I have had a very busy weekend so far. One exiting thing was I found a 1969 Packard Bell parts TV. This was a top of the line TV/Stereo console set, model CP 984. The TV is non-working, some examination shows a burnt wire in the yoke coil. It has a different chassis with a lower tube count (more solid state components) but does share some common parts to include the fly back transformer which had been replaced at some point.
I also had found on eBay NOS high voltage sockets for the high voltage rectifier & focus rectifier tubes and plate caps with 30K wire for the horizontal output and shunt regulator tubes. I thought lets eliminate all sources of possible HV leaking. Finally I picked up some additional meters for testing. While the chassis was on the bench I did jumper the circuit break to test. I used a variac hooked up to a hair dryer and found that slowly turning up the voltage the breaker trips at about 3.5 amps.
So after installing a different flyback transformer, sockets, and plate caps with associated HV wire I got the chassis back inside the TV along with lots of meters to monitor things, here are my testing notes:
- TV draws 2.8 amps from the AC line
- Horizontal output current dialed in at 220ma
- HV adjusted to 23K volts
- Reading -51 volts on the horizontal output tube grid vs -46 per the schematic
- 0.850ma shunt regulator tube cathode current, spec states should be at or above 0.850ma
- After about 5 min the shunt regulator tube cathode current started climbing up to at least 1.2ma, shortly after this I got visible/audible sparks from the shunt regulator tube plate cap, I then cut the power
- I repeated this for a second time, all measured good and again the shunt regulator tube cathode current started to climb, I believe I could start to hear some hiss so I turned off the TV
- I feel like I either fixed one problem and now am on the next, or I increased the difficulty for HV to leak and now fails more visibility
- At no point did the breaker trip, but I was watching and cut the power likely ahead of the breaker tripping
As I look at the shunt regulator circuit there are not too many parts. I have already replaced C114, but I just now measured R215 which calls for a 1000 ohms, 5% resistor. It measures high at 1,164 ohms (16% off), could this be the issue, or could it be further changing value as it heats up?
The other concerns it around the cap I used to replace C114, I was able to match the temp code from one of the replacement parts noted from the SAMS document so I hope I am ok here.