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

[Chris K]

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

[Chris K]

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?

[Yamamaya42]

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.

[Username1]

.

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

.

[Chris K]

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.

[Yamamaya42]

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
https://frank.pocnet.net/sheets/093/6/6BG6GA.pdf

huh?

[bandersen]

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?

[Yamamaya42]

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!

[bandersen]

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

[Chris K]

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

[Yamamaya42]

Originally Posted by Chris K

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.

[Chris K]

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.

[Chris K]

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?

[Yamamaya42]

Originally Posted by Chris K

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.

[Chris K]

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.