What could Account for This? RCA 9TC-240 Power Issue
 

I saw on his YT channel Bob had a series on a similar chassis. I replaced the power resistors, the electrolytic caps (more on this later!), the tubes in the HV box with known good ones and most of the paper caps. The voltages coming out of the bleeder network are pretty close to what they should be. I actually brought this TV up on original components when I started restoring it and I got a small, dim raster on the 10BP4.

But the TV is only drawing 1.5 amps when it should be drawing about 2.2 A. The high voltage is low at around 5Kv. I do not have any light on the screen of either the 10BP4 (tested OK) or on the technician 5" CRT. The brightness control ohms out close to perfect.

Any suggestions on where I should start looking to run this down? I cleaned the tube sockets in the HV section and checked the pin tabs for good connections and possible shorts but found all OK.

On the electrolytic caps, check out my other thread about the Adapt-A-Cap issue.

TIA all
Chris

Just a quick addition. I haven't traced this out yet but there is an RCA jack on the back for a phono audio input and a sliding switch for selection of TV or Phono. I know similar switches on the Dumont and cloned versions of the chassis with the continuous tuner allowing you to select TV or FM does shut down the TV when FM is selected. I've looked over the schematic for the RCA and it doesn't look like the switch wiring is drawn. If there's a switch that's broken in the phono position, could it be a possible explaination, albeit a remote one, for what I'm seeing?

Obviously 5kv is too low to drive the beam fully to the screen on a 10BP4 , norm is 9k, or so, so there has to be a problem with horizontal output or frequency or drive causing low HV. SOMETHING ain't right in that area, you jut have to track down what it is.

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Always check Power Supply Voltages.... Especially if you suspect current use is too low...

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I checked the screen voltage on the 6BG6 horizontal output tube and it's about 25 volts low. It should be 143 V DC without an RF input signal and it's around 118 V. The other voltages are OK.

I'm trying to make sense of this SAMS, and not getting very far...

https://www.earlytelevision.org/pdf/..._sams_74-8.pdf

p33

shows pin 8 g2 260 vdc.

pin 5 g1 8v vdc
pin 3 cathode 8 vdc

using p6 v18 as ref.
then it says
pin 6 110 vdc:scratch2:
https://frank.pocnet.net/sheets/093/6/6BG6GA.pdf

huh?

Put the negative lead of the voltmeter on pin 6 of V18 and measure the voltage on the pins of V19.
What are you unsure about?

What I'm getting at, this is another case of the infamous early SAMS errata with ref to pin 6, or it's being used as a tie point not shown on the schematic, as pin 6 is NC, a bit confusing!

Yes, pin6 is used as a tie point. They don't show tie points in the schematic. The is no error in the Sams.

And the Sams is close to the 9TC-240 I have, but not exact. In fact, I don't think the ETF has the specific schematic and service data for this chassis. This chassis is the KCS 28B-1. I'm using the voltages from the Service Manual 51 that says it covers KCS28 but is written in reference to the 8T-241, 243 and 244 models. The voltage on the HO screen is 160v with an input signal and 142v without.
Bob, measure the voltages on the 6AV6 audio amp while connected to pin 6 of the 6AL5 discriminator?

The only referred to exact schematic I have is in a service publication TV shops used (the ones in the 16-9 orientation!) that covers RCA TVs from 1946 to 1950. The info in the ETF technical section for KCS 28 and 9T-240 have some differences in resistor and capacitor values from this publication. When I measure some of these components, they tend to agree with the parts list in the TV shop manual

The SAMS says to use pin 6 of the 6sn7gt (horizontal oscillator), as the negative lead of the voltmeter, and then to check the voltages listed on page 33 on the SAMS, listed, though it's not exact to what you have, it should give you a sign of the problem, as they should be close to standards.

Got it. I was looking at the Riders and assuming V18 and V19 in the Sams were V118 and V119 in Riders. Most of the time, they match that way.

Speaking of the Sams, the schematic shows the Phono/TV switch which my TV has despite the Sams saying it's found only in the 9T-246. Inexperience on full display here, what is the switch supposed to do in the phono position and the TV position? When I flip it back and forth between phono and TV, I don't get a blip or any indication the slightest thing has happened on the amp meter (Sencore Isolation Transformer on the 0-4 amp scale). There is no change. Normal?

it looks like that when it is set to phono, it cuts out the TV audio, and shunts in the phono audio, and grounds out the CRT brightness. leaving horz and HV alone, rather dumb, the 2 sets I have with the feature had a cutoff for the horizontal output, but then they were made a few years later then this set.

Could a problem with this switch be a possible cause for no light on my screen? I'm using the 5" tube (electrostatic) not the 10BP4. When I turn off the TV, I get the fading central dot.

Depends on what that 5 incher is you are trying to use, you have not said what it is yet, if 5kv is below what it needs to have a full raster with full scan at normal operation, then obviously you won't see anything, and in phono mode, brightness is cut no matter what.

Did you unmount the focus coil? It interferes with the 5AXP4 focus

No I’ll do that

Or at least move it around as see what effect it has. Also do NOT use the ion trap magnet. You should get a glow on the CRT with 5kv

Removed the focus coil...did not have the ion trap on it and there's no change. I hate to keep coming back to this switch but, I measured the voltages on the brightness control (working...ohmed out 10 ohm to 52 Kohm) with the switch in the TV position and I turned the brightness control through the full range while measuring the voltage on all three tabs and had an invariant 135 volts DC reading. No change on the brightness control voltages.

