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Elsewhere I have recently discussed my 468 repair project. I now have an analog scope working pretty much correctly. However the digital store function seemed to have problems of no display.

I discovered that both V & H were offset from the visible screen. After some probing I thought I had discovered a problem with the Vrefs on the A16 board and replaced U559 and tested it out of the scope on a bench supply, but imagine my surprise when I bolted it all back together to find the Vrefs were down again; instead of +/-7.5V the +Vref was ~5V and -Vref according low.

I then discovered that the Vref problem disappeared when I unplugged P262, or BOTH of P138/P343 on the A11 V preamp board. According to my schematics, the V-shift "ghost" pots should be floating and connected ONLY to the A16 board via P138/343, when in fact both the ghost pots appear to be already attached to +5V and GND on A11 and delivering a shift voltage to A16 within that range. Thus the A11 is over-riding the A16 Vref voltage.

Have I got some incompatible A11 & A16 boards in my scope?

The scope serial number is 710418, but I would not be surprised if any of the boards came from other scopes and may not be contemporary.

Scanning through the sevice manual I see two different configurations in the pre-amp boards which explains the affore-mentioned issue but the question is what should I do to fix the problem?

The short answer might simply be to remove the offending connections and leave only the slider connections intact. I have in fact just experimented and plugged P138/343 onto A11 using only the connector centre pins (ie I've plugged them in at 90 degrees) and the system actually works (well for V mode cursors... not yet got T-mode cursors visible).

The A16 has a model number of 670-6377-02. I have a newer board version 670-6377-06 in a parts mule; might this be a better solution..... not sure whether that board actually works because the mule has a number of functional issues.

Any suggestions out there?

There are two versions of the vertical preamp board. The older one has dual pots for vertical position, the new, single-section pots. I've uploaded this picture: /g/TekScopes/album?id=274014

Depending on the version of vertical board, a jumper may or may not be required at the location shown.
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Bob Haas

Many thanks Bob; I did already discover that my other Storage board had an absent W250 jumper so I took the initiative to disconnect the one on the board I'm actually using. It worked as it should so we're good to go! I wasn't sure whether there would be any other problem lurking if I made this simple change.

I've noted that while the V-mode cursors are actually present, the T-mode cursors are not visible, even though the system (the LED display) is actually responding to physical changes via the cursor control knob. I guess I need to probe around a bit to work out why the t-mode cursors are not visible..... probably something mis-adjusted.

Try the CRT grid bias adjustment. One clue that it's wrong is if a beam doesn't appear when the intensity control is full counterclockwise (or should I say "anticlockwise"?) and the Beam Finder button is pushed.


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Bob Haas

Mea Culpa!

I'd forgotten, or never read the handbook; the time cursors are not line cursors, like the voltage cursors, but dots on the trace!!! They are there and everything is working.... I think! (I probably had 2465s on the brain.... two of those I fixed in recent weeks).

Well that's two out of three 468s working; shall I try to get the third one working????

Perhaps I'll leave that for a while; my wife keeps making comments like "oh, who are you and what are you doing in my house" when I escape from my radio room!

Ok now had time to evaluate the scope in more detail.

I note a poor blanking on the LHS at sweep speeds of 100ns/Div or faster; I see exactly the same on my other working 468..... is this to be expected?

The A trigger Slope control does nothing (it's fixed at -ve slope); I note the other 468 has an iffy switch action as well (sometimes works, sometimes doesn't). Is this simply a faulty/dirty switch or are there other known issues with the U240 Slope Select circuit? This scope must have had a frontal impact on control knobs in its lifetime since I've had to replace/repair some bits.

Other than that it seems in good shape; bandwidth is as expected and calibration seems reasonable, also the timebases seem to be working well. Storage works to the expected bandwidth and the DVM works.

Now, I have fitted a GPIB board because I had one from my mule parts but I don't have the 16-way cable to connect it to the A19 board. Neither do I have the switch and LED fitted on the Storage board (it exists on the other 468).

I have no use for GPIB, but for completeness should I bother to fit the GPIB board and cable or is it academic without those bits on the storage board as well?

The z-axis response adjustment may help. You may need to turn up the intensity. You should see the leading edge of the triggering signal in any case.

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Bob Haas

I did the Z compensation; it improved the blanking a bit but not completely; I suppose it improved it 2 extra clicks of the timebase knob. I guess it's not particularly a big deal. If I really wanted to use that degree of timebase speed I'd be using my 7904A/7B92A anyway.... about 40x faster!

Many thanks for your suggestions Bob.