Re: 7854 Repair & Restoration


Brian Bloom
 

I finally found a diagram showing a lead welded to the top of the 12
pin TO-8 package used for U50. That lead is suppose to be welded so
maybe the weld came lose. I verified on my various 400 MHz 7000
mainframes that the case of U50 is suppose to be grounded but it is
not clear how ground is used by the hybrid and the schematics do not
show anything.

>Given this, I still think you have a bad solder joint or
intermittantly open resistor *near* U50.

Next chance I get I will visually inspect the solder joints, then carefully remove the vertical amp board and clean & re-seat the leads of U50 & U150. If any solder joints look suspect, I will reflow them.

As for the lead to the top of U50 - I'll confirm whether mine was/is welded to the can. It just looks like a brass ball point contact pressed against the case. The contact looks pressed into the spring loaded bracket that holds U50 down.

>>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.

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