interesting tough dog on CTC-15/16

[DaveWM]

I was reading up on a RCA color service manual about width problems on RCA roundies. the problem presents as low cathode current on the HOT, and too much neg on the control grid. Seems the engineers designed in a safety brake on the HOT to keep the HV down in case of a shunt tube failure involving tapping into a source of neg voltage that would cut off the HOT tube. the source is from the grid of the Blanker tube that is on the chroma board, nearest the HOT. There is a 390k ohm resistor that goes from the grid to ground thru a narrow trace along the side of the board. The Heat from the HOT causes a break in this trace. Other problems include some loss of brightness but it does not show up like the loss of width. A check of the horz blanker grid will tell if there is problem as it will be too neg when compared to the sams listing.

It was recommended that the trace be bridged over anytime the set is serviced.

I imagine this applies to clones as well.

per the manual there are three sources of bias to the HOT, the rectification of the osc drive (neg) the blanker tube grid (neg) and a 10meg resistor to the b+ (positive). So if the PP and wave form from the osc tub look correct at the grid of the HOT, but the total bias voltage is not, then suspect one of those other bias sources.

[old_coot88]

Hah. That's interesting to know. I never ran into that condition 'back in the day'.
Something that should be fixed preemptively. The other oft-cited condition endemic to CTC-15/16 and '25 (and clones) is the two ground stakes on the far right end of the chroma board which should always be reflowed.. or they'll go open (not if but when), killing the heaters on the 6GU7s, resulting in beam current avalanche in the CRT.

[stromberg6]

Thanks! Going to add this to my notes.
Kevin

[ctc17]

This is about a 16x I have been fighting with for some weeks now.

Low high voltage, 19kv max, below the threshold of the regulator so there is no high voltage regulation.
The picture size bounces when there are scene changes. (no regulation)
The cathode current is low.
The width is short.
Drive may be excessive.
Adjusting efficiency coil has NO effect at all.

All caps, resistors, tubes have been tested subbed or replaced.
I havent looked at the blanking circuit yet. Later today. I know blanking issues can cause all kinds of problems on these sets.

[ctc17]

This was the problem with my set. The trace is open

[timmy]

could this neg trace also apply to a ctc12 clone as well ?

[ctc17]

look at the schematic and see if theres a 390k from the grid of the blanking amp to ground and if that also connects through a 1.8meg or so to the grid of the horizontal output.
If somthing happens there the drive goes to negative and the hv drops. It also effects the blanking and blanking working properly makes a huge difference

[DaveWM]

not used on a CTC-12 as mentioned you need to check if a clone.

[timmy]

well it does have a 390k that comes off the grid of the blanking amp tube to ground. there is also a 10meg off the grid of the hot to ground.

[DaveWM]

then yes of course it could be the problem, since that is the bias parts being discussed. the CTC-12 does not use these. a Clone may have some differences. the
problem 1st cropped up on the CTC-15 series.

[ctc17]

Measure from pin 2 of the blanker to ground. Should be around 250k. If its 500k or more its open. Make sure the set is off for an hour before you measure that.