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.

[vol.2]

Originally Posted by Alex KL-1

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.

Thanks. I went ahead and probed Pins 1 and 2 of the Pin Amp IC, and it seems like the AC component that's supposed to vary the parabola on the IC's output is not properly varying when I adjust the Pin Amp potentiometers.

I am now thinking it's most likely the fault of the IC, but also maybe it's some other component in the circuit holding the IC down at the input or output.

Does anything jump out to check?

[zeno]

Be sure the V out is getting + 24 VDC. I hate shot gunning BUT if
it has @24 V change the IC & all related 'lytic caps, ALL at once.
While things are removed do resistant tests. That will fix 95% of them.

Zeno
LFOD !

[vol.2]

Originally Posted by zeno

Be sure the V out is getting + 24 VDC. I hate shot gunning BUT if
it has @24 V change the IC & all related 'lytic caps, ALL at once.
While things are removed do resistant tests. That will fix 95% of them.

Zeno
LFOD !

Yes, it's getting 24V out of the transformer now. I also scoped Pin 6 on IC303 (East West IC) and it has the indicated 20.5V supply. In fact, all of the Pins around IC303 look hunky dory except Pin 7, which is suppose to be the parabola.

I changed all the caps on the DA board already because they were the early surface mount kind and were all dead. So unfortunately I can't do resistance tests with them out of circuit.

I've got a new East West IC coming on monday, so I'll try my luck with that and cross my fingers. Otherwise, I might have to remove the caps and start looking for an open resistor or something.