I got this 1955 RCA tube set from the original owners who had kept it all these years. I tube went bad years ago so the tv got put away. Anyways this set seemed like a good first project but things have not been going well at all. When I first brought it home I could actually get a white box to appear on the screen, so I want ahead and tested every tube. It had a bad verticle output tube that was replaced, and a bad audio 6av6 tube that is still missing (Kinda forgot about it). I replaced every capactor and had hoped all would be well, but no. The flyback has a low pitched whine and the screen does not light up at all (though the rear of the tube does). I can get an arch off of the top of the flyback tube and the plunger, but still no luck with the screen. I will pull the chassis out again and look through the manual again but nothing has been obvious.

I was hoping this would be an easier project to learn from. I repair solid state sets all the time but these tube sets have been a struggle.

The whine is likely the horizontal osc being off frequency. Most RCAs had an adjustable H osc coil. If that coil was adjusted to compensate for failing capacitors and you replaced those caps then it has to be readjusted for the new caps. Best to adjust it with an oscilloscope (I look at the sync on the composite video then match its freq in the osc, then get osc the waveform right).

If your set has an Ion trap and it has been removed or misadjusted that will result in no picture. Another cause of no picture is incorrect gun voltages on the CRT (may want to check).

Is there way to get around using Ann oscilloscope? I do have one, though it never has worked right. I did mess with that control and had no luck. I’ve been at it for over 12 hours now and honestly it’s been frustrating. I’m about to go off to college and have wraped up almost every project besides a few troublesome tube sets.

How do I check for an ion trap or check the gun voltages? Sorry for the questions but I’m a bit new to tube TVs. I’ve been doing solid state and vintage audio but the one thing that never works out is any all tube tv.

Also would these things cause the crt to not light up at all? I’d feel way better if the crt lit up.

The CRT voltages are usually show on the schematic or tube voltage chart.
Basically the voltage between the cathode and first grid (control grid) has to be in the right range, grid not too negative but not positive with respect to the cathode. Also the second grid needs to be a couple of hundred volts positive.
Ion traps are used on some CRTs in the earlier years. They resemble sheet metal clamps on the neck of the CRT near the socket. Do a search in the forums here for further info.
Don't get too wrapped up in working on this, you will only be running in circles if you do not have enough info.
Do a search for RCA Tube Manual RC-22 pdf and download it. That is the 1963 edition. RCA Tube Manuals have a section on how tubes (including CRTs) work. A good place to start. I've downloaded some other editions but the search function doesn't work well.
With RF circuits you almost always are working with sine waves so a scope doesn't help much. With TVs many areas require the right wave shape so a scope can be very useful. One with a vertical bandwidth of at least 5 MHz is recommended.

Some RCA schematics have adjustment procedures that assume you don't have a scope... Consult your copy of the sams for the set for that...IIRC it varies from chassis to chassis.

If your college has a tech/physics major/minor they probably have scopes that you may be able to get permission to use. May be worth it to bring the chassis if you can't finish it.

Everything I mentioned in my first post is a possible cause of no screen light...so is a dead CRT, but I assume you have tested it.

Some RCA schematics have adjustment procedures that assume you don't have a scope... Consult your copy of the sams for the set for that...IIRC it varies from chassis to chassis.

If your college has a tech/physics major/minor they probably have scopes that you may be able to get permission to use. May be worth it to bring the chassis if you can't finish it.

Everything I mentioned in my first post is a possible cause of no screen light...so is a dead CRT, but I assume you have tested it.



I do have an update on this set. I spent hours trying to adjust it with no luck, but something interesting did happen. After shutting the set down and walking away for a while, I powered it back up. The flyback sounded like a modern tv and the screen lit up, then as the set warmed up it went back to the normal darkness. It really has been a frustrating project.

I have not tested the CRT yet actually since it did used to kinda work and my CRT tester never has an attachment that works. Yesterday I did get 4 different CRT testers in various conditions so they need to be tested.

Also I got a few oscilliscopes. I see test points in the schematic but am not sure how to actually do the tests. Do i remove the chassis and poke around with the scope? I don't see any way to hook up a scope and adjust the horizontal at the same time any other way. Sorry for the questions. This is the first tube set I've had.

I do have an update on this set. I spent hours trying to adjust it with no luck, but something interesting did happen. After shutting the set down and walking away for a while, I powered it back up. The flyback sounded like a modern tv and the screen lit up, then as the set warmed up it went back to the normal darkness. It really has been a frustrating project.

I have not tested the CRT yet actually since it did used to kinda work and my CRT tester never has an attachment that works. Yesterday I did get 4 different CRT testers in various conditions so they need to be tested.

Also I got a few oscilliscopes. I see test points in the schematic but am not sure how to actually do the tests. Do i remove the chassis and poke around with the scope? I don't see any way to hook up a scope and adjust the horizontal at the same time any other way. Sorry for the questions. This is the first tube set I've had.

Weak CRTs typically get better the longer they are on in my experience.

