Tektronix 454A loose BNC

The mounting nut on the Channel 1 BNC connector of my Tektronix 454A was loose.

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It’s painful to get access as the channel 1 and 2 input circuitry is shielded. The screws are not Phillips; they are instead Pozidriv. HP tended to use Pozidriv screws as well, so some time ago i picked up a set of Xcelite Pozidriv bits for my Series 99 handle.

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You have to disconnect the shaft for the Channel 2 Invert switch to get to one of the screws on the shield.

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This is the one that is normally hidden behind the Invert shaft.

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The other side is clear. Both screws tend to get dislodged when removing the shield, so take care that they don’t fall someplace inaccessible.

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You’d like to think the shield would lift straight out, but the front edge is beneath the lip on the front panel and the mounting tabs for the two hidden screws catch on components in the shielded compartments. I can’t offer a precise method for removing it; I just jiggled it around until it suddenly came free.

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The cast shielded compartments are beautiful. Nobody made scopes like Tek. Look closely and you can see where the two screws from the hidden mounting tabs fell. If they fall off the side and under the shield box, you'll need to go fishing for them.

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Tightening up the mounting nut for the BNC input connector is a bit of a puzzle. You can’t get a wrench on the nut due to the protruding lip on the front panel casting, and the input AC coupling capacitor is soldered right to the BNC pin and prevents access to the connector from the back. 

You could unsolder the coupling capacitor and pull the DC/GND/AC switch, but I suspect lead dress is quite important in this area so I wanted to avoid disturbing the wiring as much as possible.

I managed to tighten the nut using a pair of long-nosed pliers. It’s not as snug as I’d like, but it’s as good as it’s going to get without removing the front panel.

Over on ARF, Johnnysan suggested using a flat-bladed screwdriver like a chisel to nudged the nut tight. I wasn’t able to get it any tighter that way, but it would have been a better approach than using long nose pliers, as the coupling capacitor interfered with the pliers.

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I put a dab of red Glyptal enamel on to lock the nut to the threads.

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Getting the hidden screws back into the mounting tabs is a bit of a challenge.

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I used a bit of masking tape to hold the screw on the driver while starting it.

© Steve Byan 2011-2016