CPD1604S Vertical Collapse

[vol.2]

I replaced the IC303 with another part, and the other transistors in the 300 range, which are largely related to the Pincushion circuit. After getting it back together, I checked voltages and found that some of them are correct now that were slightly off before, but I have one new issue that I didn't have before.

I found that I have lost width control, and there is some kind of short or something going on that I have not been able to track down the cause of at all.

The Pincushion IC is connected to the Vert position circuit through Q255, the collector of which is connected to Pin 2 on IC303. It is supposed to be 2.2V at the Collector of Q255, and then end up at 6.5V at Pin 2 of IC303.

Basically what's happening now is that the voltage at Pin 2 of IC303 changes with the Vertical Position pot, which is obviously wrong. I've checked all the transistors and diodes in the area and can't find an issue. I also checked IC205 for shorts between pins and found nothing.

There is 22V at the Emitter of Q254, which will change the voltage present at the bottom of D257, and then raise the voltage at the base of Q255, which finally raises the voltage at the collector of Q255 and that voltage shows up on Pin 2 of IC303.

It's also possible that something in the switching section of the circuit is functioning incorrectly as perhaps if the various diode arrays were getting the wrong input somewhere, it might lead to a weird mixed outcome for the cumulative output. The connection between the Vertical position circuit also is impacted by the switch which goes between the presets and the external V/H size and position controls. The switch is not a simple on/off, but a ground/not ground signal sent to the anode of D609, which turns Q615 on or off.

FYI, the service manual wants me to take all voltages and waveforms with a 640x400 70Hz color bars pattern, which is what I have been doing.


I replaced Q254 and Q255 with brand new exact replacement transistors (all of the SMD transistors and diodes are still available cheap for this PCB thankfully), and I tested D257 and it looks like a working diode. It's a Diode package that has one anode and two cathodes.

The junction betwen D257 and Q254 is 22V when it should be 7.7V and I really can't figure out what's causing it at all.

Any ideas?

[Alex KL-1]

Originally Posted by vol.2

I replaced the IC303 with another part, and the other transistors in the 300 range, which are largely related to the Pincushion circuit. After getting it back together, I checked voltages and found that some of them are correct now that were slightly off before, but I have one new issue that I didn't have before.

I found that I have lost width control, and there is some kind of short or something going on that I have not been able to track down the cause of at all.

The Pincushion IC is connected to the Vert position circuit through Q255, the collector of which is connected to Pin 2 on IC303. It is supposed to be 2.2V at the Collector of Q255, and then end up at 6.5V at Pin 2 of IC303.

Basically what's happening now is that the voltage at Pin 2 of IC303 changes with the Vertical Position pot, which is obviously wrong. I've checked all the transistors and diodes in the area and can't find an issue. I also checked IC205 for shorts between pins and found nothing.

There is 22V at the Emitter of Q254, which will change the voltage present at the bottom of D257, and then raise the voltage at the base of Q255, which finally raises the voltage at the collector of Q255 and that voltage shows up on Pin 2 of IC303.

It's also possible that something in the switching section of the circuit is functioning incorrectly as perhaps if the various diode arrays were getting the wrong input somewhere, it might lead to a weird mixed outcome for the cumulative output. The connection between the Vertical position circuit also is impacted by the switch which goes between the presets and the external V/H size and position controls. The switch is not a simple on/off, but a ground/not ground signal sent to the anode of D609, which turns Q615 on or off.

FYI, the service manual wants me to take all voltages and waveforms with a 640x400 70Hz color bars pattern, which is what I have been doing.


I replaced Q254 and Q255 with brand new exact replacement transistors (all of the SMD transistors and diodes are still available cheap for this PCB thankfully), and I tested D257 and it looks like a working diode. It's a Diode package that has one anode and two cathodes.

The junction betwen D257 and Q254 is 22V when it should be 7.7V and I really can't figure out what's causing it at all.

Any ideas?

Hmmm... multiscan monitor... a lot of things to see...

22V? The IC are responding ok to the logic on it's inputs (the IC250)? Or, if something ahead the P302 are throwing this voltage (eg. with RV257 closed)?

Likely, some cascade of events can occur.

