The Panasonic CT-27G7DF I repaired 2 or so years ago I decided to keep in my shop.
It has performed flawlessly for 2 years but at the end of last week the vertical deflection went out.
I had replaced the vertical IC an LA7838 preemptively 2 years ago along with the caps in the vertical section when the MPU went south.
So with the waveforms listed in the service manual I dusted off my scope.
I checked the vertical input from the MPU it was fine, checked the vertical pump up the voltage was there but no waveform, checked the vertical drive pin for a sawtooth nothing, and of course there was no vertical output waveform on the vertical output pin.
Both supply voltages VCC 1 (Switchmode PSU dervied) and VCC 2 (Flyback Derived) were there.
Since the sawtooth is generated inside the IC and the supply voltages were there I had no doubt in my mind that the IC was toast.
So I replaced it and applied some Arctic Silver CPU grade thermal compound to the new IC before fastening it to the heatsink and the set is working again. The vertical drive, pumpup, and output waveforms were back (see attached pics if interested).
In image order all waveforms are at 5 ms timebase.
Now I'm left to wonder why this IC failed? I bought about 7 of them from MCM Electronics (a division of Newark) so I can be pretty sure they're not counterfeits.
Checking the supply voltages VCC1 measured 7.63V, VCC2 the pump up supply (VCC2 pin) measured 27.29.
According to the datasheet technically the operating range of the IC is between 8-14 volts 15 volts max on VCC1. While the VCC2 range is 10-27 volts 30 volts max.
So VCC1 is a bit low but not low enough to be a concern IMO.
So I broke out the thermocouple on my Fluke and decided to take the temperature of the vertical ICs heatsink.
I turned the TV on and waited for the temperature to stabilize. Once it did the heatsink reached a scorching 155 degrees F! It wandered between 155.2 and 155.7 to be specific.
Now this isn't really surprising since this little pipsqueak SIP package can dissipate up to 8 watts!
The ICs operating temperature is between -40 - 185 degrees F and it has thermal protection if things get too toasty which I think would cut off the internal vertical amplifier stage.
I've never measured the temperature of a vertical IC before but since it's within operating parameters it doesn't raise any red flags for me. If this is a normal temperature than I understand why these ICs are common failure points.
The old vertical IC had no bulges, bubbles, or any type of heat discoloration. All of the magic smoke was intact
.
So to make a long story short not finding anything out of the ordinary I closed the set back up and I'm chalking this one up to an IC failure with no external cause.
If you guys have any opinions I'd like to hear them.
It has performed flawlessly for 2 years but at the end of last week the vertical deflection went out.
I had replaced the vertical IC an LA7838 preemptively 2 years ago along with the caps in the vertical section when the MPU went south.
So with the waveforms listed in the service manual I dusted off my scope.
I checked the vertical input from the MPU it was fine, checked the vertical pump up the voltage was there but no waveform, checked the vertical drive pin for a sawtooth nothing, and of course there was no vertical output waveform on the vertical output pin.
Both supply voltages VCC 1 (Switchmode PSU dervied) and VCC 2 (Flyback Derived) were there.
Since the sawtooth is generated inside the IC and the supply voltages were there I had no doubt in my mind that the IC was toast.
So I replaced it and applied some Arctic Silver CPU grade thermal compound to the new IC before fastening it to the heatsink and the set is working again. The vertical drive, pumpup, and output waveforms were back (see attached pics if interested).
In image order all waveforms are at 5 ms timebase.
- Vertical drive (Pin 7) Sawtooth waveform 1 volt per division.
- Vertical pumpup (Pin 9) 20 volts per division.
- Vertical output (Pin 12) 50 volts per division.
Now I'm left to wonder why this IC failed? I bought about 7 of them from MCM Electronics (a division of Newark) so I can be pretty sure they're not counterfeits.
Checking the supply voltages VCC1 measured 7.63V, VCC2 the pump up supply (VCC2 pin) measured 27.29.
According to the datasheet technically the operating range of the IC is between 8-14 volts 15 volts max on VCC1. While the VCC2 range is 10-27 volts 30 volts max.
So VCC1 is a bit low but not low enough to be a concern IMO.
So I broke out the thermocouple on my Fluke and decided to take the temperature of the vertical ICs heatsink.
I turned the TV on and waited for the temperature to stabilize. Once it did the heatsink reached a scorching 155 degrees F! It wandered between 155.2 and 155.7 to be specific.
Now this isn't really surprising since this little pipsqueak SIP package can dissipate up to 8 watts!
The ICs operating temperature is between -40 - 185 degrees F and it has thermal protection if things get too toasty which I think would cut off the internal vertical amplifier stage.
I've never measured the temperature of a vertical IC before but since it's within operating parameters it doesn't raise any red flags for me. If this is a normal temperature than I understand why these ICs are common failure points.
The old vertical IC had no bulges, bubbles, or any type of heat discoloration. All of the magic smoke was intact
.So to make a long story short not finding anything out of the ordinary I closed the set back up and I'm chalking this one up to an IC failure with no external cause.
If you guys have any opinions I'd like to hear them.
Attached Files
