Newbie's First Questions RCA TS 721 Restoration - Page 11 - Videokarma.org TV - Video

Notimetolooz · #**151** 07-23-2022, 11:04 AM

See if there is any places that might be draining off the HV. Ceramic cap C142 could be leaky. Check the resistors around the horizontal output. Measure the HOT voltages except the plate cap.
I wonder about the accuracy of HV probes but yours wouldn't be off that much.
Make sure the outer coating of the CRT is grounded, it forms a filter cap on the HV (along with C142).
You might check the adjustment of the horizontal drive C136B. Do not use it to adjust the HV however, you could damage the HOT that way.

Chris K · #**152** 07-23-2022, 11:27 AM

Quote:

Originally Posted by Kevin Kuehn

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.

The voltage spec sheet shows 290 for Pin 10 and I've got that. Previously I replaced the HV resistor with a new one so that's probably ok. The horizontal and vertical yoke windings ohmed out perfectly and it is in circuit. The 1B3 is old stock tested good I got from Tube Depot so I'm stumped. Maybe I'll replace it with the original and see if that makes a difference. I did order another one to try because they are so common in these sets it won't go to waste. The HV measured the same on and off the CRT. It's gotta be the tube right? The damper is new as well. I'll keep troubleshooting and see what I can come up with. I do see in the Riders schematic the B boost generated by the damper is 330V but I have a tube socket voltage chart from the Sams that references 290V.

Kevin Kuehn · #**153** 07-23-2022, 12:31 PM

Quote:

Originally Posted by Chris K

The voltage spec sheet shows 290 for Pin 10 and I've got that. Previously I replaced the HV resistor with a new one so that's probably ok. The horizontal and vertical yoke windings ohmed out perfectly and it is in circuit. The 1B3 is old stock tested good I got from Tube Depot so I'm stumped. Maybe I'll replace it with the original and see if that makes a difference. I did order another one to try because they are so common in these sets it won't go to waste. The HV measured the same on and off the CRT. It's gotta be the tube right? The damper is new as well. I'll keep troubleshooting and see what I can come up with. I do see in the Riders schematic the B boost generated by the damper is 330V but I have a tube socket voltage chart from the Sams that references 290V.

Another possibility is that the horizontal oscillator is running so far off frequency that it's not at the resonant frequency of the flyback transformer. Although if this low HV has persisted since you had a stable picture on the set that idea could not be valid.

Chris K · #**154** 07-23-2022, 12:43 PM

Quote:

Originally Posted by Kevin Kuehn

Another possibility is that the horizontal oscillator is running so far off frequency that it's not at the resonant frequency of the flyback transformer. Although if this low HV has persisted since you had a stable picture on the set that idea could not be valid.

OK...now that might be something...can I blame Bob Andersen? (JUST KIDDING!!!). We are talking about adjustment to the horizontal sweep oscillator? When I watched Bob's restoration of one of these 721s, in an effort to get something reasonable on the CRT, he practically bottomed out the core of L121 so I figured what the hell??? It was part of how I managed to get a picture originally and I know it's nowhere near the original position, although I didn't bottom it out. How would one go about adjusting L121 and the resonance with the flyback?

Chris K · #**155** 07-23-2022, 12:51 PM

Quote:

Originally Posted by Notimetolooz

See if there is any places that might be draining off the HV. Ceramic cap C142 could be leaky. Check the resistors around the horizontal output. Measure the HOT voltages except the plate cap.
I wonder about the accuracy of HV probes but yours wouldn't be off that much.
Make sure the outer coating of the CRT is grounded, it forms a filter cap on the HV (along with C142).
You might check the adjustment of the horizontal drive C136B. Do not use it to adjust the HV however, you could damage the HOT that way.

C142 has worried me since I started this. The capacitance is on the money but I haven't checked for leakage. Where does one find a 500pf cap at 10,000 volts!? I should probably check the damper cathode voltage first. I've messed with C136B too during the wild L121 party. Crap! I have taken the CRT out and put it back in so many times, the Aquadag is wearing where the ground loops contact it so I can spray those spots with the Bonderite graphite spray I have.

Kevin Kuehn · #**156** 07-23-2022, 01:46 PM

Quote:

Originally Posted by Chris K

OK...now that might be something...can I blame Bob Andersen? (JUST KIDDING!!!). We are talking about adjustment to the horizontal sweep oscillator? When I watched Bob's restoration of one of these 721s, in an effort to get something reasonable on the CRT, he practically bottomed out the core of L121 so I figured what the hell??? It was part of how I managed to get a picture originally and I know it's nowhere near the original position, although I didn't bottom it out. How would one go about adjusting L121 and the resonance with the flyback?

