Tektronix 468 Repair

[willmac]

Being flushed with success after repairing an old Telequipment D75 oscilloscope I have started on a much more complex project - to restore a Tektronix 468.

This scope was given to me with the D75, as 'probably not working 10-15 years ago'. It has been stored in a damp shed and is in terrible cosmetic condition. Everything about this scope is complex in comparison to the D75 - the service manual is 640 pages+ so I wonder if I am taking on too much for someone with extremely limited electronic knowledge. However - I think it is worth trying with your help.

The oscilloscope had suffered a major impact, causing the case to be badly dented in. This prevented the case from being removed, so I cut out some of the aluminium case and pulled the rest back to a roughly square state. This allowed the case to be slid off. After this I expected damage to the PCBs, but none was immediately apparent - in fact the internal state was fairly clean.

I powered the scope up and got nothing much happening, bar the fan. Checking the power supply on the underside main board, I found some blown fuses. I replaced these and got some life. The current status is:

1) POST appears to work as expected. I get main panel LEDs light, the marching zeros in the 7 segment display and no error code remaining after POST.

2) I get something like a square wave trace of about the correct amplitude and frequency from Channel 1 and 2 from the calibration loop, after cleaning it enough to get proper contact. The other controls seem to respond roughly as you would expect for an analogue scope. The top of the square wave shows a lot of ripple/mushy stuff. Certainly not clean and tidy.

3) I get all the main voltages just about within the tolerances given in the manual.

4) I get no digital stored display at all. No waveform trace displayed, no cursors etc. If I press beam find when in the digital mode I get a single bright horizontal trace, which is unresponsive to any controls. The volts/time readout does give me some results, which might be feasible numbers for the trace that isn't being displayed. (hope that makes sense).

5) Measuring ripple in the main voltages , I get good clean readings with no serious ripple that I can measure with primitive equipment, except for the 55V rail. On this I see some strange fuzziness which my D75 scope cannot properly resolve or trigger on. My DVM also shows that the 55V has some ac component but I can't find any way to actually measure this.

I am wondering about the best tactics to start working on this scope.

I know from reading the manual that the 55V supply is very important so I think that sorting out the main power supply to the point that I get a really clean ripple free 55V should be a good start.

Beyond that I think that finding out why the digital display is totally dead might be the next step. The analogue and digital parts of this instrument are quite tightly integrated together so I think this might be quite tricky.


I would be grateful for any suggestions on general approach - or should I just step through the manual page by page, test point by test point? I don't mind if that is the best approach but I'm guessing someone who knows these scopes well may have a more direct route.

[MrBungle]

Probably shorted tantalum capacitors on the digital boards on them. Check the rail voltages on the digital sections.

[willmac]

Thats a good possibility, I will check them. I did notice that the main transformer is getting a bit warm - maybe too much current being drawn?

For checking capacitors I only have my DVM. That will find a shorted cap of course, but do I need to get an ESR meter?

I understand that tantalum caps are well known for bad failure modes. Would a wholesale replacment programme be warranted?

[cmjones01]

The 468 is fairly complex for a relative beginner, but starting with the basics should get you a long way. The ripple on the 55V supply is probably a problem, and is likely to be due to tired smoothing capacitors on whichever the feed to that rail is. The manual's circuit diagram will tell you.

And, as MrBungle says, shorted tantalums are the next thing to look for. Tektronix equipment of this era is peppered with them like little booby traps. My method for dealing with them is to go round finding them by eye, since they're quite distinctive coloured blobs and easy to spot, and measuring across them with a continuity tester. If any of them shows short-circuit, you know that rail has a short across it, but it may not be due to the capacitor you're measuring but another one. If you have a Toneohm or similar you can narrow the fault down to the capacitor which has gone short-circuit without unsoldering anything, but often it's just as easy to unsolder one leg of the capacitor and test again. Repeat until the fault clears. It's usually possible to do this without removing the PCBs from the case, which makes the job easier.

