Tech-Master NIB TV Kit #5516

[Crist Rigott]

Originally Posted by bandersen

Glad you got the CRT focus sorted out and nice work! One thought - you might want to add a thermistor to give the tubes a soft start. Something like a CL-90 has a 120 ohms cold and around 3 ohms hot.

Thanks Bob!

Do you think I'll still need the thermistor even though the tubes have that 11 second delay built in?

[dieseljeep]

Originally Posted by Crist Rigott

Not quite. Originally I had a 10 ohm 50W resistor (R69) in place of the 5 ohm 5W resistor that came with the kit. I used a 1N4007 instead of the selenium rectifier so I had to go up in resistance from the 5 ohms. Using my iso transformer and variac the 10 ohms gave me about 125 to 128V for the 130V bus (B+). Then I remembered that I'll be powering the TV using the wall outlets of 124vac. I had to go up to 15 ohms to get about 132V B+ voltage.

Thanks.

Your line voltage is quite high! Ours is usually around 121 volts. The distribution transformer is on the lot next to me. A newer area, 15 years old. I imagine, the voltage is that high to avoid brown-outs in the summer time when everyone has their AC on. Your warm season is a lot longer than ours.
It's great to see that everything is working out fine. A lot of effort was put in and the results show it!

[Crist Rigott]

With the TV plugged into the wall (124vac) I'm getting 135v B+ and 13.2 to 14.6Kv depending on the brightness setting.

This is with a 10 ohm resistor in the filament string and a 15 ohm resistor in the power line going to the rectifier.

[Findm-Keepm]

Originally Posted by dieseljeep

Your line voltage is quite high!

124V is still within limits of ANSI C84.1.....127V max. I'm at 127V...

[bandersen]

Originally Posted by Crist Rigott

Thanks Bob!

Do you think I'll still need the thermistor even though the tubes have that 11 second delay built in?

Not sure what you mean by that ? Is there some device providing a slow 11 second filament power up ?

[dieseljeep]

Originally Posted by bandersen

Not sure what you mean by that ? Is there some device providing a slow 11 second filament power up ?

According to the tube manuals, all the newer issue 600ma tubes have controlled heater warmup of 11 seconds.

[Crist Rigott]

Originally Posted by bandersen

Not sure what you mean by that ? Is there some device providing a slow 11 second filament power up ?

On the 25DN6 tube data sheet they talk about controlled warm up period.

"Thermal characteristics of the heater have been controlled such that heater voltage surges during the warm-up cycle are minimized provided it is used with other types which are similarly controlled."

Then further down on the data sheet it states:

"Heather warm-up time. (Approx.)" 11 seconds

Most all of the tubes used have these statements in their data sheets.

[bandersen]

Yes and typically tubes with an 'A' suffix like 6AU6A are designed for series string operation. It's still a good idea to add a thermistor for additional protection. They do a better job than the tube on it's own.

[Crist Rigott]

Originally Posted by bandersen

Yes and typically tubes with an 'A' suffix like 6AU6A are designed for series string operation. It's still a good idea to add a thermistor for additional protection. They do a better job than the tube on it's own.

10-4. Thanks. I'll order some.

[Crist Rigott]

Here is what I used instead of the cork they supplied with the kit.

It is a very close cell foam that I get at Hobby Lobby. This is about 1/16 thick. I used some spray contact cement to glue them in place.

I also used the same stuff on the "inside" of the CRT strap so there isn't any metal to glass contact.





I also made a "stand-off" from 1/16 phenolic sheet to help manage the HV lead from getting too close the the chassis and the lead from the fly-back to the HV rectifier.

[Crist Rigott]

There was a post either in this thread or on the other forum, suggesting that I might need some more filtering. Well I clipped in another 220uf cap parallel with C43B. There was no change to the B+ supply but the picture seemed a bit more stable. Hmm..... so I added another 220uf to C43A and again the B+ stayed the same abd the picture remained stable.

So if I add these E-caps to each C43A and C43B it will bring each up to 440uf. Does anybody see anything wrong with doing that? Would a lesser uf work?

I'm including a picture of that section of the schematic.

[Crist Rigott]

I've been doing some investigating the focus problem I had with the help of another person offline.

Basically I cannot duplicate the focusing problem I had before I repaired the broken wires on the CRT.

If it was a broken wire I should be able to duplicate it by disconnecting pin 6 from B-. When I do, the picture remains in focus! NOTE - On the schematic, the blue wire from pin 6 shows it goes to chassis ground. This is an error on the drawing. It really goes to B-.


I've tried disconnecting wires on pin 2 and pin 10 with the same results, It still stays in focus. I even went back to the kit supplied 25CD6 and still stayed in focus.

I'm at a loss as to what caused the focus problem. I do know that after I fixed the the broken wires and reattached the base, the CRT stays in focus.

I'm including a section of the schematic so you can see what I'm talking about and maybe offer some reasons or suggestions.

[tom.j.fla]

The extra caps will be fine. Back in the day would add extra filtering to get rid of audio and video hum. Have been following your journey with the build as I have a unit in my collection that was built 60 years ago. Not a real good job on the construction on the set I have. Thank you for posting the docs for construction of the set as my set had no info, so now I will start to fix mine. All the best,Tom.J

[Crist Rigott]

Originally Posted by tom.j.fla

The extra caps will be fine. Back in the day would add extra filtering to get rid of audio and video hum. Have been following your journey with the build as I have a unit in my collection that was built 60 years ago. Not a real good job on the construction on the set I have. Thank you for posting the docs for construction of the set as my set had no info, so now I will start to fix mine. All the best,Tom.J

Tom, are you saying you have the same chassis? That would be terrific! What size CRT does it have?

[tom.j.fla]

Yes I do have a T-M 5516 and it has a 17 inch CRT. All the best,Tom.J