Re: 7854 Repair & Restoration


On 10 Mar 2016 17:48:33 -0800, you wrote:

I just found vertical amp board that was from a 7844 or 7904 - marked 670-2768-02. The driver and output cans are both held by the same type of plastic clamp. The clamps each have two special screws holding them to the board, with a spring at one end for compression. There is a contact in the center of each one, with a lead going to the board (ground?). There is also a sheet metal layer about 0.032" thick added to the component side of the board in this area, apparently to serve as a ground plane and for cooling. So, the compression force is probably essential also for good thermal contact of the cans to the plane. I can't tell if there's any thermal grease underneath. The contacts are definitely not welded to the cans - in fact, the clamps have notches on one end to allow them to swing away for access to the cans without removing the screws.

I have always been reluctant to mess with the various hybrids for fear
of damaging them but I took a closer look at U50 in my 7854 and
confirmed your description of the construction and analogaddict013's
observations about the lead attachment. I have not messed with one of
these since my 7834 and there I only removed the vertical CRT
amplifier and did not mess with U50. The notched end of the plastic
clamp can easily be pushed down relieving it of the screw head so that
the clamp can be slid to the side. Doing this reveals that the top
lead attachment is *not* welded to the TO-8 can and just makes contact
under light pressure. Earlier I found a figure showing the lead
welded to the top of the TO-8 package but that must not have been
literally accurate.

This provided an opportunity to run a test however. I slid the clamp
to the side disconnecting the top lead and powered up my 7854. The
display on the CRT was completely normal. I think this confirms that
the problem with analogaddict013's 7854 where the display shifts to
the top of the CRT when U50 is tapped is not caused by the top lead
connection to U50.

I did not try to remove U50. Is it soldered into place or does it use
collet socket pins embedded into the printed circuit board?

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