Re: Clean and Lubricate Pots in Tek 475


 

Ed:
The A&B pots were designed to be modular. What I mean by this is that they are designed to fit together such that you can make a 1,2,3 section, or probably more, multiple pots out of the sections. The 311-1397-00 is one example of what Tektronix did with these. So you start with a front module that has the shaft and is the "thread" end of the assembly. Then you just add the first variable resistor section which fits into the front shaft section and mates with the "driver" on the rear of the shaft section which is also "keyed" so that you cannot assemble these incorrectly. Then the second variable resistor section is designed to "mate" with the first variable resistor section with a little driver piece to provide the link between the first resistor and second resistor shaft. There are no "rubber" gaskets between the resistor sections in this design. Then this can continue until you have the number of sections you need. A&B obviously made resistor sections in different resistance values so you could make up a control with say 5K in the first and 10k in the second and so on. I have no idea exactly what values A&B made available. Then there is a back plate where the 4 screws go through to the front shaft section in the front and hold the whole assembly together.
I found that each resistor sections would turn very easy and the front shaft was turning easily also, but when the whole assembly was put together it was stiff and springy. I tried to find the source of the stiffness with no success whatsoever. I left the screws loose and still no relief. The back section shaft was not touching the rear plate so no problem there. You cannot disassemble the individual sections but it is easy to get the Deoxid into them and that got rid of the "scratchiness". These controls were not stiff and springy at the beginning when I got the 475 in 1980 or so but something inside of each resistor section must somehow add up to the whole problem. I also tried to clean each section in IPA and then reassemble but to no avail. I then tried to "lubricate" the section shafts with the same results. I guess what I could do is destroy one section by "tearing" it apart by force and see what is inside of the section shafts to see what A&B put inside. These are the black pots.
The newer Bournes pots are riveted together and must be factory ordered as to the resistance values you need for each section. These are the blue pots. A little research told me that Bournes bought A&B sometime in the past. The shafts of the A&Bs were not bent in any way and were connected to the front panel of the 475 with the usual fiber shafts that Tek liked to use when the pots were mounted on the PC boards way back in the instruments. The Bournes are an exact replacement in the PC board after removal of the older A&B pots, as you would expect.
There is NO way to remove the vertical position pots in the 475 vertical preamp board without removing the board from the instrument. Then it is easy to unsolder them from the PC board and work on them.
Bill

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