CPD1604S Vertical Collapse

[Alex KL-1]

Originally Posted by vol.2

Yes, I had to change all the low value eletros. All the surface mounts spilled their guts, and the few through hole caps were quite bad. I checked the orientation of the caps many times, I didn't make any mistakes.


I tested the Pin Amp IC:

Pin 5) correct. 20 Vpkpk square wave @ 31.15kHz

Pin 6) correct 20VDC on it

Pin 7)14VDC offset from schematic is present, but the 0.5Vp-p waveform is screwy and wrong. It looks more like a sawtooth than what the schematic shows, and it's only about 250mV, not 500

Pin 8) 3.4 V pkpk sawtooth. Very close to the 3.6V pkpk shown in the service manual.

So from this I can conclude that something is fishy around the chip or inside the chip. Pin 7 is connected to ground through a 150k resistor, but in this set, it's also connected to the rest of the circuit through a 470k resistor with a cap to ground between them. I have no idea why it's done, but I can guess that maybe it's some kind of DC restoration to bring the level of the chip up to the rest of the circuit on pin 7 or possibly to enable some of the other geometry controls in some way.

Pin 7 is connected to Pin 5 through the one resistor, and the waveform on Pin 5 looks correct, so therefore, I'd basically assume that there's either something wrong with the chip, or with one of the components going to ground between the Pins.

R376- 150k - to GND from Pin 7

C340- 0.0015 - to GND from Pin 7

R375- 470k - between Pin 7 and Pin 5

Does this result say anything to you about the circuit or the failure I'm seeing; it's like something in the circuit is bring down the 500mVpkpk parabola I'm supposed to see to ~280mV and squishing the top down into noise

I attached the expected waveforms from Pins 5,7 and 8 of the Pin Amp IC, Pin 6 is the supply is supposed to be 20 V and it is.

Makes all sense. We can imagine the waveforms differing a little according with CRT variances, but certainly not a radical difference. At least with my experiences with modern TV when I repairing it (and monitors), is that the waveforms always resemble the originals when it are working well. Besides the suspicious DC voltages.