I have two scopes that are giving me fits. A 2445 and a 2465 DMS. They both have the same failure mode - F1330 on the power supply which supplies
-15 UNREG to the A9 high voltage board keeps opening up. I have isolated the problem to the A9 board. I disconnected the -15 UNREG to the A9 board and supplied -15 volts to it with an external power supply current limited to 500mA. I Then checked the feedback circuit for proper operation which uses a voltage divider from -1900 volts to U1956A using another external power supply injected at R1945 and R1944 and verified that Q1981 and U1956 are functioning correctly. There are no shorted semiconductors on the A9 board. The only thing that I can see that is left is the operation of U1830 high voltage module or the CRT. If I disconnect the CRT the problem persists.
I guess one question is whether it is a valid test to disconnect the CRT. On a working scope would this cause the A9 board to misbehave and blow F1330?
Has anyone seen this failure mode and figured out what the fault was? Is this a common failure mode in the 24x5 scopes?
Many thanks for any advice.