Newbie's First Questions RCA TS 721 Restoration

[Yamamaya42]

I literally spent a few months bringing my Fada 895 (rca 630ts clone) back from the dead, starting with just a chassis, 12LP4 and safety glass faceplate, which spent a large part of it's life nonfunctional custom built into a wall in my Grandparent's home in Pennsylvania, early 70s to late 90s, when they sold it and I removed it and had it shipped to my home, chassis was slightly damaged in the move, it was then kept in storage for more years till I was ready for the challenge to restore it.

I guess the reason the 12LP4 is so strong is that it ha not been used since the 70s.

I spent many many hours going over documentation working out how the silly thing worked!
Including RCA630TS Service manual, rca_630ts_sams , both sams and riders for Fada 895.

The thing is a pain to work on, but worth the time and effort!

[Notimetolooz]

The resistor wattages are based on what the voltages (and resistance) should be. A one watt resistor could be used if the actual dissipation is 0.625 W but it will run hotter than a 2 W.

If you plan to work on more TVs in the future a tube tester might be more economical that replacing all the tubes at the start. Also that way you don't throw out tubes that may have worked fine in the TV.

The forum will be here when you get back.
Stay away from sharks.

[Yamamaya42]

The +225v is critical for the horizontal and vertical deflection, and thus HV, if you have something on the screen as you mention, then the +225v line has to be working partly, but there must be some error along the line of it in it's point to point wiring, won't be easy to track down.

[Notimetolooz]

It isn't that bad.
The 6K6 voltages can all be explained if R185 is connected to the -85V supply point instead of the +225V.
The 6AT6 could be something different. The grid pin 1 should be a bit negative, if it is near zero it could be passing too much current. That could cause pin 7, the plate, to be pulled too low. Check R180. Another test would to measure the voltage on pin 7 if you remove the 6AT6, it should go to about 225V. Check R179.
It would also be a good idea, one that is always a good idea to do early, is to measure the voltage supply points in the power supply. +225V, +150V, -3.5V -14V and -85V.

[Chris K]

Thank you thank you thank you. Great information here! I'm going to get all of my packing done when I get home ASAP and give this one more try tonight! I will check all of the supply voltages. I've got HV and a stable picture so my guess is it's a misconnection in the electrolytic caps or in some of the cracked wiring I replaced. I definitely have +225V DC.

[Chris K]

Originally Posted by Yamamaya42

The +225v is critical for the horizontal and vertical deflection, and thus HV, if you have something on the screen as you mention, then the +225v line has to be working partly, but there must be some error along the line of it in it's point to point wiring, won't be easy to track down.

I've been in this chassis and traced the wiring for so long, I feel like I know it by heart!

[Chris K]

Originally Posted by Notimetolooz

The resistor wattages are based on what the voltages (and resistance) should be. A one watt resistor could be used if the actual dissipation is 0.625 W but it will run hotter than a 2 W.

If you plan to work on more TVs in the future a tube tester might be more economical that replacing all the tubes at the start. Also that way you don't throw out tubes that may have worked fine in the TV.

The forum will be here when you get back.
Stay away from sharks.

Water up there is in the high 50s. It'll be Land Sharks I'll need to be careful of!
A tube tester is on my wish list...absolutely and I love these old TVs from the late 40s and early 50s. They are built like tanks!

[Yamamaya42]

I guess somehow, someway, R185 got mis-connected from the terminal block with r150 and the red wires to the junction of the cans of c127/128 (yel wires) . :O
There may be other lines wrong as well?

[Chris K]

Originally Posted by Yamamaya42

I guess somehow, someway, R185 got mis-connected from the terminal block with r150 and the red wires to the junction of the cans of c127/128 (yel wires) . :O
There may be other lines wrong as well?

Could be and I had a problem with the 40uf C131 electrolytic section. I did a direct replacement but...the positive side of C131A is where the 260V DC from the 5U4 (pin 8) comes off into the system. The 260V is also connected to the positive side of the 40uf section of C128. I have the negative sides of 3 electrolytic sections C128B, C127A and C131A going to the negative side of C128A. The positive of 128A goes to ground. I have a feeling this is where I messed things up. There was no connection between C131A+ and C128B+ so I put a jumper in and the set began to work developing HV and a picture for the first time. But I did do something that wasn't there before Maybe this is where things got messed up. One more question...the 62 ohm field coil. Where is that? Doing the trace, the yellow and black wires seem to go to the speaker! Is that correct??? I didn't touch anything here I swear!!!

[Yamamaya42]

Originally Posted by Chris K

Could be and I had a problem with the 40uf C131 electrolytic section. I did a direct replacement but...the positive side of C131A is where the 260V DC from the 5U4 (pin 8) comes off into the system. The 260V is also connected to the positive side of the 40uf section of C128. I have the negative sides of 3 electrolytic sections C128B, C127A and C131A going to the negative side of C128A. The positive of 128A goes to ground. I have a feeling this is where I messed things up. There was no connection between C131A+ and C128B+ so I put a jumper in and the set began to work developing HV and a picture for the first time. But I did do something that wasn't there before Maybe this is where things got messed up. One more question...the 62 ohm field coil. Where is that? Doing the trace, the yellow and black wires seem to go to the speaker! Is that correct??? I didn't touch anything here I swear!!!

The answer to that is in the parts list.
Speaker 4x6 elliptical E.M. … electromagnetic, there is no perm magnet , it uses a coil. :O

[Chris K]

Originally Posted by Yamamaya42

The answer to that is in the parts list.
Speaker 4x6 elliptical E.M. … electromagnetic, there is no perm magnet , it uses a coil. :O

Thank you!!!

[Yamamaya42]

c128A is on the -85 line, and thus should be isolated with positive side to chassis.

C131A & C128B are 2, 40uf on the main bias line in parallel, and could have been replaced with 1 80 or 82uf (as 82uf is very easy to get) cap.

[Electronic M]

Field coils are typically on speakers. Permanent magnet technology wasn't mature enough until around WWII for permanent magnet speaker magnets to work, so instead electromagnets (field coils) were used. It wasn't until the early 50s that the last makes stopped making field coil speakers.

In the post war era supply shortages made it so some makers used both types depending on what they could get any given week. Case in point I've got a early post war Zenith AA5 table radio/phono that I used to have 3 of. They stamped the chassis for octal tubes, but used the newer style miniature tubes on some sets and the mix of which were octal and mineature varied depending on supply.

[Chris K]

Originally Posted by Notimetolooz

It isn't that bad.
The 6K6 voltages can all be explained if R185 is connected to the -85V supply point instead of the +225V.
The 6AT6 could be something different. The grid pin 1 should be a bit negative, if it is near zero it could be passing too much current. That could cause pin 7, the plate, to be pulled too low. Check R180. Another test would to measure the voltage on pin 7 if you remove the 6AT6, it should go to about 225V. Check R179.
It would also be a good idea, one that is always a good idea to do early, is to measure the voltage supply points in the power supply. +225V, +150V, -3.5V -14V and -85V.

Everything coming off the bleeder resistors is perfect

[Yamamaya42]

you will have to work out why you are losing the +225V at the point of the bleeders to the point of the 1k resistor r185, where you saw the negative voltage. SOMETHING in the red wiring from the two points is messed up.