Here's this area:

https://imagizer.imageshack.com/v2/1...922/0ngP4C.jpg

what are the voltages on the other pins of the CRT G1 and G2? you say cathode is 135 and wont change?

Short #3 on the brightness control to ground and try

yeah. go for broke! :D

that will force it full on! or at least pin 2 of the brightness control should drop to near zero when you turn it to that direction if it's good.

That did it! I have light on the screen. No horizontal deflection but there's a line down the center that responds to the brightness control.

I'll get the voltages on V19 from pin 6 of V18 in a few.

Thanks guys. You Let There Be Light!

OK the voltage weirdness is as follows. Black lead from DMM on pin 6 of V18 6SN7GT Horizontal Oscillator and voltages on V19 6BG6 Horizontal Output

Pin #: 0.0V
Pin #2: 0.0V
Pin #3: 4.6V Should be 8V DC
Pin #4: 0.0V
Pin #5: -10.0V Should be 8V DC
Pin #6: 4.7V Should be 110V DC
Pin #7: 6.3V AC
Pin #8: 240V DC Should be 260V DC

There's a jumper installed on V19 between pins 3 and 6 on my TV. Is that supposed to be there? There has been some fiddling with this TV over the years. Why isn't pin 6 over 100V DC?

No horizontal deflection but you have high voltage?!? That can only happen if the horizontal yoke is open or not connected.

Well there's good news. Tube is correctly oriented because when I adjust height, the line shortens in the vertical aspect. I'll check the connections.

The fact that you have HV, vertical sweep but no horizontal is an indication of a possible problem with the horizontal yoke windings and the path from it to the flyback. al that must be check very closely for it;s a tuned circuit, and when it gets out of wack you are lucky to get any HV at all, the fact you have vertical sweep but no horizontal is a big pointer in the direction of the problem, I dont see where it's stated that the resistance of the horizontal part of the yoke should be, but it has to be someplace, that is a good place to start.

13.5 ohms. I will check

15 ohms on the horizontal yoke. I measured from pin 6 of the 5V4 damper to the other side.

Flyback tests fine

That is well within tolerance, the problem must be some place where the yoke hooks to, a bad cap or resistor, or solder joint.

Yes the solder joints in the HV cage do look terrible so I'll go through them tomorrow. I don't like soldered in fuses much. I'll add a fuse holder for the 0.25 amp fuse as well. Headed for bed. Thank you guys so much. We'll get it run down.

one side of the horizontal part of the yoke goes to the plates of the damper tube, the other goes to a power point same as the focus coil it look like.

Yeah the RCA schematic for the 240 is drawn out differently than the Sams. The fuse is directly in line with the yoke and pin 6 on the 5V4 is on the other side. I also ohmed it out from the focus control. I got 13.5 and 15 ohms from multiple places and I doubt a reading that close to the actual value from 2 spots is a coincidence. But with my luck, it could be! I need to get into the HV cage and do a complete service. Haven’t done anything in there yet. Need to resolder and clean and check values.

Gettin' Closer
 

OK, I know why there's no horizontal deflection. It grew out of working with multiple schematics from various models, direct replacement of capacitors in the chassis, value for value, and confusion regarding the Sams chassis photos of the bottom.

This chassis had a number of those weird caps in it...not waxies but almost like a cylindrical ceramic with coding stripes, in fact, they look like giant resistors only light in color. I can't replace these value for value because I can't read them (color blind). I depend on the sams photos to identify color coded stuff like this.

The problem with the horizontal yoke is that one side isn't connected. If you look closely at the picture, you can see the thick yoke wire attached to the highest lug of the bottom terminal strip. There was one of those funky caps attached to it running down into the terminal strip that has the large 0.47uf cap soldered in. The sams photo does not have the cap I cut out pictured.

https://imagizer.imageshack.com/v2/1...922/xBGJwK.jpg

You can see the yoke lead soldered to the terminal but there's nothing attached to it after I cut out the mystery cap

Here is a wider view. The striped lead running horizontally across the center goes to the yoke. The large diameter yellow modern cap is the 0.47uf I put in to replace another large old cap...but not the one I previously cut out. It is C199 in the schematic down at the end.

https://imagizer.imageshack.com/v2/1...924/BrvumL.jpg

Here is the section of the schematic I'm using. The sams is depicted so differently.

https://imagizer.imageshack.com/v2/1...924/MC6Jmf.jpg

So, I need to connect the yoke lead to something. Like I said, someone has been in here before so I'm not sure what I saw when I opened it was what rolled out of the RCA factory. The other side of the horizontal yoke is connected in the HV cage fine.

so that must be the .047 cap on the RED/BLACK wire to the fuse line that is missing, it gets hit pretty hard, not sure what DC rating is called for, must be rather high.

this is prob overkill. but..
598-474MWR102K mouser part.

Those caps you describe are simply paper caps inside plastic like bumblebee caps.
I'm confused - you say the .47uF cap pictured is C199?
This is the one that connects to the yoke. What do you have it going to?

There are two .47, 200v volt caps in this set. C190 and C199 in the RCA service info. One is used for AGC the other is for the yoke .

Check out the RCA wiring diagram. C199 is located in the middle at the bottom.

The yoke connects on the left side of C199

https://live.staticflickr.com/65535/...bf79f89a_z.jpg

C199 on the schematic picture I posted. Lemme take a look at this. I did not have a cap bridging those terminals when I started.