Finally the EHT multiplier comes in the post!

EHT Multiplier Replacement

So the Tek475 has been sitting on my desk with the vertical board pulled out for quite some time now. Finally the part I've been waiting for came in the post. I was partly hoping to see a red-cross styro-foam eski like you see on the medical programs on TV but not - just a plain box.

I de-solder the connection to the board and undo the plastic nuts but there isn't enough room between this and the metal case to get it out. I loosen off the screws holding the main interface board in including the ones on the power transistors at the back and I am able to bend it just enough to easy the EHT multiplier out safely.

I've undone the cover over the plate above where the metal tube goes and I disconnect the EHT from the plastic connector which is on the end of the wire coming out of the multiplier (making sure I short it to the case just in case) . The connector is a right -angled affair and the wire goes underneath the metal tube containing the CRT and there isn't enough clearance to get the connector round. So how do I get it out? Ok I could cut the wire (its dead anyway) but then how do I get the new one in? Hmm.. I could pull the metal cover containing the tube back but I really don't want to damage the tube. God I don't really want to *touch* the tube.

Off to the Tek group. Somebody quickly replies and explains that the newer version of the manual describes the tube removal process. As it turns out I've already downloaded this (or the US DOD equivalent anyway).

The process does in fact require removal of the tube. It actually turned out to be easier and less scary than it sounded. You first reach in with a pair of pliers and pull off all these fine wires connecting to the deflection plates and grids etc. They are stiff enough to stay in place so I won't have to remember where to put them back. You undo the back cover and then the round cover over the end of the tube. You pull the electrical connector off the end of the CRT.

Then you undo the screws over the front screen cover and remove the blue filter etc.

Then comes the scary bit - you push on the back of the CRT and ease it gently out of the front of the scope. You have to totally withdraw it from the scope and then rest it somewhere padded. The anode wire comes with it so take a look where it goes before you totally pull it out so you know where to feed it in a minute!

The CRT internals look amazing. I wish I had a clue what half of them are. They look like something out of Star Trek! The black spot looks like a bit of a worry but I'm sure it is just the effect of the cathode heater.

Ok so I still can't get the wire through. I undo the bolt holding the metal tube that the tube rested in so I can slide it partially out the back of the scope. It doesn't move easily but eventually I can overcome the friction of the plastic sleeve it is sitting against and it moves. Now I can easily remove the EHT cable!

I route the new EHT cable through the same way. There is a sort of recess round the case it sits in so I pass it back the same way.

I push the metal tube back in, re-fit the bolt and then put the CRT back in. I stuff the anode wire back into the hole and push it in a bit at a time while pulling the anode through. This takes a couple of goes as somewhere along the line I snag the wires for the graticule illumination and have to re-start.

There are these plastic corners that seem to be to stop the tube rattling in place. One has totally crumbled when I removed the tube. I figure it is just padding so I make up a new pad with some folded paper just thick enough to hold the tube snugly.

The cap goes back on, the the CRT, the covers front and back and now it is time to re-connect the deflection plates. This isn't fun - the wires are thin and the metal tangs leading out of the CRT are thin and relatively short. I don't want to bend them as I imagine snapping them off would be *very* bad. After much cursing, stealing my wife's head lamp and some tricky manouvering they are back on. The worst ones were the horizontal deflection plates which are down a hole through the middle of the main interface board. It's like a high-voltage game of operation! Eventually all these go back on.

I flex the interface board again to get the EHT multiplier in. I solder it back in place, connect the anode lead into the angle connector and decide this might be a good spot for a quick test. I turn it on and immediately you can feel the EHT fire up and the screen quietly sizzles with static. After a few seconds the entire screen is glowing which is a good sign! Means the EHT works and the CRT works!

The next step is to re-assemble the vertical board from the pictures I took before. All the coax goes back in, the connectors go back where they were and the de-soldered bits get re-soldered.

The knobs proved to be trick to put back on as they are quite tight on the fibreglass shafts. Unfortunately I broke one of the connectors. This was easy enough to fix with a bit of epoxy (Araldite).

So then I fire it up and... well nothing. No trace. I hit the beam find button and guess what? We have a spot! More good news! But still more work to do obviously as there is no deflection. Well that's Ok I'm happy :)