Model 4P Non Gate Array - PAL or Crystal temperature issues

[Tibs]

I've got a temperature related issue that screws up the timing on everything in the system. It starts with video slowly getting out of whack, then the whole thing dies. Either cold spray or power off for a few fixes it. Actually the FDC starts to get wonky first and I haven't looked to see where it gets its signal from since its not managed by U126, just CPU/Video/RS232. There is a capacitor and a resistor in this circuit that I'll check as well.

I have narrowed it down to either the 20.2752 MHZ oscilator or U126 (PAL). It's really hard to ensure I don't get cold spray (I use compressed air) on both so I'm not 100% sure its the crystal or the PAL for that reason. It is 100% a temperature issue I know that because the system works great for about 5 minutes then things start failing eventually video smears and system lock up occur at the same time.

If this is the PAL the system is toast unless someone has cloned it. Can oscillators be temperature sensitive? I guess it could be a broken seal on Y1.

I'll eventually get the scope out and report if the 20mhz signal is drifting as it warms up, since more than just video goes bad I'm really suspecting its U126, a 16R6A PAL or the source to it. There are a few support items that could be failing too, will report out what I find.

 

[TRS-Ian]

I have spares of the PAL chips on failed motherboards here if you can narrow it down to which is causing your problems.

 

[Tibs]

Thanks for the offer Ian.

Turns out it is the oscillator that is suspect to be bad and I'll see about finding the oscillator out there on the internet. I only have a 12.xxx one. After it heats up it starts to mess up and pressing on it some makes the frequency change so I think the case may have lost its hermetical (is that even the right way to use this word?) seal.

Here are a few pictures:

Good signal (Pin 1 of U126):


Bad signal:


Screen:


I froze a q-tip and when it cooled Y1, it started to work, then after a bit went out again. I have no clue what that tape looking stuff is.

 

[ldkraemer]

It could also be a bad solder connection. It might be worth a try to re-flow those connections. It will just take a bit of time.

You can use a wooden lead Pencil to put inside the 180 degree curve of the flat cables that plug into the Mother board to
easily remove and install them. That's the easy part.

Larry

 

[Tibs]

I took your advice, and it looks like the part corroded and all four contacts broke free from the box. I'll have to replace it.

I've had a heck of a time finding specifically 20.2752, I have thought about trying a 20. I've got a feeling the RS232 and video might not be happy.