I've revived a lot of Tek 7000 series plugins from the same era using this technique.

Chris

[MrBungle]

As the traces are pretty long and have relatively high resistance you can usually get away with measuring ohms across each cap and then snipping out the lowest one until you find the dead one. Sometimes they are visibly dead and sometimes not. You need a relatively decent meter for this though - 4.5 digit with low ohms mode.

The old Tek 465/475 scopes had wet tantalum capacitors as well. Those are fun. Had one blow up in my face

[RogerEvans]

Apart from shorted tantalums and tired aluminium electrolytics another common problem at this vintage, especially when kit has been idle for many years, is poor switch contacts. Try to get some IPA in with a fine paintbrush and exercise the switches many many times. I had a 7S14 sampling plugin, very rare and very densely packed, totally dead until the switches had been cleaned three times and exercised probably 100 times, then bit by bit the functionality returned. The push button switches are by far the worst and I think the 468 has them on the digital 'add on' board.

Roger

[willmac]

The push button switches are by far the worst and I think the 468 has them on the digital 'add on' board.

Yes it does, lots of them. I have been using contact cleaner on these, is that OK?

[RogerEvans]

The official advice is 'IPA only' but I confess to being sufficiently desperate on some occasions to resort to aerosol cleaners, but then try to wash it out later with IPA.

This is just a thought - does the 468 have any transistors in plugin sockets? Some of these were very unobtrusive unless you looked carefully. They can be dislodged if there was a bad impact, worth giving the digital board a close look and a prod.

Roger

[MrBungle]

I use contact cleaner. As long as it's residue free you should be fine these days. WD40 do a contact cleaner that is safe for plastics. Don't use normal WD40!

My 475 hasn't had a bad switch on it though and that's a really old Tek.

[willmac]

I have started to look for Tantalum caps and there are quite a few around. Some boards have many some none or few. As far as I can see, none of these capacitors have silk screened numbers, and the parts list in the manual does not identify them as tantalum capacitors. By a process of elimination looking at manufacturers codes I think I can identify them in the parts lists. For example, manufacturer code 56289 is Sprague, who I can see made tantalum caps, and the part number 196D685X..... corresponds to their internal numbering system. Tektronix calls this part a CAP., FXD, ELECTLT:6.8UF,20%, 35V, which agrees with the Sprague number. I suppose Tantalum capacitors are actually electrolytic capacitors, but I just assumed they would call them out a bit more specifically. -- Is my identification guess reasonable?

A second question about these Tantalum capacitors. Every one has a tiny bead fitted to each leg. These beads do not appear in the manufacturers drawings so Tektronix must have added them. I'm not sure what these beads are for. They could possibly be for insulation, but no other components with legs have such beads. Perhaps they are ferrite beads - but they are quite brightly coloured. Obviously I will replace these on the new parts but what are they and why?

Finally RogerEvans query on transistor sockets - I haven't found any yet in this scope with sockets. This is in contrast to the D75 I repaired where all the transistors were socketed. In that case I usually had to replace sockets and transistors together because I couldn't get the old transistor legs out of the sockets without breaking them. Not so convenient!

Thanks for all the help - I am still a bit daunted by this project, but learning a lot as I go along, so no big concern if it doesn't all work out perfectly.

[MrBungle]

Those are ferrite beads on the legs. I tend to just slip them back over the same leg when you replace. The sprague tants are trouble! Sometimes they don't even look blown up until you get them out and see the big burn mark on the bottom.

The 4xx series aren't obviously sockets but a lot if not all of the transistors are in tiny little sockets that are soldered into the board holes. They usually just pull right out.

I had the D83, which is a bigger D75. Lovely scopes. Still keep trying to nab one on ebay. My wife has told me I have enough scopes though

[willmac]

Originally Posted by MrBungle

The 4xx series aren't obviously sockets but a lot if not all of the transistors are in tiny little sockets that are soldered into the board holes. They usually just pull right out.