It sounds like a component is drifting at warmup and messing up the H osc operation, or shifting the bias to the H output...Shifting CRT base bias could also cause that if the HV holds steady through the blackout.

First, verify that one of your scopes work. Most ones made in the last ~40 years have an internal square wave generator with a terminal on the front panel...Hook a probe to it and an input and check that you can make that square wave appear on the screen with a decent wave shape and amplitude that makes sense relative to the scale on screen, the voltage setting and multiplier of the probe.

If your scope is good there are a few ways to do the procedure. Some RCA sets have 2 cores in the H osc coil one accessible via the back or top of the chassis the other only available underneath the chassis...If there are two slugs you will probably need to adjust both. Assuming a transformer powered set you can pull the H output tube and adjust the osc with the chassis on the bench (on a series string reconnect only the heater leads of the tube using clip leads).

If you ever need to scope something with the chassis in the cabinet there are
options for that too...You can get tube socket savers/extenders with test points on them to probe test points on tube pins. You can also get alligator clip leads to bring test points out from under the chassis (and avoid putting your probes at risk of mechanical damage). If you can't get the scope close enough to the set for the probes to reach you can get BNC extender cables for the probe(s)...Make sure to get the right impedance cable (most scopes/instrumentation use 50-ohm impedance cables).

Sealtest,
Just one oscilloscope is usually enough.
What scopes do you have? Older scopes work somewhat different than newer ones. I can steer you in the right direction for "how to"s on them online. Many of the ones that come up in a search deal with very modern types.
Do you know if any of them work?
Do you have probes for them?
TVs are very complex in there operation, until PCs came out they were the most complex electronics in the home.
Getting the screen to "light up" is called getting a raster. That is about half way to getting a picture. Not to be confused with getting the heater in the CRT to light.

If you can't get the scope close enough to the set for the probes to reach you can get BNC extender cables for the probe(s)...Make sure to get the right impedance cable (most scopes/instrumentation use 50-ohm impedance cables).
That's not exactly right. Some high frequency scopes do have a setting for the input that is 50 ohm, however the more typical scope input is 1 Megohm.
Using the 50 ohm input would load down the normal points a scope would measure. Normal scope probes use special low capacitance coax.

I
Also I got a few oscilliscopes. I see test points in the schematic but am not sure how to actually do the tests. Do i remove the chassis and poke around with the scope? I don't see any way to hook up a scope and adjust the horizontal at the same time any other way. Sorry for the questions. This is the first tube set I've had.

Usually you have to pull the chassis out to get to the points you want to measure. Basically you attach the scope probe to a point in the circuit. There are a few spots that are accessible without pulling the chassis, it depends on the chassis. They did make tube test adapters that allowed you to connect to the tube terminals without accessing the bottom of the tube sockets but those are hard to find now and expensive when you do find them. I don't like the phrase "poke around" when dealing with high voltage tube circuits.

The other thing to watch out for, all though not relevant in this case but worth mentioning, is when attempting to use a CRO on a live chassis set. The set under test must be connected to the mains via an isolating transformer otherwise bright blue flashes occur! ie path to earth via the CRO probe and CRO. Usually results in damage to the probe, the CRO input and the set under test. It's particularly spectacular in Aus as we have 240V mains.

Sometime either today or in the next day or so I am going to clear my bench and take the chassis into my shop and try all that you all mentioned. I really do appreciate the help with this. Tv repair is a big jump from vintage hifi restoration like I used to do.

As for oscilloscopes I have a Kenwood CS-4025 which is no good for this set since it only goes to 250v. The Tenima 72-720 goes to 600v so it should be best. Both are calibrated and tested.

I did not know about using an isolation transformer. Very good advice. I just got one out of a tv repair shop that needs testing (condition is poor).

Actually the Kenwood would be fine if you use a 10X probe. The DC voltage on a signal can be high in a TV but the AC voltage isn't usually that large on most points. Use the AC setting of the input to remove the DC. There are some point you should never try to measure, even with a meter, unless you have the special probes, the Horizontal Output plate and the HV rectifier terminals.

Makes sense. I might use both scopes just to compare the two.

But here is a new issue. I removed the tv chassis and took it into the shop. I have it connected to a isotap transformer all ready to go, but now the high voltage does not come on at all. I've been looking for loose connections and made sure the transformer is putting out the right amount of power but no luck. Does this chassis have to have the picture tube connected to complete some sort of circuit?

It's possible that the plug for the yoke has a jumper between two pins which breaks the B+ going to the sweep circuits.