Back on your post #33 it sure looks like you had the horizontal locked(not rolling). Unless you've readjusted L121 since then it's not likely your problem. I haven't looked but Sams generally has instructions for adjusting the horizontal freq. Unless you have a scope or frequency counter that you know how to use it'll have to wait until you have the CRT back in. You ultimately are the best judge of what you tampered with since you had it working.

Electronic M · #**157** 07-23-2022, 02:19 PM

Doorknob caps were used in legacy X-ray machines until recently. There may still be stocks on mouser or digikey, but they're expensive. The cap isn't necessary if the dag is grounded. I'd unhook the cap and stick the loose end in a glass cup or jar...If the HV comes up the cap was bad.

Notimetolooz · #**158** 07-24-2022, 09:01 AM

Here is a link to a Crist Rigotti thread where he re-stuffed a doorknob cap while working on a 621.
He was lucky enough to find ceramic caps that could be hidden inside the old casing.
Of course he has some great skills also.
http://videokarma.org/showthread.php...ght=621&page=4

Chris K · #**159** 07-24-2022, 10:50 AM

Thank you very much...I will check it out. I've seen some of Chris's stuff and I have spoken with him regarding my Philco restorations. Very very helpful guy.

Chris K · #**160** 07-27-2022, 01:35 PM

I have ordered a parts chassis for relatively cheap. I've received detailed pictures of the underside and, except for the replacement of the power resistors, it's all original. I've decided to use the original wiring as a template for what I have screwed up. I've also done some scanning and manipulating of the Riders wiring diagram and I have a large and comprehensive copy of it. I blew lots of cash on replica cloth insulated wiring from Tubedepot.com (beautiful stuff) in 18 and 20 gauge and I will be replacing all of the wiring in my set as well. Thanks everyone for all of your help in this thread. For now, I'm going to plod through this one wire, one capacitor and one resistor at a time and I won't be firing it up for a while. I'm sure I'll be back with questions soon enough!!!

Chris K · #**161** 08-05-2022, 10:47 AM

Hey all...back at it with the 721. I discovered my HV probe was faulty reading voltage at about 50% of its actual value. I also acquired the parts chassis and with other purchases, I now have 3 working transformers, 3 flybacks, 3 focus coils and 3 different ion traps! The traps, 2 passive and 1 electromagnetic, all work well. The parts chassis I purchased is a very early model, serial #1265 while the chassis I'm restoring is around #65,000 but they're similar enough that they share around 95% parts compatibility. It looks like the EM ion trap is the earliest beam bending effort, the passive magnetic collar and ring the second and a clamp style, square bar magnet type, the last type used on this set. I got one hell of a shock last night moving the focus coil around on its pivots and I'm wondering what that's all about. I had my fingers on both the body of the coil and the metal support structure so I need to look for whatever might be touching the chassis on the underside.
Focus of the picture is poor despite rotating the focus control to its limits. The best I can get it is still soft. Does anyone have any suggestions as to what I should be looking at other than the variable focus pot???
New doorknob HV cap also!

Chris K · #**162** 08-05-2022, 05:11 PM

I think I solved the focus issue. I moved the coil around...moved it backwards. Picture has a very bright area all the way left to right near the top. This is supposed to be a crosshatch pattern. I guess I need to stop fiddling with coils and the like and ask how do I diagnose and dial this in and what kind of diagnostic equipment do I need?

Notimetolooz · #**163** 08-06-2022, 09:26 AM

The bright areas are probably due to the beam lingering in those areas or retracing them.
I'd be more concerned about the fact that it looks like a negative image.
It could be the vertical sweep frequency being far off. Try adjusting vertical hold and similar things. There may be a coarse vertical adjustment.
Sometime a bad video detector will cause a negative image.
I don't think I have every seen anything like that screen.
Make sure the crosshatch signal is not so high it overloads the set.

Kevin Kuehn · #**164** 08-06-2022, 10:54 AM

A video of that screen would be more telling. The diagonal shading makes me think your horizontal isn't locked. I don't see any defined horizontal video, only the wavy vertical along with the raster scan lines. The raster lines themselves seem quite well focused center to bottom.

Chris K · #**165** 08-06-2022, 12:32 PM

Quote:

Originally Posted by Notimetolooz

The bright areas are probably due to the beam lingering in those areas or retracing them.
I'd be more concerned about the fact that it looks like a negative image.
It could be the vertical sweep frequency being far off. Try adjusting vertical hold and similar things. There may be a coarse vertical adjustment.
Sometime a bad video detector will cause a negative image.
I don't think I have every seen anything like that screen.
Make sure the crosshatch signal is not so high it overloads the set.

My photo is actually rotated 90 degrees to the left.