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-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Raymond Domp Frank
Sent: 14 August 2018 19:05
To: [email protected]
Subject: Re: [TekScopes] Concentric A and B time-base knobs/interlocking

Hi Colin,
Do you know if the time base speeds of A and B correspond to those indicated on the knobs? Did you try observing waveforms with both A and B? I'm thinking you may have made an assembly error: The max. A and B rotation and the freedom of rotation angles between A and B are mechanically limited by the knob assembly.

There is a less-intense portion which can be moved by the "Delay Time Position" control, but it's length isn't controlled by the setting of the
B (delayed) time-base knob, only it's position.

I have no idea how familiar you are with this model and 'scopes in general so please don't be insulted if you know all this: Just to make sure we're talking about the same thing, the smaller, grey knob controls the speed of the B timebase, the clear part behind it controls the A time base. The Delay Time knob controls the time after A's start until B starts. If all is well (assembled), A can never be faster than B.

...no bright-up indicating where the B (delayed) time-base will run from and to

This makes me think we are talking about the same A's and B's but the bright-up should be there.

...it's length isn't controlled by the setting of the B (delayed) time-base knob, only it's position.

That sounds strange: The B (delayed) time base knob (the smaller grey one) should control the length of B on A and not influence the starting position.
Sounds like assembly error.

Check the speeds of "A" and "B" relating to the indications on the time scale by applying a signal and see if they make sense.

Raymond

-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Raymond Domp Frank
Sent: 14 August 2018 14:59
To: [email protected]
Subject: Re: [TekScopes] Concentric A and B time-base knobs/interlocking

On Tue, Aug 14, 2018 at 03:39 PM, Colin Herbert wrote:

The "A" part of the display is given and is brighter than just showing the A
time-base on its own, but the supposedly intensified "B" part in too dim to
see, even if I turn the intensity up to maximum.

Colin,
Not sure if I understand exactly what you mean:
The "A" part of the trace (that's the part movable by the delay setting) is supposed to be intensified, not the "B" part.

Are you sure "B ends A" isn't set? It's the fully CW position of the Holdoff knob, which is just to the right of the Calibrator loop.

If that's not it, ISTR that there's a separate adjustment for the A and B part (one being the normal Intensity setting). Unfortunately, ATM I have no time to look it up in the Service Manual.

Raymond

-----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

On Mon, 13 Aug 2018 at 12:13 Thomas Dodge <tdodge2404@...> wrote:

Thanks for your email. Yes, it is a TDS 684A. I actually got it running
yesterday, and it is working very well. I think it had been in the surplus
place for a long time, and so it took a while for everything to initialize,
but it works great. It is in very good condition. Do you know where I can
get an original service manual for it? I will take a look inside it and
look at the condition inside. Thank you very much.

-------- Original message --------From: Thomas Dodge <tdodge2404@...> Date: 8/13/18 11:13 AM (GMT-06:00) To: [email protected] Subject: Re: [TekScopes] Scope
Hi Roger,
Thanks for your email. Yes, it is a TDS 684A. I actually got it running
yesterday, and it is working very well. I think it had been in the surplus
place for a long time, and so it took a while for everything to initialize,
but it works great. It is in very good condition. Do you know where I can
get an original service manual for it? I will take a look inside it and
look at the condition inside. Thank you very much.

Tom

On Mon, Aug 13, 2018, 8:49 AM Roger Evans via Groups.Io <very_fuzzy_logic=
[email protected]> wrote:

Tom,

Do you mean a TDS 684A?? I have one of these and it has serious problems
with leaking of the SMD electrolytic capacitors on the acquisition board
and subsequent corrosion.? There are not too many to change and you can
find the service manual which details the procedure for removing the
acquisition board.? Look very carefully around the other SMD components
near the attenuator assembly, this is where I have the most obvious
corrosion rather than near the electrolytics.? Best to fix these before
they cause damage to the PCB tracks and vias.

Roger