Sony XBR Questions

[Electronic M]

Originally Posted by JohnCT

CRTS on Sonys can get cranky. If the grey scale shifts, the AKB will mute the screen or causing blinking, but if the cathode goes too low, it will draw too much current and shut the chassis down.

Try gently flexing the neck board of the tube and gently tapping the neck of the tube itself. Sometimes, there could be a piece of wayward cathode material that gets caught between the other tube elements. If flexing the board seems to change things, take a close look at the IC mounted to the heatsink (some have one IC for each gun, others have one larger drive IC to cover all three). These ICs run hot and could develop cold joints.

If the tube was the problem, I used to use my modified B&K 467 to try to clean the out by pushing the shorts clear button repeatedly while tapping on the neck. If that didn't help, I'd put the B&K on clean setting and repeat while tapping the neck.

John

Mine is the 1 IC per gun version.

If flexing the IC doesn't change behavior (IIRC it did and the heat sink ground joints were the worst last time) I'll try my B&K 466 on it as a last ditch (ISTR hearing most Sony CRTs die if you try rejuvenation).

[JohnCT]

Originally Posted by Electronic M

Mine is the 1 IC per gun version.

If flexing the IC doesn't change behavior (IIRC it did and the heat sink ground joints were the worst last time) I'll try my B&K 466 on it as a last ditch (ISTR hearing most Sony CRTs die if you try rejuvenation).

Going bright red could also indicate a heater to red cathode leakage/short. These shorts can't be blasted because the heater would be damaged, but the heater can be "floated" as a workaround. The heater voltage is provided from a winding off the flyback that provides 6.3 volts but it's anchored to ground.

So, if the heater and any cathode make contact, the cathode will be pulled low towards the heater's ground reference. The lower the cathode voltage, the higher the current drawn and the brighter the screen goes.

What I used to do is to wrap three turns of ordinary insulated wire around the core of the flyback and feed it to the CRT socket at the filament pins. Often, a trace would have to be cut in order to get the CRT filament away from ground.

If done properly, you'll still get 6.3V**, but it will now be a floating ground supply. If the heater and cathode short again, the floating heater supply will get pulled up to the cathode potential of 200 plus volts above ground, but it will still maintain the 6.3 between the pins. Because the filament is no longer anchored to ground, a heater cathode short can't pull the cathode low.

Heater to cathode shorts usually will respond to gentle tapping on the neck of the tube, but not the circuit board flexing.

As far as Sony's blowing up on rejuv, I've done a bunch without issues, but any rejuv on any tube is a last resort option. If you need to, there's nothing to lose.

Sometimes, the clean setting will heat the cathode enough to either burn off any crud that's causing the runaway red condition or even melt the cathode and stabilize it.

If the tube is cranky and it's not a filament issue, then try a cleaning.


** With regards to wiring a floating ground supply: you must be careful not to exceed 6.3VAC filament voltage as even a couple of tenths will shorten the balance of the tube's life considerably, and this goes for all CRTs, not just Sony. Also, because the filament supply is sourced from the flyback (15Khz, not 60hz), cheap meters will give false readings of the voltage. A True RMS meter will read the high frequency AC voltage accurately. This is important.

Once you wrap the wire around the flyback core, zip tie it into position as moving it up and down will "tune" it and affect the voltage. Add an appropriate low value 5 watt resistor in series to get the voltage to 6.3 or just under.

John