Major triumph today. Been battling a 'ticking' K7400 on and off for a long time. I went through EVERYTHING that is normally the culprit....
-) New flyback and full cap kit
-) HOT does not read shorted in or out of circuit (replaced it anyway)
-) Re-flowed every pad on the entire chassis
-) Checked D106 A, B, and C - Good
-) D708/D709 - Good
-) C718/C723/C724 - Good
-) Q101 - Good
I researched this symptom and after going through all the above items, it still was stuck in over-protection/ticking mode. However, what led me to the fix was the HOT read good out of circuit (.545), but in circuit, it read .181. Normally, it will read somewhere around +/-.500 from the collector to ground. This was reading .181 to ground. What was even more strange is with the HOT removed, the collector's solder pad should have been open to ground, but it was not. It still read .181 to ground. I eventually had to break it down component by component, which took a LONG time. I removed one component at a time along the HOT collector circuit until I found what was causing it to read so low/strangely. C121 was bad. It wasn't shorted, and it wasn't open. When I removed it from the circuit, my HOT collector began to read .545 to ground. With C121 installed, the HOT collector read .181 to ground and caused the chassis to go into protection mode (ticking).
So I installed a new C121 from a donor chassis, went over everything one more time, reinstalled the chassis on the tube and powered it up....SUCCESS! Got everything dialed in and looking great. Man, this was a pain. Feels good to get it going. I've had a lot of K7400s come across my bench that I just could not figure out. This one took all my patience and experience to crack. Anyway, I just wanted to share in case anyone else has a 'ticking' K7400 that they can't get figured out. Replace C121!!



-) New flyback and full cap kit
-) HOT does not read shorted in or out of circuit (replaced it anyway)
-) Re-flowed every pad on the entire chassis
-) Checked D106 A, B, and C - Good
-) D708/D709 - Good
-) C718/C723/C724 - Good
-) Q101 - Good
I researched this symptom and after going through all the above items, it still was stuck in over-protection/ticking mode. However, what led me to the fix was the HOT read good out of circuit (.545), but in circuit, it read .181. Normally, it will read somewhere around +/-.500 from the collector to ground. This was reading .181 to ground. What was even more strange is with the HOT removed, the collector's solder pad should have been open to ground, but it was not. It still read .181 to ground. I eventually had to break it down component by component, which took a LONG time. I removed one component at a time along the HOT collector circuit until I found what was causing it to read so low/strangely. C121 was bad. It wasn't shorted, and it wasn't open. When I removed it from the circuit, my HOT collector began to read .545 to ground. With C121 installed, the HOT collector read .181 to ground and caused the chassis to go into protection mode (ticking).
So I installed a new C121 from a donor chassis, went over everything one more time, reinstalled the chassis on the tube and powered it up....SUCCESS! Got everything dialed in and looking great. Man, this was a pain. Feels good to get it going. I've had a lot of K7400s come across my bench that I just could not figure out. This one took all my patience and experience to crack. Anyway, I just wanted to share in case anyone else has a 'ticking' K7400 that they can't get figured out. Replace C121!!





