Sony TV9-90 Low Frequency Response problem

[JohnBHanson]

I have one of these and it has a problem that appears to be low frequency response. However the problem is there on both 405 and 625 line picture and also when a signal is input to the positive side of C504.

The response is that a white horizontal bar will revert to black after about 1/6th of the way across the screen. Looking at the circuit is the only significant caps that I can see in the video path are C504 and C505. These are 50uF and hence at quite a large value and hence low impedence at line frequency and hence I have initially discounted them.

Are there any other components that are likely to cause this problem?

[ukcol]

In the TV9-90 the video is AC coupled to the CRT cathode via a 0.2uF capacitor so there will be some drop off in low frequency response compared to designs that maintain the zero frequency component of the video signal.

I seem to remember noticing the poor L.F. on that model and its 405 only predecessor.

[JohnBHanson]

Is there a fix, or can the capacitor value just be increased. If so what voltage rating would be needed for a replacement.

I have heard that there was a Sony mod to improve the lf response. Anyone have any details?

Thanks

[ukcol]

My guess, and it is a guess, is that if the problem could been solved simply by increasing the value of the capacitor without causing any other problem Sony would have done it.

Sony did issue manual supplements from time to time, if there is a mod someone out there will know.

[ronbryan]

Have you checked the DC restoration diode D501?

Ron

[JohnBHanson]

Yes, checked that and all is fine, however there is a 10k resistor in parallel with it. Looks original though.

[ronbryan]

Is there any change if you lift off one end of the 10k resistor? I have the circuit for 9-90 serial numbers 80001 up and the shunt resistor is not shown on that issue even if it is a factory mod.

Ron