Ha-ha. I told you TVs are complicated.
The horizontal yoke winding is used in conjunction with the flyback to generate the CRT high voltage.
Those TVs with the CRT separate from the chassis are a pain.
A good time to remind you that CRT are delicate and dangerous. Separating the CRT from the cabinet means great care must be used. Probably best not to remove the CRT from the cabinet. Back in the day TV repairmen usually drove a panel truck to haul the TVs. Another trick was to use special small test CRTs that could work well enough with the original yoke and CRT socket.
He could leave the CRT and just take the yoke and the chassis to his shop.
If you haven't checked out "Phil's Old Radios" website, he has a number of interesting articles about TVs.
https://www.antiqueradio.org/8XP4PictureTube.htm

What make and model is the set?
HiFi audio is straight forward compared to even a B/W TV, a schematic is practically mandatory unless you have a lot of experience with TVs. A TV that old might have a free schematic available if it is a major brand.
SAMs Photofacts many times will have scope waveforms to look for as well as adjustment procedures.

I don't plan to remove the CRT from the cabinet. I hang out at the local E-waste facility a lot and have seen many people drop CRT sets with the mentality of "I don't have to be careful with this thing since I'm throwing it away". One loud boom and a broken CRT later they know not to do this.

Anyways I am going to try scoot the chassis out of the cabinet just enough to try and get at the bottom. I will be very careful of course.

The tv is a RCA KSC92-a chassis. I have the manual now and is has waveforms in it, but without the CRT all the voltages go wonky and I could not get most of the chassis to have proper power. It does mention the procedures for HV adjustment but they all have the CRT showing raster so they may or may not apply. I just don't understand how an oscilloscope will help when turning the controls for the HV never gets it where it needs to be. The pitch changes but never goes high enough.

If by adjustment for the HV you mean the Horiz oscillator adjustment procedure, the scope is needed to adjust the wave shape of the osc. There are two slugs in the osc coil one is primarily for the shape and the other is primarily for the frequency, BUT they interact with each other.

With RCAs (when adjusting the H osc) I tend to scope video and set the scope up to show 2 to 4 H synch pulses and count the scope scale divisions between the leading edges of the tips. Leave the scope time scale untouched and connect it to the horizontal. I then adjust the horiz oscillator coil slugs to get the peaks of the waveform the same distance (read divisions on the scope screen scale) apart as the video sync peaks were while maintaining proper wave shape illustrated in the sam's...It is not hard to do once you've done it once or twice.

There may be other controls like Drive, linearity and width that need to be tweaked for best/correct operation.

IIRC the yoke plug on that is octal. You can extend the yoke by robbing an octal socket from a junk set, taking the base off a dud octal tube and some wire and making an octal extension cord...You can do the same with the HV lead (you'll need a big screen BPC CRT set to rob the HV cable from), and the CRT socket (brighteners will give you all you need except the wire).

Yes I mean the Horiz oscillator adjustment procedure. I have looked in the manual and tried to follow. The process doesn't seem too bad by any means, there is just the fact this is my first time trying this and so far there have been many snags. Tube sets have always been so cool and satisfying when you save one from the dump, but fixing them has been another story.

Good info on the adjustment. I am trying real hard to get this sorted, but has there ever been a case where the Horiz oscillator can not be gotten close enough to be adjusted with a scope? That is the worry with this set so far. I can adjust and change the pitch of the HV but it is never even close. In the manual for this set there are many troubleshooting processes but none that have the Horiz working while also having no raster. At this point it looks like it may be best to try harder to find someone local who can give some hands on guidance with this.

Yes this set has the drive, width, etc. Adjusting those did not change anything, but they probably just made it worse. Oh well it is a learning experience.

Sounds like it's time to search for more junk sets. So far the couple tube sets I have are the only ones that have shown up in years of searching the area. Before doing anything it looks like this set has a lot of slack and extra wire so I will scoot the chassis out with everything hooked up and see how many test points are accessible. Something to do tomorrow or in the next couple days.

The extension cables are a good idea and getting some parts from brighteners is good also.
In other posts you said that you have stuff from closed TV repair shops. You might find things like extension cables and test CRTs among the items. You could find brighteners there also. Maybe replacement yokes too.
I didn't mean to imply that a scope should be used for every troubleshooting problem. You can determine frequency with your scopes to within about 5% which is plenty good enough in cases like these.
The way that the flyback circuit works it can only produce the high voltage if the frequency is within a certain range. I've seen parts destroyed because the frequency is way off. The horizontal output circuit is one that handles a lot of power and parts are under a lot of stress even when working normally.
If the horizontal oscillator is off frequency it could be due to many things, bad resistor, capacitor, coil, tube, bad connections. You seem to have some evidence that the problem changes with temperature so when you test these components individually they might not show that they are bad. You said you replaced all the capacitors, did you use the right type in each location? The CRT itself doesn't take a lot of current so having it disconnected shouldn't effect chassis voltages very much. Things like the yoke and the focus coil (if your set has one) need to be connected.
Since I don't have the schematic, if you could scan the section containing the horizontal area and post it that would help.

I just don't understand how an oscilloscope will help when turning the controls for the HV never gets it where it needs to be. The pitch changes but never goes high enough.

The scope can visually verify that the frequency and shape of the horizontal oscillator frequency waveform is within the ballpark, and you will see how the controls change it's shape and frequency.