Cleaning Tek 7603 Intensity Pots

From the TekScope mailing list:

Hi!

Some time ago, I asked about the correct way of cleaning the intensity pot of my 7603. I got a few helpful answers and some hints about the archive.

I had a few free days and decided to tackle the task. The intensity pot at my 7603 was jumpy, and the readout-intensity pot worked only erraticaly. So I removed the right part of the cabinet and examined the assembly closely. There are two pots and a switch on a coaxial axle. They seemed to be sealed (or at least without visible openings) and the only way to get some contact cleaner into them seemed to disassemble the whole thing. So I proceeded to remove the whole thing (two very small hex-keys for the knobs and a flat wrench for the nut). After unsoldering the three wires I proceeded to remove the four long screws holding the whole thing together.

First error: don't remember where the flat spring included was exactly positioned.

After removing the four screws, you'll get three distinct modules. The intensity pot, the readout intensity pot and the switch connected to the readout intensity pot. I tried in vain to find a hole for spraying some contact cleaner into the pots… the things are just closed. So I took my dremel and a small drill and drilled a 1.5mm hole into the pots, using a dab of vaseline to catch the chippings. After that, I could use my favourite contact cleaner (Oszillin T6, an old standby for cleaning pots and contacts in tv and radio sets) on them. This worked spendidly. The pots aren't completely sealed, but I couldn't find a way inside without damaging the thing.

After that, I dried the pots by letting them sit in the sun for a day, closed the holes with a piece of electrical tape and reassembled the whole assembly. It's handy to make photos and/or notes when disassemble it… y'know, hindsight is an exact science. Well, I found out that there is only one way to reassemble the thing (so that it's working correctly… there are several ways of assembling it slightly wrong After putting in back into the scope and resoldering the wires the scope works perfectly now, both intensity pots work perfectly smooth now.

To make the whole thing short, here's a step by step list how to do this.

• remove the two knobs using correct fitting hex keys (very small, it might be helpful to use a drop of WD40 in the screws first… mine were very tight).
• remove the nut and the washer holding the assembly
• unsolder the wires
• DO NOT remove the four small screws at the end of the assembly…
• drill a small hole (I used a 1.5mm drill) at the middle of the pot body. I used the left side when looking from the front of the scope, the wire terminals looking up.
• wash the pots with a suitable solvent. I used Oszillin T6, Deoxid or something similar should work, too. Don't use something like Kontakt 60, it is not easy to wash the pots correctly after that.
• close the holes with a good tape
• reassemble everything.

This should work for the 76xx scopes with their coaxial intensity/readout intensity pots.

Grüße aus Hohenlohe,

Martin Jangowski