Vsync issue on PAL Atari 2600 woody

[Paul Green]

Hi,

 

I have a little experience in repairing 2600s but this one is proving beyond me

 

The problem is on the main PCB, the switch board is fine (I have tried with a spare).  All 3 main chips (Tia, riot, and the CPU) are good as I have tried on a spare 2600

 

I noticed that I got an exceptionally faint picture from the RF modulator.  So after one circuit tracing I thought the 4050 could be the issue.

 

I replaced the 4050 with a socket an a cheap copy from China and I get a picture in pal but I'll has vsync problems from mild (1 scroll a second) to bad (about 10 scrolls a second) depending on the game.  I am using video output via a standard video amp board which is again proven good. All the colours and brightness are fine.

 

I thought this might be the Chinese 4050, but when I replaced with a TI chip which again works (tried on a spare) I get no picture at all.

 

I have traced all the Tia video output, and every pin is connected and I can trace the vsync signals from Tia to 4050 thorough the resistor network and out.

 

I believe this console was either over or reverse volted at some point In it's life.

 

The only thing I can think of is that the oscillator circuit has drifted out of specifications which is making the vsync off.

 

Can anyone make any suggestions or fixed similar?

[emmanuelf]

At this point in the debug, you need a scope, preferably one with video trigger.

[Paul Green]

I was thinking the same.  I don't currently have access to a scope however but the thinking cap is on!

 

Thanks for the reply

[ChildOfCv]

On 12/9/2019 at 10:53 AM, Paul Green said:

Can anyone make any suggestions or fixed similar?

Check that there is a signal on the sync input to the 4050.