The Trinitron IK circuit ("IK" meaning "Cathode Current") is a protection circuit appearing on all Sony Trinitron designs starting around the mid-1990s. It is their implementation of AKB (Automatic Kinescope Bias). Its purpose is to avoid overdriving the cathodes on the CRT, and when it goes into protective shutdown, it produces the classic "no picture with sound" issues.
When a Trinitron fails to come out of IK shutdown, it may be a sign that the CRT is wearing out or the chassis is in need of an expensive repair.
The circuit
Pictured is the C board (neck board) from the schematics for the Sony KV-9PT60:
The RGB drives from the jungle chip arrive onto the board and drive the three RGB output transistors by biasing the +180V supply towards ground. The IK circuitry detects current flow through three PNP transistors, which all feed into a fourth PNP buffer transistor. The output of the buffer transistor circuit is fed back to the jungle chip via a resistor (in this case, a 220 ohm resistor at R776), which reads it as an analog voltage level. If the jungle chip does not get the sense voltage level it expects, it will disable the RGB drives, and the screen will remain blank.
Bypassing
Remember: this is for diagnostics only, and is a total hack. Do not leave this in place when you're done working on the set, because if the vertical circuit fails, there is a chance it will send the electron beam into the shell of the CRT, heating it up and eventually destroying it.
Before you apply the IK bypass:
- Verify voltage levels on the neck board with a multimeter and oscilloscope.
- Check that the CRT heater is working (orange neck glow) and that the anode high voltage is being produced (it's often enough to check flyback pin outputs).
- Make sure there are no shorted components between the CRT and the jungle chip's RGB drive outputs, including the cathodes on the CRT itself.
- Input a signal and turn the SCREEN (G2) knob on the flyback to see if any picture is produced at all.
Bypassing:
Isolate the IK sense line from the jungle chip and jump the jungle chip's RGB drive outputs directly to the IK pin via 1N4148 diodes[1], with the cathodes of said diodes all pointing towards the IK pin.
When the SCREEN (G2) pot on the flyback is turned up, you should see a picture. If you can't get a picture even with the bypass, suspect that the jungle chip or a component around it has failed.
If you can see something, but the colors are off, it's likely that the CRT is on its way out. You can try recalibrating the set to work around this, but there's not much life left in the tube.
Causes of IK shutdown
- Worn-out or bad CRT[2]
- Bad jungle chip
- Vertical circuit or vertical circuit feedback to jungle chip bad
- Bad capacitor between the jungle chip and IK buffer output