Tektronix 568 & 230

This has been on my to-do list for way too many years. I got the 568 as part of a lot the better part of 10 years ago. After that, I found a 230, then another. It took another little bit of time to cobble together a pair of working sampling plug-ins, and the special interconnect cable.
Armed with all the pieces, I just couldn’t for the life of me to get the 230 spring to life. With 2 units of cards, the best combination I could get was some cursors moving on one channel, but nothing else.

Starting from the top, I checked the rails first. All was within tolerance. Next we start with the Buffer Card, where all the incoming signals from the scope land. In order for the 230 to start making measurements, it needs the horizontal sweep (the ‘low speed’ sweep), the sweep gate, and the clock pulses (one per sample). It also needs the mV & time settings on the vertical & horizontal plug-ins, and of course, the signal to be measured, but it doesn’t need those to spring into life.

All signals coming into and out of the Buffer Card were as expected. I started poking around at the Clock & Synchronizer Cards, because those generate a whole host of timing signals & gates that actually trigger the measurements. I honestly starting losing my place, because there are a lot of signals that need to be in the right state for the MEASURE signal to fire. I got tied up looking at the !END signal (! meaning compliment, shown with a line over the signal name in this documentation), which was needed to to start, and it started seeming like an infinite loop of sorts.

Oh, and a word on logic levels. This predates TTL logic, and uses active low logic, where a True state is represented by a low level between 0 – 2V, and a False state is represented by a high level between 6 – 12V. The convention in the manual is ‘High’ and ‘Low’, where High is False and Low is True. so for example the MEASURE signal goes Low for a measurement, and the !END signal goes High to indicate the end of a measurement cycle.

To make things even more confusing (and forgive me if I explain this incorrectly), negative logic changes the meaning of NAND and NOR gates, and they’re drawn differently in the schematics. See the NAND gate below. Electrically (shown in the second column), it behaves as a NOR gate, but in negative logic, it behaves as a NAND.

All this to say, there are a few things that make this a difficult machine to troubleshoot. So I cheated. I bought a 3rd, working unit. It arrived exceptionally well packed (so very grateful for that), with a nicely made interconnect cable (I already had a factory one), and it worked as advertised. Finally, I was able to see this beauty in action!

But I said I was going to fix a 230, so I set about starting to swap cards, this time, with a known good unit.

Swapping the Clock & Synchronizer Cards made no change. This supported my suspicion that perhaps they weren’t getting the right signal from the memory circuit. Swapped the memory, no change. The Start / Stop comparators send signals back to the clock, so I tried those next.

It’s always the last thing you try. With the B channel Memory & Zone Generator, I swapped the A Zone Generator with the known good card, and the unit spring to life. Through mechanisms I’m still trying to understand, if you have a bad Zone Generator Card installed with a good memory card, it can lock up the unit into the state I’d been seeing it in. And as luck would have it, I had three bad zone generator cards.

The Zone Generator Card has (2) logic ICs, and 36 transistors, a mix of NPN & PNP, all individually socketed.


The sockets are a blessing and a curse; It’s easy to swap transistors, but the sockets themselves also become a point of failure.

Sometimes, you want to sit down and dig into a schematic, come up with a hypothesis on why it’s not working, and start probing around to prove or disprove your theory. Other times you just want to brute force a problem, because it’s 11:00 PM, and manual labor is sometimes easier than critical thinking. So I set out to test every transistor in one of the bad cards to see if I could coax it back into the living.

I believe these to be unremarkable transistors, since the application is mostly simple low speed switching. In my model there was a mix of Tek model #s and manufacturer #s:
NPN: 2N3904 / 151-0190
PNP: 2N3906 / 151-0188
I decided to test them on my 575 Curve Tracer at their operating voltage, fearing that perhaps the chinesium battery powered one might miss a fault, since these are operated across 50V rails.
I found 2 culprits:

And with those replaced, the unit sprung to life!

So now we have 2 Zone Generator cards that at least let the unit run measurements, but both have problems. The Zone Generator card generates the areas on the displayed waveform where measurements are made, and show those at highlighted portions on the trace. A zone can be .5cm, 2cm, 4cm or 10cm long (or 12cm, but I’m not getting into that now). When the zone is .5cm, the average of that zone is returned; when it’s longer, the peak is returned.

