Motorola TS-912 No Raster
 

Working on a TS-912 chassis (roundie). Temp clipped in power supply caps and replaced H output tubes. I have HV but no raster. The CRT cathode volts are way high and the 695 Sams shows a very spartan brightness circuit, all cathodes tied together and completely isolated video feed from the video output via a .5uf cap. The DC path is a 180k resistor on the CRT side going to the middle of the brightness control that is connected to ground and 275v . The reality is there is a neon lamp and a 10k resistor across the .5uf cap and the brightness control isn't wired like the Sams shows. The 180k resistor is a 240k 2 watt resistor. 2 watt seems a bit hefty for just biasing the CRT cathodes.
I'm going to have to pull the front controls out and chase wires.

Has anyone got an idea how this should be wired? Is there an updated schematic or Motorola update to explain this diversion? From what I have been able to peek at the brightness control is not wired like the Sams indicates. Also the neon bulb is lit. I would think this was for a discharge condition of some kind.

Look'in for some good insight on this.

Schematic clip added and folded for easier viewing.

That set is a real gem! I only seen one like it. It is rare and a short production model. Looks like they used shortcuts to see how cheap they could build a color TV.
The set uses strange tubes as used in a European or British TV design.
It's worth hanging on to for awhile. :D

What voltage swing do you get at the wiper of the brt. control, rotating stop-to-stop?

Also, make sure you have focus voltage. Low or no focus voltage will sometimes cause no raster at all in a color set.

I can assume the CRT has been short/ emission tested?

one could guess that NE bulb is in there as a safety feature, but that's just a wild shot in the dark.

the RCA-CTC-10 also has something like it in the same area.
https://www.earlytelevision.org/pdf/...sams_517-2.pdf

SORT of what you discribe

To be more precise, it sort of looks like an odd cross between an RCA CTC-10 & 11, while cutting as many corners to save money as they could...

Cathode 245 - 341
G1 155
G2 670

Focus 4.5 KV
2 nd Anode 24 KV


Cathode needs to come down. Really wish I had a better schematic for this. If the jug was smaller and compactrons were used this could have been the forerunner to a Porta-Color with it's bare bones approach to circuitry.

Yes the CRT was tested by the owner, a VK user, and he says the emission was fairly good. These numbers just don't look good at the CRT.

You will most likely have break out an o-scope, and see in there is waveforms on the cathodes and G1s that's anything near what is shown in the SAMs , from what I understand, ( could be wrong ) there has to be SOME sort of signal there for you to see something on the screen, with the exception of G2 and focus, which is just direct bias driven.

G1 is where color information would be added. With no color or signal the present voltage should be OK. The cathode voltage should be something less than G1 I believe.

Example, this is what is going to my GE color roundie G1, with color turned down, (21fjp22) https://imgur.com/BSJ5KpG
and color up
https://i.imgur.com/5eI2bqn.jpg

IF I pull the horizontal blanking tube or the Y-AMPS, I-E anything to cause it to stop that pulse from getting to G1, I lose raster, BUT, can still get a dim line when setup switch is in service mode, ( no vert )
I have seen the same thing happen if I pull the 1st video amp tube, I lost all raster, same with the video output tube, it had loose socket contacts, and had a habit of losing heater voltage, AGAIN, when it did, I lost raster, but voltage did not change much at the cathodes or G1, this tells me that voltages alone are not enough to get a full raster, at least from what I have seen, are you able to get a line on the screen if you set it to service mode?

A good example is this,


https://www.youtube.com/watch?v=FpOD9g2dKjY

the crt was getting HV and some voltages 400 and up, but due to a LV power fail, it was missing signals that it needed to display anything, and would only show a line in setup mode.

No kidding. the G1s are 'waay too negative with respect to cathode, so the guns are biased 'waay beyond cutoff.

W/O seeing the full schematic, or knowing how the set is wired, one can only guess, looking at the CTC 10/11, the cathodes are balanced with 360v pull-up and ground through power resistors respectively, but, it's also controlled by the video output, and thus the reason I'm saying check waveforms, for w/o any output from video output, cathodes will not run correctly.

In the schematic clip you'll notice the only dc path is thru the brightness control that has 275 v and ground. The wiper goes thru a 180k resistor to the cathodes. The signal path is thru a .5uf cap. In a perfect Sams world the cathode potential should vary and at some point give raster. The reality is, the set doesn't follow Sams. One end of the bright cont has never had a connection soldered to it, spurring on my suspicion the schematic is not correct. Thats why Im hopeing for a better schematic or bulletin.

:saywhat::eek:

Then that's why the control is not working. The low end has to go to ground per the schematic. Once the low end is grounded, voltage at the wiper should swing smoothly from 275V down to near zero when you rotate the control (provided the resistance element has continuity end-to-end).

I would disconnect one end of that neon bulb too, just to keep it out of circuit for now. Its sole function is probably just a safety 'spark gap'.