[vol.2]

Originally Posted by Alex KL-1

Hmmm... multiscan monitor... a lot of things to see...

22V? The IC are responding ok to the logic on it's inputs (the IC250)? Or, if something ahead the P302 are throwing this voltage (eg. with RV257 closed)?

Likely, some cascade of events can occur.

I'm not sure how to check if there's an issue with IC250. I checked for shorts, and there's no shorts between any of the pins; all pins are no connection to each other.

From the datasheet, there doesn't seem to be a way that 20+ volts is produced by Pin 10 unless there is some internal short?

This is why I though the 20+ volts is coming from somewhere else further down.

Transistor arrays are new to me. This one looks this way on the inside:

[Alex KL-1]

An array is a simple form of IC not having power supply sometimes, so rarely have any processing. Basically the cause can be external to it (sometime trowing 20V there).
Anyway, the pin 9 must have higher voltage than the others, or the internal diode conducts.
The problem must be with any of the outputs if it don't reacts with the input.

[vol.2]

Originally Posted by Alex KL-1

An array is a simple form of IC not having power supply sometimes, so rarely have any processing. Basically the cause can be external to it (sometime trowing 20V there).
Anyway, the pin 9 must have higher voltage than the others, or the internal diode conducts.
The problem must be with any of the outputs if it don't reacts with the input.

And so I measured the logic circuit approaching IC250 and I found a problem with inputs on IC604, which directly feeds the network of transistors leading to the inputs of IC250.

Pin 1 of IC604 is LOW when it should be HIGH. This is causing Pin 5 to be LOW when it should be HIGH.

I traced the input for Pin 1 IC604 back, and it's connected to the collector of Q613, and also to the 12V line through some resistors.

The Base of Q613 is connected to the output of D609.

D609 is switched on and off by the Auto/Manual switch for the horizontal and vertical size and position. The switch goes between GND and Open Circuit.

D609 measures correct on it's other side (toward Q615), but it reads 2.7V on the side towards Q613 when it should read 0V.

I can see that the input of Pin 1 on IC604 being wrong will cause certain problems, but I can't tell if this problem is because D609 give incorrect voltage, or because the 6.5V which should be present on the other side of Q613 is not there for some other reason.

I also cannot see why this possible "cascade" problem would create 24V on Pin 10 of IC250. If I had to make a guess, I would tend to believe there is an issue creating the 24V on Pin 10 of IC250 and that is leading to, or also causing the loss of 6.5V on other side of IC250.

I measured all other logic ICs in this circuit (IC601, 602, 603, 605, 606) and I found that all voltages were correct on all pins of all ICs.

I replaced Q613 with a new part of the exact same transistor (they are still available) and that didn't fix it. The old transistor measured to be a bit weak and measured different from new part, so I thought it was worth a try.

I have replacement ICs for logic coming in the mail, luckily they are all also available and very cheap.

This is what I am seeing, with incorrect voltages in red:

[Alex KL-1]

So, the AUTO switch, connected to GND, can extinguish Q615 and Q613, unless have some fail in this line.
The IC 604, it follow the truth table in the datasheet? Too busy at moment to analyze it but, a question: perhaps 0V in the pin 1 causes 0V in the outputs?

[vol.2]

Originally Posted by Alex KL-1

The IC 604, it follow the truth table in the datasheet? Too busy at moment to analyze it but, a question: perhaps 0V in the pin 1 causes 0V in the outputs?

Yes, IC304 definitely follows the truth table of its datasheet. The incorrect voltage on Pin 1 causes 0V on Pin 5 output.



Pin 1 should be HIGH, and the 4 outputs it controls should all be HIGH, instead it becomes HLHH

So the 2.7V on base of Q613 maybe is the problem. Or possibly the lack of 6.5V on the collector of Q613 causes the 2.7V to appear on the base.

I don't know the chicken or the egg unfortunately


Edit: I'd like to add that I isolated IC250, Pin 10 to try and see where the 23V is coming from that shouldn't be there, and without Pin 10 connected to the rest of the circuit, it no longer has the 23V on it, but I still see 23V all over Q254 and the Vertical Position potentiometer. Not sure why it's wrong, but I also can't see how it's connected to the issue with IC604.