Newbie's First Questions RCA TS 721 Restoration

[Kevin Kuehn]

Originally Posted by Chris K

C116B negative lead goes to ground? I'm not seeing that. It's not tied to the -85V center tap as well?

Yes indeed you are correct. That was my very bad assumption the 621/721 were the same in that area.

[Chris K]

No problem...I'm grateful for the help!

[Chris K]

So here is why my CRT isn't working. The CRT voltages are correct except for the cathode that's supposed to be 220V DC and I'm getting -65V. I've got 220v going into terminal 1 of the brightness control and -65v coming out of the second terminal. Terminal 3 has -78v. The corresponding wires on 1, 2 and 3 are red, yellow and orange. The yellow goes out to a terminal strip where it meets the yellow wire to the CRT cathode. Orange goes to a close terminal strip where it meets R133 and a wire back to the capacitor negative of C131. ARUGHHHHHHH!

[Kevin Kuehn]

Originally Posted by Chris K

Orange goes to a close terminal strip where it meets R133 and a wire back to the capacitor negative of C131. ARUGHHHHHHH!

I believe the wire that's going back to the negative side of C131 should instead be going to chassis ground. Notice on the schematic that the bottom side of R133 is tied to chassis ground. The negative side of C131 is at -85. Possibly you inadvertently put that wire back at the wrong location?

[Chris K]

I looked at it and it was hooked up incorrectly but when I removed it, the CRT cathode voltage went from -65 to +30v DC. I'm using both the wiring diagram and the schematic in the Riders version. The issue is my set isn't exactly set up the way the wiring diagram depicts. There are some subtle differences. I also have the Sams for this but my set is much closer to Riders. Unfortunately, I've made so many modifications I've lost track of where and how the wiring was originally configured on the electrolytic caps and yes, I didn't take adequate photos to reference the baseline. I'm close on this. I think it's just a matter of a connection or 2 as I haven't made any wholesale changes except for the electrolytics. One of the issues is the wiring diagram is different from my set connections in the area of the audio output transformer and R133. I can take some photos and post showing the issues I'm having.

[Kevin Kuehn]

Originally Posted by Chris K

I looked at it and it was hooked up incorrectly but when I removed it, the CRT cathode voltage went from -65 to +30v DC.

I believe you should get a positive voltage that varies between approximately 28-225v on the cathode as you rotate the brightness control. 30v sounds like the brightness control is all the way counter clockwise?

[Chris K]

Originally Posted by Kevin Kuehn

I believe you should get a positive voltage that varies between approximately 28-225v on the cathode as you rotate the brightness control. 30v sounds like the brightness control is all the way counter clockwise?

You are absolutely correct Kevin...only I have something hooked up backward!!! Full clockwise rotation of the brightness control is giving me 30v and full counterclockwise rotation, or what should be minimum voltage, is reading 230v DC!

[Kevin Kuehn]

Originally Posted by Chris K

You are absolutely correct Kevin...only I have something hooked up backward!!! Full clockwise rotation of the brightness control is giving me 30v and full counterclockwise rotation, or what should be minimum voltage, is reading 230v DC!

Although that seems backwards it may actually be normal for these early RCA sets. There's a note on my 621 schematic saying counterclockwise is full brightness, but I don't see a similar note on the 721 schematic. In any event are you getting a raster now?

[Chris K]

Originally Posted by Kevin Kuehn

Although that seems backwards it may actually be normal for these early RCA sets. There's a note on my 621 schematic saying counterclockwise is full brightness, but I don't see a similar note on the 721 schematic. In any event are you getting a raster now?

