the photo. I have replaced a lot of resistors. I have checked all the tubes. As you can see it has a foldover problem. Can anyone give me advice on how to repair this? Thanks again! Alex

[cbenham]

Originally Posted by Acesales1

Hello everybody- This my first post so I hope everything goes well. Thanks in avance for any help. I am working on a 1948 Teletone 7" electrostatic deflection set. I have completely recapped the set and brought it as far as you see in the photo. I have replaced a lot of resistors. I have checked all the tubes. As you can see it has a foldover problem. Can anyone give me advice on how to repair this? Thanks again! Alex

Welcome! I hope you get the right tip to fix your set. Do you have a Sams schematic for it? If it is a TV149, it is SAMS 56, folder 22 dated 02/49.

Have you swapped the vertical output tube with the horizontal output tube [both are 12SN7s] to see if the problem moves from V to H? There are 4
12SN7s in the sweep section; swapping them around might pinpoint a weak one.

Might also be low B+. There is a selenium rectifier in the power supply that could be weak.

Use caution when you work on this set with it powered up; an isolation transformer would make it safer for you. There is no power transformer in the set to isolate the chassis and *one side of the AC line* is connected directly to chassis ground when the set is on. Also, be careful, there's 5000 volts on
the deflection plates of the 7JP4.

Best luck, it looks like it makes a bright picture.
Cliff

[Eric H]

Hi Alex, I have this same chassis sitting on my bench!

The only thing I can think of right off is did you replace the 6000 volt caps with regular caps or ceramic discs?
The Ceramics won't work right in the vertcal circuit and could cause this problem.

Also you might check all those 2.2 meg resistors on the board that the vert cent control is on.

I replaced the 6KV caps with the white ASC brand tubulars from Allied. I didn't use ceramics. I've checked and or replaced all the resistors on the HV board. I replaced the selenium rectifier with a 1 amp, 1000PIV diode. All electrolytics have been replaced. I have a copy of the schematic that I blieve is from Riders TV vol II. It shows B+ at 440V, I measure 392V at the main line. I've tried two different 25Z6's with the same voltage. Yesterday I changed the channel and horizontal deflection went to half the screen size! I finally located the problem in the hor. oscillator tube. Sometimes these things make you want to pull your hair out! Thanks again for any info! Al

[bgadow]

Welcome! Vertical problems can also be traced to bad vert lin or height pots. I've had a couple that had a bad spot and caused similiar problems.

[Tubejunke]

Did you replace all of the capacitors with equal or higher working voltage and similar/higher capacity? If you ran into any capacitors with color bands are you sure you got the decimal point in the right place when you identified them? This has often thrown me off. You have to watch out for mfd and mmf. or pico farad ratings here and take note that most schematics make some reference to the difference but it can be easily overlooked.

If I were in your situation I would go back through my work and make sure all of the values are correct. While your at it look at those resistors you say you replaced. A 4.7K and a 470K are only a color band apart. Also there is the chance of a new part being bad. You may want to measure the value of at least the components in the vertical section.

Last you are right, sometimes these sets will make you want to make you want to pull your hair out. I STILL have a 59 Zenith Space Command that I recapped and have had no luck whatsoever with. It should work like new since most of the parts are new. No dice!!

Part of the problem is that many of us are aproaching repair from the parts replacement standpoint as opposed to having a good understanding of exactly what should be going on in various circuits. Also we don't have a lot of the equipment that a professional would have had when techs actually worked on tube sets.

Now I am not being high and mighty here. I am definitly a parts replacer. Yes, I have been into vintage tv and radio for at least 25 years. I have learned a lot in that time about the thory of operation. However I never learned to use things like signal generators, oscilloscopes and the like. I think some of the things that us parts changers get stumped on could quickly and easily be sorted out by a person who can use the tools of the trade. For me its mostly a volt/ohm meter. You can do a WHOLE lot of repair work with just a meter but if it were that simple then a meter what be all that the pro's use.

Found it! Open connection in the vertical linearity circuit! I'm finished pulling my hair out!! Now for the final project. I noticed when I turn the TV on the CRT filament and a few others glow like light bulbs. What can I do to insure against filament failure?? Thanks again! Alex

[David Roper]

Take the CRT OUT of the filament string, install a small transformer with just sufficient ampacity for the 600ma CRT filament...and let the rest of the tubes fend for themselves!

[wa2ise]

Originally Posted by David Roper

Take the CRT OUT of the filament string, install a small transformer with just sufficient ampacity for the 600ma CRT filament

Another method is to use back to back zener diodes (high wattage, 8V) across the CRT heater.

Be aware that some transformers can produce stray magnetic fields that can interfere with the CRT deflection.

[Reece]

I like the idea of the separate filament transformer for the antique CRT. As to the other tubes, you can put an inrush current limiter in series with their heaters. Check on Mouser.com for example for Inrush Current Limiter. They have a high resistance when cold but quickly drop resistance as they heat up and bring on the tubes gently. Don't have the schematic for your set. Are all the tubes in one long series string or is there more than one string across the AC line? You might need more than one ICL. The one I use is Mouser P/N 527-CL90, G.E No. CL-90. It can handle 2 amps and has an initial resistance of 120 ohms, so it's the softest start of them all. But you might be limited by the 2 amp capacity depending on the heater string(s) that you have. Need to add up the amp requirements of all the tubes in each string. The next one down is the CL-80 that can handle 3 amps but is 47 ohms, so a little faster start. These things are all less than 2 bucks each and good insurance. They look sort of like a small ceramic cap. They will get hot so you want to mount them on a terminal strip under the chassis and "out in the air" away from any RF components that might drift if heated.

Reece

[Steve McVoy]

I've seen dozens of 7JP4s with low emission, but none with open filaments. I think that the probability of burning out a tube from the turn-on surge is very low.

[bgadow]

I know that the discussion about what to do with that filament can lead to a heated discussion. I remember an old ARF thread that went on for pages and pages! I don't think a consensus was ever reached. I've used a filament transformer on a Pilot, and have a couple other electrostatic sets that I need to tackle one of these days. My most pressing need for an inrush limiter is a little 3-tube AM transmittter-those filaments really take a beating at turn-on. I experimented with various light bulbs in series as well as some limiters out of a junked tv set but nothing worked perfect. Maybe I'll try what Mouser has.

Well I finally finished the TV. I added a Radio Shack 12.6v 1.2A transformer which powers the CRT alone. I put a two ohm 25 watt resistor in place of the CRT filament. I also added a 1.5 amp fuse in the main AC line for safety. I promised this TV to a friend, so when I delivered it I ended up buying a Sentinel 7" set in a wooden cabinet. Before I bought it I plugged the Sentinel CRT into the good working Teletone chassis to check the tube. It had an excellent picture so I told him I would take it! One more project! Thanks again for all the help. Al

[David Roper]

12.6 volts?? The CRT's filament runs on 6.3 volts, what means are you using to drop the voltage?

[fsjonsey]

Originally Posted by David Roper

12.6 volts?? The CRT's filament runs on 6.3 volts, what means are you using to drop the voltage?

He'a probably using the center tapped 12.6v transformer, at least I hope he is.