Thanks for your kind response and suggestions.
I took some pictures for you, there is one X7R decoupling capacitor which has some black grime around it but I do not suspect this to be the problem. I tried to clean this off with Isopropyl and a Q-Tip. What ever it is, its not coming off easy. I will try with a tooth brush again.
Also on resistor R2014 there was a slight green discoloration on one solder joint, not sue what that could be. I tried to clean this off as well. I did notice the solder joints in this DAC area look not really appealing to me, they look dull as if they were cold joints. I am not sure if back than Tektronix used lead-free solder. The joints look actually really crappy to me.
I tried to re-solder the worst joints using tin-lead solder and rosin flux but for some reason the solder joints still look not great. I am not sure if there is some contamination preventing a decent solder flow.
Anyway, it seems I am onto something. When I poke around this DAC circuit and aggravating using cold spray, I can sort of provoke the problem to come and go intermittently. At this point I can not put my finger on it what exactly is causing this as this is getting a little weird.
I checked the DAC signals B1 to B12, they look good to me altho I can not verify the streaming logic values. But for the moment I think they are just fine. I looked the the DAC output Pin 18 and Pin 19 (D, E), I see various analog levels as I would expect, however the overall amplitude seems unstable. I am not sure what the peak to peak amplitude should be, maybe you guys know. Sometimes I see about 1.4Vpp sometimes a lot less about 0.6Vpp or even less than that. I can observe the trace brightness, readout brightness and trace position so as delta time cursors going crazy on the CRT. This is consistent with my understanding how the 1.36V and -1.25V rail is used in this scope. If this is unstable so is everything else.
If I manage to get it stable for the moment, I get the scope to pass all self tests. Good news.
Poking around scoping points A, B and C, I can see bad flicker noise several milli volts.
So I
Re-soldered R2016, did not impact the problem
Replaced C2010 with an electrolytic cap, did not impact the problem
Replaced R2016, did not impact the problem
Re-soldered R2012, did not impact the problem
Double checked the 10V Reference voltage at R2016, looks good.
DAC Pin 20, +5V looks good,
DAC Pin 17, -15V looks good,
DAC Pin 13, 0V looks good.
So at this point I am convinced the DAC circuit is the offender. I am leaning to replace all the passive components around the DAC chip including the pot R2010, but then I will have to recal this DAC circuit which I would like to avoid having to do.
What are the odds that the Pot went bad ?
What are the odds the DAC chip is flaky ?
As I noticed that the DAC signal amplitude (D, E) seems to increase if I put my finger on the DAC chip for a while.