Before Thanksgiving, I got stalled trying to align the sound and video IF traps on a 1949 RCA KCS 28B chassis in an RCA 9-TC-240 cabinet TV. After making some progress (With a tremendous amount of help from Penthode, Tom C and Bob A) I began to have issues with the HP 8600 Sweep Generator and the HP 8601 Digital Marker combination. Without describing the long ride to standstill, and after several "Ah Ha!" moments that I thought solved the issue, I all but gave up after the last failure. The furthest I got was narrowing the issue down to the 8600 marker. Sometimes the issue would disappear with banging or circuit board flexing only to show up an hour later or a couple of days later. I figured it was a ghost in the machine I'd never find.
Six weeks of actual troubleshooting using a multimeter and a schematic instead of a club led me to the 5V circuit on the regulator board. It was low at 4.5V even with the adjustment pot on the board set for maximum voltage. The voltage is regulated by a 10 gold pin HP can IC, part #1820-0196.
It's proprietary...nobody but HP made it. When I froze it, voltage went to +5V DC. The machine worked perfectly until the voltage returned to 4.5V and the problems reappeared. I then froze it to restore function and instead of waiting for it to warm up, I immediately adjusted the voltage down using the regulating pot and as soon as I got to 4.5V, the marker wouldn't function correctly. I repeated this several times and was able to reproduce the phenomena every time. The regulator is a 500 ohm pot and it measures perfect. Below is the offending board and the SOB of a "chip in a can" part.
After fruitless searching for weeks, I finally came across the chip NOS on eBay for $10. So it should be here in a few days , I'll instal and test the unit. If it's now reliable, it's back to alignment on the RCA.
While thankful for all of the contributors in the other thread (and there were mare than the 3 I mentioned), I want to give a huge thanks to Penthode who rode with me through this half year process giving advise and imparting his experience and wisdom while taking a heartfelt interest in seeing me succeed. Besides the thread we exchanged emails as well. I hope he'll be pleased I have not given up!
Additionally, I bought an HP 8643A Signal Generator. It's a beast. Other than a dead memory battery, it was tested and certified and appears to work well. I can sweep with it but if I'm going to look at a scope XY trace, I need to figure out where to tap into the generator to access the horizontal sweep output. The scope gets the Y input from the test point in the TV, the sweep signal goes into the input lug of the particular tube but the scope needs an X input. It's probably on the inaccessible back of this station wagon on my bench!
Thanks everyone!