开云体育

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Raymond Domp Frank
Sent: 16 July 2018 00:52
To: [email protected]
Subject: Re: [TekScopes] Concentric A and B time-base knobs/interlocking

Colin,
Usually, this is a symptom of the core of the plastic drum for the B-timebase being loose from the shaft that controls it from the front. Often forcing the knob beyond the extreme position breaks the injection molded bond between the aluminium shaft and the drum's core. Also, the plastic of the drum tends to become brittle with time, as I've seen in one case, where the complete assembly had disintegrated.
This problem and its solution have been described in this group in the past but I haven't tried looking things up.

I have used two methods for repair:

1. The safe way of removing the trigger and time base boards, disassembling the switch assembly and repairing the bond with cyanoacrylate glue. This is quite doable if you pay attention to what you're doing. Note that your 'scope may contain a spring-loaded cam-and-pin to prevent a few extreme combinations of the A- and B-timebase, as noted on the front plate. This part of the assembly is not present on later (?) units and AFAIK only prevents the selection of less-accurate combinations.

2. The adventurous way, as follows (read all steps through so you understand the procedure before doing anything):

a. Don't start by opening the 'scope, let alone disassembling anything!
Instead:
b. Remove the red variable time base button
c. Note the length of B-timebase shaft coming out of the 'scope in the pushed-in (no B-timebase) position and pull the B-timebase shaft out of the 'scope. It should come out easily if the problem is as suspected.
d. Try pushing the shaft in and out a few times. It should not bend with the surrounding (A-timebase) tube and see if you can get the same length of staff sticking out as noted in c.
d. Remove all grease from the shaft
e. Cover the shaft with a thin layer of silicon grease but leave about 1 cm at both ends free
f. Thinly cover the top end and ribbed tip of the shaft that is supposed to sit inside the drum with a layer of cyanoacrylate gel
g. Take a deep breath and push the shaft in as far as it was before (point c). You may want to rehearse this without grease and glue first.
h. Wait until the glue has set.

If you correctly followed this procedure, with a tiny bit of luck the shaft is now strongly stuck to the drum and *not* to the surrounding A-timebase tube, because the silicon grease prevented that.
Obviously, you basically only get one shot at this because if you pull the B-shaft out and push it in again, the glue-covered end will pass through the remaining grease in the outer (A-timebase) tube, killing the effectiveness of the glue.

Unless you put an inordinate amount of glue on the end (almost impossible) or did not completely cover the shaft with silicon grease (possible), it'll be easy to pull the B-shaft out again from the A-shaft if the procedure didn't work and you'd still have to follow procedure 1.

I have had complete success with both procedures in several cases (3 or 4 with method 2. alone, no retries or going to procedure 1.) and no failures, apart from the case where the plastic drum assembly had almost disintegrated and the switch assembly had to be replaced by a donor unit.

Please report back which procedure you followed and its results. Good luck!

Raymond