Full clockwise is full brightness on this set. I haven't tried the set with the CRT hooked up for a bit but before I started messing with changes to the wiring to get sound from the speaker (successful) resulting in the voltage issues with the CRT, I did have a good picture using my pattern generator so there's no question the TV works. Unfortunately, the CRT on the 721 is not strapped to the chassis. It is fixed in place only when it is in the cabinet held by 4 brackets mounted on the inside of the cabinet front. It's almost impossible and downright dangerous to work under the chassis with the CRT in place. I've searched for the 5" technician CRT created for this and other TV s of the era with the same issue but I haven't come across any as of yet. Even if one became available, I've already spent a significant amount of money on this thing (just purchased $100 of cloth wiring in original colors to replace all of the crumbling wires in the set!) and my wife is justifiably concerned with the cash that's going to Digikey, Mouser, Tubedepot, Amazon etc in service of restoring this TV. My goal is to take the time over the course of the rest of the year to completely restore this TV to 100% factory fresh. The CRT tests good and they are readily available should I run this one into the ground! I even have a NOS, backup transformer I intend to install! But for now, a good picture with sound is goal #1!

[Notimetolooz]

Yeah. It isn't unusual to read a first post from someone new to a forum, never worked on vintage electronics, that just got a set for $40 or so. They think they got a great bargain. It is hard to break the news to them that they are going to have to spend more (maybe a lot more) to get it working.
I think you had some idea of what may be ahead.

[Tom9589]

Your are right. The cost of replacing capacitors and resistors can mount up on vintage electronics. I bought a Heathkit scope for $10 and then spent almost $70 for parts. I also restored a 1077B B&K TV Analyst and the parts cost was even higher.

In both cases, the results were well worth the price and effort.

[Kevin Kuehn]

Back in the day there were CRT and Yoke extension cables available, but now not so easy to source. A CRT extension can be made using the connectors from a inexpensive CRT brightener. The Yoke probably uses common tube base plugs/sockets. Another issue could be the ion trap if this set uses a electromagnet, but that could be solved by using a permanent magnet variety. All and all it can seems like a daunting amount of effort and dollars when the objective is to restore one old TV. So yes it's understandable that a wife could view such a process as being on the borderline of insanity.

[Chris K]

Originally Posted by Notimetolooz

Yeah. It isn't unusual to read a first post from someone new to a forum, never worked on vintage electronics, that just got a set for $40 or so. They think they got a great bargain. It is hard to break the news to them that they are going to have to spend more (maybe a lot more) to get it working.
I think you had some idea of what may be ahead.

I did and with this TV and its antiquity status, I was never under the delusion it was not going to be expensive. But this is a hobby and a growing passion and feeding that is the reason I do it. I have friends and family members who are firmly rooted in the practical and would look at the money spent on this and conclude "Are you crazy? You could have had a 60" flat screen for what you've spent on this!" Others would say "How much are you going to sell that for when you're done?" I do it for the same reason some people put crazy money into restoring a classic pre-war car to bone stock specifications or restore a pocket watch to gleaming perfection...pride, satisfaction and being able to say "I did it!" No question my wife and I are blessed to be in a position to spend fun money like this and she is incredibly tolerant and understanding of what my "unprofitable" passions consume in money and time. But if it makes me happy, we're happy and vice versa. I'll keep at this beautiful example of early technology until I get it right and I have no doubt I'll look at my crude work years from now and...probably do all of it over again!!!

[Chris K]

Quick question...my HV is low...about 3.5Kv and it should be 8.5Kv. New damper, horizontal output tubes and the flyback ohms out fine. What else should I be checking???

[Kevin Kuehn]

Originally Posted by Chris K

Quick question...my HV is low...about 3.5Kv and it should be 8.5Kv. New damper, horizontal output tubes and the flyback ohms out fine. What else should I be checking???

Do you have the yoke in circuit? I don't see if you've mentioned if you have 330v on pin 10 of the CRT socket? 330v is the B Boost generated by the damper, and if it's not present or low could explain low HV. B Boost would also be measurable at pin 8(cathode) of the damper. Otherwise a weak 1B3 HV rectifier or drifted high R187 HV filament resistor could explain it.