On Thu, Jun 14, 2018 at 01:53 am, <edward@...> wrote:
Thanks to Fabio's detailed and clever reasoning (you're very gifted Fabio), I
have found the issue with the Alt mode. The problem turned out to be a
defective U4391 (74LS02) that can be found on diagram 8 (Horizontal Display
Logic A&B Sweep Generator). When measuring pin 3 on this IC there was no
change in state (stayed low) when operating the ALT button in the "Horiz
display" section on the front control panel (after checking the switch first).
Replacing this IC from a butchered 465B restored the Alt mode and the scope
now reliably displays two traces :)
Note that your problem might be caused by a different component failure or
break in the Vert Alt Sync path, but if you follow Fabio's excellent
diagnostics, you will find the problem. Good luck, and another big THANKS to
Fabio, I owe you a drink!
Off course you could be right that the IC really was bad. Did you put the original back to confirm ?
Another likely culprit would be bad IC sockets which are known to cause various intermittent problems.
465B from that time are known to have them and if not replaced more problems will probably occur in the future.
To identify the sockets check here: http://w140.com/tekwiki/wiki/Bad_TI_IC_sockets
Note that some of the text and the large picture is incorrect. The smaller pics are right.