Meck XB-702 restoration

[jr_tech]

Originally Posted by bandersen

Yeah, 60Hz all over. It's crazy to see the test pattern snap into perfection for a brief moment when power is turned off.

Perhaps you could turn off power to the DC supplies while leaving the heaters on
and vice versa, to narrow down the source of the 60 hz interference.

jr

[Kevin Kuehn]

Whenever I see 60hz all over a chassis I think ground loop, which can be as simple as how you're hooking the scope into the circuit. For instance not having a good ground very near the end of the probe in the near vicinity of where your taking the measurement. I'm just making a general comment for those following along that may not be aware of that situation. It's more important when the piece of equipment is floating on an isolation transformer.

[old_coot88]

Originally Posted by jr_tech

Perhaps you could turn off power to the DC supplies while leaving the heaters on
and vice versa, to narrow down the source of the 60 hz interference.

jr

Seems like the reverse would be preferable; kill the heaters, so there'd be several seconds of watchable display while they cooled

(EDIT.) OOPs. You aready said it. Dumb me.

[Kevin Kuehn]

Here's a set with a similar problem at about 1:00 in the first video.

https://www.youtube.com/watch?v=Ygpw87pqTz0

https://www.youtube.com/watch?v=aSEMVpJ1hzk

[Notimetolooz]

Another thing is that the problem seems to go away when you move the CRT away from the chassis.
I just had a thought.
When you look at some of the signals with the scope, like the sync, does the hum look stationary with respect to the sync pulses or does the position slowly move ?

[bandersen]

Originally Posted by Kevin Kuehn

Here's a set with a similar problem at about 1:00 in the first video.

https://www.youtube.com/watch?v=Ygpw87pqTz0

https://www.youtube.com/watch?v=aSEMVpJ1hzk

It sure does. That's a series string, floating chassis if I recall correctly.

Originally Posted by Notimetolooz

Another thing is that the problem seems to go away when you move the CRT away from the chassis.
I just had a thought.
When you look at some of the signals with the scope, like the sync, does the hum look stationary with respect to the sync pulses or does the position slowly move ?

The problem does not go away when I move the CRT away. At least not horizontally. Perhaps it's a little better vertically.

The hum moves slowly with respect to the sync. If I trigger my scope from the line, the hum locks in place.

[Notimetolooz]

Originally Posted by bandersen

The problem does not go away when I move the CRT away. At least not horizontally. Perhaps it's a little better vertically.

The hum moves slowly with respect to the sync. If I trigger my scope from the line, the hum locks in place.

Oh, I must be thinking of something else. I had a thought that if it did go away when you moved the CRT away that just maybe it was the sweep on the CRT plates that was appearing on the sync signal on the scope and the 60Hz sweep interference on the sync wasn't really the problem. However since you can lock on the line 60Hz the interference on the sync signal does come from that source.