sparks flying in RCA 721

[thomash85715]

Hi: We are trying to revive this TV and found numerous problems. The CRT was laying on a filter cap and a IF can having dislodged from its flimsy and crude mounting ears. It's a wonder it was still intact. We got it sort of correctly in place and brought it up on variac. About 2/3 of the way up, the rectifier tube started arcing so we shut it down. Filmanents were coming on. Many tubes are non-RCA including kine which is tung sol. But caps all look original from 1947. We got our work cut out for us. I have not located a fuse; is there supposed to be one somewhere or did they just intend to sacrifice the 5U4 if something failed? Cabinet is a little rough but overall the thing is definitely a find and worth some effort. Does anyone disagree? Thank you. --Tom in Arizona

[Chad Hauris]

Too often these sets did not have fuses and will really destroy themselves in the case of a short...have a Hammond organ where it literally melted the glass on a 5U4 due to a short when with its previous owners. It is always a good idea to install fuse protection in the power line when repairing...for a TV of this type 3 to 4 amp fuse should do.
You will need to replace all the paper and electrolytic caps to get it working right; especially on electrolytics this old there is almost no chance they are still good.

[Phil Nelson]

As Chad suggested, at least replace the electrolytics before you power it up again.

Get your hands on a schematic, check things off as you replace them, and be patient. If you burn up something like a power transformer through overeagerness, you will really kick yourself.

Phil Nelson

[bgadow]

I agree that the crt mounting on these is poor. Maybe when they were new those ears gripped better, but my 721 was the same way when I bought it. All early RCA's used that kind of arrangement while most of the others strapped the crt tightly to either chassis or cabinet.

[fujifrontier]

I don't understand the big deal about the transofrmers. why can't you just get a new one?

[Chimes]

Originally Posted by fujifrontier

I don't understand the big deal about the transofrmers. why can't you just get a new one?

Finding a new transformer with the correct voltages and current rating will likely be very difficult if not impossible for a vintage TV set, and if you do, the price will be high. Power transformers are not cheap.

[fujifrontier]

oh. sorry. i thought they were kinda like any other component... dial up rat shack and get one

[Tubejunke]

Originally Posted by fujifrontier

oh. sorry. i thought they were kinda like any other component... dial up rat shack and get one

What can you find at Radio Shack but cell phones, satelites, and a staff who wouldnt know a capacitor, tube, or a transformer from a CD. The elusive Store Manager might know what you are talking about. Then they would politely tell you that they don't stock it, and can't get it. Now ONCE there was a time.....

Also, everybody has probably heard my whole RCA transformer story. I needed one, and found one. Not expensive but not far from what I have in the set. Its all relative to how much one likes a given set as far as expense I think.

I love my TC-127 and can't wait to get the juice hooked up and give it a whirl.

Phil was really right about being patient. I'm not sure, but if I were more patient I may not have had to do the transformer replacement. I want to believe that the damage was done before I got my hands on the set. I never applied 110v longer than maybe 5 seconds, 2 or three times, The rectifier heater was completely open from the get go and after pulling the trans. and looking inside the thing was REALLY smoked. Signs of prior damage...

[Dave S]

I put a copy of the service manual up at http://www.njarc.org/manuals.htm for you. Good luck!
--Dave

[fujifrontier]

Originally Posted by Tubejunke

What can you find at Radio Shack but cell phones, satelites, and a staff who wouldnt know a capacitor, tube, or a transformer from a CD. The elusive Store Manager might know what you are talking about. Then they would politely tell you that they don't stock it, and can't get it. Now ONCE there was a time......

calm down you know what I meant

[Chad Hauris]

Originally Posted by fujifrontier

I don't understand the big deal about the transofrmers. why can't you just get a new one?

A fuse in the power line will protect a difficult to replace component and prevent a fire hazard. I take it you have not seen some of these unfused pieces of equipment after they have been destroyed by a short...there can be severe damage.
I don't get what you are trying to say

[fujifrontier]

Originally Posted by Chad Hauris

I take it you have not seen some of these unfused pieces of equipment after they have been destroyed by a short...

I don't get what you are trying to say

i wasn't trying to be flippant about it. i thought you could just go get another one, like you do with caps.

and no, i havent seen any pics.

[Phil Nelson]

Originally Posted by fujifrontier

i thought you could just go get another one

Unfortunately, no. TV designs were varied, and the power transformer for each one supplies a specific set of voltages and current ratings. Plus, they have different sizes and shapes, so you might find one with the right specs that doesn't fit in your chassis. Flyback transformers are even more specific to each particular model, as I understand it.

These are not generic parts, in other words. The only source is typically another vintage device. And the cost to replace one might exceed what an unrestored TV is worth.

The article at http://antiqueradio.org/scot02.htm describes how I burned the power transformer on a Scott 800B and ended up adapting another old transformer to replace it. You don't want to go there!

That said, don't get the idea that your TV is a ticking time bomb. There are simple precautions, including replacing electrolytics, that you can take to avoid damaging it. There are also straightforward procedures for powering it up and testing the power supply, once you have replaced the electrolytics.

[fujifrontier]

i see