Re: 7854 Repair & Restoration


On 10 Mar 2016 17:15:23 -0800, you wrote:


So then... about the CRT circuit... DS47 is supposed to have a slight glow? Isn't that chain of neons supposed to be circuit protection?

I wonder if other scopes of the 78**/79** series have the same thing going on..
I have never noticed such a glow but I have not specifically looked
for it either. As far as I know, those neon bulbs should not be
glowing at all except during a fault.
I'm really curious why Francis had circled the diodes in the focus-grid dc restorer. I guess I should just ask him.
I asked Francis about this and the glowing neon - he said he vaguely remembers that he noticed it as well, so he checked all of the voltages around the Focus-Grid DC Restorer circuit and everything had seemed fine. He didn't look any further into it.

I've heard of these old neons going ' soft ' and the voltage at which they conduct lowering too much with age. Have you? After dealing with the vertical board and verification that problem is fixed, I'll check the voltages and waveforms of the related circuits, and if all checks out maybe I should just replace the neon and see what happens... I think I do have some of those type somewhere.
Unlike the bulbs in the z-axis DC restorer, the bulbs in the focus DC
restorer do not hold the focus lead close to the cathode voltage so I
am not sure what purpose they serve.

I pulled the high voltage shield off of my 7854 and in a dark room,
none of the bulbs I could observe showed any activity. They were all
completely dark. When I first powered the oscilloscope up, I could
hear the crackling from the high voltage.

If there is a problem, hopefully work on the vertical amplifier around
U50 will solve it. If not, then I think it will be easier to just
shotgun rebuild the DC restorers instead of trying to diagnose them.

On 10 Mar 2016 17:27:36 -0800, you wrote:

If you guys are referring to the TO-8 and bigger canned hybrids in the vertical amplifiers, as I recall, on some models they had a plastic clamp type thing with a contact and wire lead that would both press the can leads into the socket pins, and make electrical connection to the can top, but only by contact force from compression, not a weld.

Maybe there are two different versions of the package. All of the 400
MHz 7000 mainframes mostly use the same ICs and U50 is identical in
all of them but I have never had one of the vertical amplifiers apart
far enough to see if that lead is welded or attached with compression.

I do not think it matters though. There is no ground connection to
U50 that I have been able to find which would cause the observed
vertical deflection behavior. I think the lead is for shielding

Knowing exactly where in the circuit that wire connects would probably
reveal what it is for.

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