The first card had an issue where the 0% zone would jump around, and not get to all the locations. We can see in the schematic, there’s a binary coded input coming from the zone position control. Because of the way it was jumping around, I suspected either the 2 or 4 bit, corresponding to Q864 or Q884. Q864 was bad, and replacing it gave me a fully functioning card.

The second card had an issue where the entirety of the 100% zone would be highlighted, so I could only see a movable cursor when only the 0% zone was selected.

This trace shows 2 good ZONE signals, and the incoming SWEEP GATE on the bottom, which I’m triggering from (thank god for 4 channel scopes). The negative pulses represent the position and width of the two zones.

The trace below shows a bad ZONE signal.

I’m seeing this signal coming from M938, but also going into it. Moving upstream, I check Q933. That’s OK. I pull and re-seat M938, and that seems to fix the issue. Now I have a cursor, but it has the same skip issue as the other card. The 100% zone transistors in question are Q964 & Q984. Both of these are good, but re-seating them fixes the issue. Like I said, those sockets are a blessing and a curse.

So now I have 2 more working Zone Generator cards, and a 2nd mostly functioning 230. It’s still wildly out of cal, and there are some quirks with the Nixie display, and cross channel time measurements, but I’ll get to those eventually.

Tektronix Analog Sampling

I’ve been putting this off for a while, but after a series of easy wins, I thought maybe I was up for the challenge. I have had in my possession for a number of years, a decent collection of analog sampling plugins and accessories from Tektronix, including 2 digital readout units, which allow for automated voltage and time measurements. Getting those readout units working is the goal, but first I have to start with a reliable sampling system.

I’m doing all this on a tektronix 568 mainframe, which has all the connectivity for the readout and automation systems, but these plugin pairs would work just as well on a 561 or 564 if you just wanted basic sampling functionality.

First I’d like to catalog the state of all of these plugins. I think the majority of them don’t work, so I want to find the units that work the most, and start from there. If I have a working pair, it will help evaluate the other plugins, as sometimes it’s hard to tell if the fault lies in the timing unit or the sampling unit.

The plugins

3S76 – Dual Sampling #2443
This one seems to work on the A channel. I have had it successfully send a trigger to the timing unit, and was able to position a signal on screen. The B channel is not working. After some time, I do get a sweep on screen for it, but I can’t position it. Getting the B channel working might be a good step in understanding how repair other sampling units. More on the repair below.

3T77 – Sweep #2810
This seems to work. Needs trigger sensitivity up pretty high, and it’s not what I’d call rock solid, but so far it seems like the best timing unit of the lot.

3T6 – Programmable Sweep #110543
This also seems to work well. External trigger provided on BNC J123 on the rear of the 568, via the gremar connector on the plugin

3T77 – Sweep #2250
Doesn’t work.

3T77 – Sweep #0881
Doesn’t work.

3T77 – Sweep #2246
Doesn’t work.

3S5 – Programmable Dual Sampling #40199
This seems to work on the A channel with the 1 known good S2 sampling head I have. The B channel is not working
Side note: the channels are lettered on these early sampling scopes, and numbered on traditional scopes. No idea why, but it holds true on the 661, and all of the 560 series plugins. On the 7S14, it’s back to numbers.

3S5 – Programmable Dual Sampling #30151
A channel works, with internal triggering. The B channel is not working.

3T2 – Random Sweep #30320
Doesn’t work.

3S76 – Dual Sampling #1209
Doesn’t work. With normal/invert switches set in the middle (a troubleshooting step in the manual). I get a sweep that can be moved vertically on Channel A, but not on Channel B, suggesting there are a few things wrong with this unit.
The sample diodes seemed to pass a multi-meter diode test, for what that’s worth.

Update: I stole a Nuvistor V1073 & transistor Q2163 from this to repair #2443

3S2 – Dual Sampling #40267
Both channels work, although I wasn’t able to get internal triggering working. Unfortunately I only seem to have one working sampling head at the moment.

3S76 troubleshooting

Set up with plug-in extender
Waveform at test point 4
Waveform at test point 6
Measuring bridge voltage on channel A – all good.
Measuring bridge voltage on B channel – no good
Sampling diode bridge