Sony CRT TV Resurrection
I got a Sony KV-27V20 CRT off the marketplace for $50 CAD. Yes, I overpaid for this, but it was a beautiful CRT, so didn't want to pass on it. It was also a small price to pay for a CRT I wanted to keep. When I met the seller, I powered it on and everything was in working order. I didn't have a console to test the geometry etc. but the CRT was in great cosmetic condition. Therefore, I took the chance. Brought it home and powered it on and tried the composite input and all was well. There were few geometry issues, but nothing out of the norm.
I will be making a sperate post on the RGB mod performed on this CRT. I modded the CRT for RGB and powered it on. It powered on with usual blinking lights, the bong sound came up and poof. I heard the CRT power itself off with the typical static noise you hear. After that point onwards, the CRT wouldn't power on. There was no sign of life to it. No standby light, no sound, it was totally dead!
[picture of disconnected yoke wire]
So, went back and looked at all the wires, and I have forgotten one important wire to connect before powering the CRT on. This is the wire that goes to the deflection coil on the yoke. Someone on reddit enlightened me. If a CRT is turned on without this deflection wire plugged in, it will blow the HOT (Horizontal Output Transistor) - fun!
[picture of a HOT]
As forgetful as I am, this was definitely not the first time I powered on a set without attaching the deflection coil wires. However, other sets managed to survie from this disaster. But, this Sony set chose to die on me.
Checking for shorted HOT
This is a fairly common problem with CRTs. There are lots of information on the internet on this topic, but I still struggled with some of the most fundamental questions. I went through dozens of YouTube videos and forums, but I really didn't understand what was going on until I decided to experiment this myself. Here are some of the questions I had.
- What could've blown the HOT?
- How do you check if the HOT is shorted?
- What other components this could've damaged?
- How do you source replacement parts?
- How do you find the root cause and not blow the HOT or other components?
and many more... Let's answer these questions one at a time.
Before going further, it is important to understand some of the fundamental components of the CRT. I'm going to leave this image here with the particular CRT I was troubleshooting so that you can visualize the situation. This way it is clear what is a HOT, deflection coil, power transistors etc.
[picture with the anatomy of the CRT] https://repair.wiki/w/CRT_repair_and_troubleshooting
What caused the HOT to blow?
I was running the CRT without the proper load on the horizontal circuit. The load in this case was the deflection coil on the yoke. When the CRT was turned on, the current spike supplied without the load (in this case yoke deflection coil), killed the HOT.
Below are some reasons why a HOT can fail
- Due to age (replacing HOT should work here along with other failed components on the path)
- Dry joints in the horizontal circuit (if this is not fixed, then even the new HOT will fail)
- Shorted flyback or other flyback related failures (this will blow a bunch of things on the path)
- Not connecting the deflection coil wire (due to the surge, this will below a bunch of things on the path)
- High B+ voltage
- Horizontal driver circuit running at incorrect frequencies
- Bad capacitor in the supply line horizontal driver transformer
- Shorted horizontal yoke deflection coil
- Wrong HOT part
So, as you can see, there are many reasons why a HOT can fail. Mine was a very particular case, but nevertheless it was a good learning experience to share.
How do you check if the HOT is shorted?
This may seem like a simple question to answer for those who are familiar with CRT and HOT. However, let's go through this.
[Picture of a HOT]