I received some replacement 7490s and now this SG503 is fixed! I had to really sit down and teach myself how the 7490 works. I know what counters do but never really learned how they work. I'm still learning electronics and teach myself through repair mostly. I love a repair like this one that teaches me a new concept. A bit different than your typical power supply issue - though I did spend a bit too much time on that before moving on to the actual problem.
I'd still be curious to know what voltages are typical at the -22V and 5.2V rails for this SG503 powered via a TM5000 (not TM500). Because I am still measuring ~4.7V at the 5.2V rail even with it working. If anyone would be willing to measure that and share with the group, I would appreciate it! Could save someone else from going down the same rabbit hole that I did (if in fact this is normal).
The 10Mhz setting is still a bit scratchy - it gets unleveled at this setting (likely due to bad contact). And there are a couple cosmetic things I need to finish. As well as calibration. But that is it for this guy!
Here's the vblog of the repair on my YT channel for anyone interested.
Now all I need is to fix my CG5011 and I will be able to calibrate scopes!
-Frank
Frank Mashockie