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?