RCA CTC16 arcing - PW700

[DavGoodlin]

I'm in the perverbial jungle here, its hot! That is - the right side of the color board closest to the horizontal output tube. Oddly, this chassis appears never to have had work in this area, between the two 6GY6 and two 6GU7 tubes. Even those two brown jumper wires are intact, but with the usual black slime and green crud at the ends.

At first power-up and after testing all tubes, the HV came up and 6JE6 cathode current was 170 ma. But no raster (video). Then the 270 ohm 3 watt cathode resistor, common to the R-y, B-y, G-y triode sections of the 6GU7's smoked and lit up. It was after I replaced this resistor and 3 more plate resistors AND grid coupling caps, that I powered up again. This time, I left the 6GU7s out and the resistor burnt up again.

Now for the strange part. I put a larger resistor, 300 ohm wirewound 10 watt in and powered up slowly on the powerstat, noting the resistor did not get hot but I saw arcing around pins 8 and 9 of the left tube 6GU7 (R-y, B-y) socket. I figured the tube socket was bad and desoldered it out and then I saw the brown wire was burnt open in the middle and arcing at socket pin 8, the cathode connection. I thought this was a heater filament wire jumper, but how could 6.3 volts short to and fry a 270 ohm 3 watt resistor?

I better get good at this, there the customer's other combo's 16 waiting for repairs and my two CTC12's behind that.

[ChrisW6ATV]

Someone may correct me, but I think I remember hearing of problems with multiple ground points on those late RCA roundie circuit boards. Maybe that is part of the problem(s).

[dieseljeep]

Originally Posted by DavGoodlin

I'm in the perverbial jungle here, its hot! That is - the right side of the color board closest to the horizontal output tube. Oddly, this chassis appears never to have had work in this area, between the two 6GY6 and two 6GU7 tubes. Even those two brown jumper wires are intact, but with the usual black slime and green crud at the ends.

At first power-up and after testing all tubes, the HV came up and 6JE6 cathode current was 170 ma. But no raster (video). Then the 270 ohm 3 watt cathode resistor, common to the R-y, B-y, G-y triode sections of the 6GU7's smoked and lit up. It was after I replaced this resistor and 3 more plate resistors AND grid coupling caps, that I powered up again. This time, I left the 6GU7s out and the resistor burnt up again.

Now for the strange part. I put a larger resistor, 300 ohm wirewound 10 watt in and powered up slowly on the powerstat, noting the resistor did not get hot but I saw arcing around pins 8 and 9 of the left tube 6GU7 (R-y, B-y) socket. I figured the tube socket was bad and desoldered it out and then I saw the brown wire was burnt open in the middle and arcing at socket pin 8, the cathode connection. I thought this was a heater filament wire jumper, but how could 6.3 volts short to and fry a 270 ohm 3 watt resistor?

I better get good at this, there the customer's other combo's 16 waiting for repairs and my two CTC12's behind that.

As a matter of practice, the first thing I did was remove the crudded up wires on top of the board and relocate them on the bottom of the board. Then resolder the ground points, to avoid a call-back. That wire problem would also destroy the 620uh peaking coils in that area.
I did that, even if the set didn't have a fault in that area. Generally on the bench for a flyback job.

[DavGoodlin]

Originally Posted by dieseljeep

As a matter of practice, the first thing I did was remove the crudded up wires on top of the board and relocate them on the bottom of the board. Then resolder the ground points, to avoid a call-back. That wire problem would also destroy the 620uh peaking coils in that area.
I did that, even if the set didn't have a fault in that area. Generally on the bench for a flyback job.

Exactly what someone else did, added a 405v source jumper under the PWB. Trouble is, the wire on the top was not removed and then it arced to pin 8, then pin 8 tracked to 9, which is grounded. A nice carbon-lamp, the "limelight" if you will.

I removed the top wires, replaced both under the chassis, using red 18ga wire from a fluorescent ballast, TFFN insulation - 600 volts. The 620 uH peaking coil was broken apart from the heat also and I tried to resolder the one leg and the other broke off, so I robbed one from the other chassis I have to do. Wish I had a spare chassis now.

After replacing the tube socket, I plugged in the tubes and powered up. Got a raster right away and set up purity, have a good color pic now but focus coil is at one end. I checked the 66 meg resistor and it was 54 Meg, close enough. The 4.7 Meg is now suspect.

Ran out of time this morning, just like Xmas but its my birthday and this was a treat so far. I recall giving the same treatment, though it was already working, to a friend's CTC16 way back in the 80s and it was a great watcher after that!

[DavGoodlin]

Originally Posted by ChrisW6ATV

Someone may correct me, but I think I remember hearing of problems with multiple ground points on those late RCA roundie circuit boards. Maybe that is part of the problem(s).

The guy who Im doing this for did very little to it, thankfully. He did resolder all the PW griplets to the chassis horns. This can cause all kind of wacky mayhem and it seemed IF strips suffered the most.

[DavGoodlin]

The focus range has improved greatly after replacement of the 66 meg resistor with another that did not drift as low. 55 meg versus 50 made a difference.
I don't know the wattage on these rare birds, so I don't know if I can reliably add a 10 meg-1 watt resistor in series and optimize the range

The cathode current on the 6JE6 was dipped at 192 mA, likely due to the replacement fly.

Raster is great but picture only good so Ill post my other question separately since its no longer arcing