7971 Failure and Teardown (pdf attached with proper formatting)
Jeff Walton
Recently, I got an older B7971 from an ebay auction that was sold as “tested” and made a
Note that all segments in the ebay listing were lit properly and appeared to be working as expected. The tube was date coded 6351 and was branded as Burroughs.
The tube was well packaged. On testing, I immediately noted that the tube had issues: The dreaded short that often occurs when the segments overlap in the corner. There have been numerous posts about possible ways to clear this problem by mechanical shock, which I’ve done a few times myself. The problem in this case was that the tube appeared to be shorted in four locations: Top right corner, bottom right corner, lower left corner and one in the middle.
Here is what I saw on testing the segments. Note that the left photo should be the top horizontal segment only and the bottom photo was powered to only the bottom horizontal segment. I attempted to dislodge the shorts but repeated shocks to the envelope only served to loosen the internal structure as one of the supporting attachments to the bottom part of the internal structure broke loose. Note, the seller refunded the tube and said to just keep it.
At this point, I was noticing that the top and bottom segments seemed to have a funny bend in them and decided that there was no way to ever make this tube work properly. I scored the envelope at the base and removed the inner structure to examine the shorts. What I found was very interesting. The segments appear to be made from two flat pieces of metal that are laminated/bonded together around the supporting piece in the center of each segment. The supporting piece protrudes through the backing material where it connects to the wires that connects to the pins. Here are close-ups of the shorts and also photos of a couple of the segments, which appears to show the laminations of the flat metal segments in a state of separation. I can’t tell if the metal pieces were coated or plated prior to assembly as some of the separations appear to be the thickness of plating material. Note that the scratches on the backing were caused by my effort to move the segments.
I have never seen before this in a 7971 but it would be difficult to see with the anode screen in place. I have successfully restored 7971’s with shorted segments, but never with more than one short.
Jeff W. |
reasonable offer. My interest in this particular tube was the fact that it was one of the older “pre circuit board” assembly with the top post. I was trying to match up some other tubes that I already had. Here is what the tube looked like from the seller:
