Tek 2465 Teardown

Well,

I have a Tektronics 2465 'scope that has some issues with it's timebase:

The sweep A is all out of bonk (that's a technical expression) - in the image above there's a 10MHz signal applied and at a 50ns sweep time per division we should see 5 sinewaves - we see 8 :-(

Sweep B is correct at 20ns however at 20ns we see two complete transitions of the signal.

I've started to take the 'scope to bits and will use this blog entry to document things as I go. Here are the initial shots of the 'scope in bits:

So here you can see both U700 and U900 removed, they are the main hybrid components in the Sweep A and B circuit. I'm going to try the 'scope with these two swapped round as they are the same component but for Sweep A and B:

So, swapping the two hybrid devices U700 and U900 made no difference; I now need to focus initially on this bit of the circuit:

I've also looked in more detail at the exact problem that I have. It seems all sweep speeds EXCEPT 5ns are impacted. I've annotated two points in the diagram above and have taken the voltage measurement at these points at all sweep speeds:

You will also note the consistency in the delta at all sweep speeds above 5ns - this is the same right up to 500ms but I just got bored writing the same thing over and over.

So, why is the voltage at the first test point so dramatically different at 5ns (the only sweep speed working) than all the others. Could it be that the B Sweep hybrid actually does the 5ns sweep on A? The B sweep can never be slower than the A sweep so there is no need for both to implement 5ns - I don't know if this is true or not.

I pulled R724 and in the process the end snapped clean off - but it measured 35.55K (it should be 35.3K) so I dont think that will be the problem. However, I've had to order some bits as this is a very non standard resistor value.

I also had a look at C712 which I have replaced. I also need to check that the +42V line is as it should be.

Once I have the replacement bits I'll update this post with my findings.

Some more pics while I wait for the bits:

Now the bits have arrived I have replaced R724 (made from two resistors in series) plus C712 and BINGO - the fault has gone and the sweep speeds are back on the money. The unit needs calibration now :-(

Final update: I've been through the step-by-step calibration routine. To access the routine you set a jumper on the control board and then press the delta T and V buttons together and whilst holding them down stick a chicken down your trousers (actually you press the slope button). Then you go step by step through the calibration routines and using the other test gear I have hear I seem to now have the 'scope very much on the money.

Local conditions.