I had the D83, which is a bigger D75. Lovely scopes. Still keep trying to nab one on ebay. My wife has told me I have enough scopes though

OK - I think I see what you mean about the tiny sockets. I thought they were just the pads where the pins go into the board.

As for the D75, yes I like this one, especially after the repair.

[willmac]

I'm busy putting together a parts list. There are a lot of Tantalum caps, so a significant amount of work to replace, but I don't really want to miss a bad one.

I could replace all these parts like for like, but I have been reading about the need to de-rate Tantalum capacitors for voltage. Should I assume that Tektronix will have already taken this into account, or is there an advantage in derating them anyway.

[MrBungle]

Higher voltage ones cost a lot more money so it's a careful trade off.

You can actually replace most of them that aren't in a signal path with decent low ESR electrolytics now which are much cheaper but didn't exist when the scope was designed. They almost always have a considerably higher voltage rating than tantalum.

[karesz*]

Hello Willmac,

I could replace all these parts like for like

Be carfull please with changing all of tantalums, especially the Sprague metallic cylinders are practically military grade tantals_ I didnt find a failed from it in my praxis and their are after i.e. 30 years tight on specs...
The problematic types of Tantals are only the diverse coloured blob types! (as Chris told it too)
BTW: before exchange all blobs; you can touch that for temperature sensing, than I would change only ones there are some "warm"...
Be succesfull with these action!
Karl

[willmac]

It is the coloured blob types that I am thinking of replacing. There are a few other tantalum capacitors that are not this type, so may be the kind you are describing as 'military grade'.

[MrBungle]

The military grade ones are the ones that blew up for me.

[RogerEvans]

There are separate power lines for the digital storage boards: +12VS, -12VS, +5VS and -6VS. Did you check that those are OK?

The vertical signal from the storage board uses some switching circuitry common with the 'trig view' switching. Does trig view work in analogue mode?

If you are still looking at the 'fuzz' on the +55V supply can you probe the raw output from the rectifier / reservoir capacitor on C829, collector of Q508 or collector of Q501. If you set your Telequipment scope to line trigger it should sync with any ripple from the mains. If the fuzz appears to be very high frequency then try adding a capacitor across C603 in case it is high ESR or open circuit. If the fuzz has no structure at all try adjusting the +55V adj in case it is a noisy pot.

Roger

[willmac]

Originally Posted by RogerEvans

There are separate power lines for the digital storage boards: +12VS, -12VS, +5VS and -6VS. Did you check that those are OK?
Roger

I think that I have checked all those voltages, but you are asking specifically whether they are correct on the digital storage board. I am not certain that I have checked all those voltages on that specific board, but will do so.

The vertical signal from the storage board uses some switching circuitry common with the 'trig view' switching. Does trig view work in analogue mode?
Roger

The trigger adjustment appears to work ok in analogue mode. To qualify this, I was using the calibration signal. By adjusting the trigger I could get the trace to lock and unlock on the screen.The trigger lamp would light when this happened. Beyond that I could not say that the trigger is working as well as it should. I could test the trigger circuitry with other signals if this would help with diagnosis.

If you are still looking at the 'fuzz' on the +55V supply can you probe the raw output from the rectifier / reservoir capacitor on C829, collector of Q508 or collector of Q501. If you set your Telequipment scope to line trigger it should sync with any ripple from the mains. If the fuzz appears to be very high frequency then try adding a capacitor across C603 in case it is high ESR or open circuit. If the fuzz has no structure at all try adjusting the +55V adj in case it is a noisy pot.

OK I will try that and report back. Many thanks for the help.

[RogerEvans]

Just to the left of the CRT is a vertical group of five pushbuttons, the second one down is labelled 'A trig view'. This is supposed to show the signal that caused the A timebase to trigger, I have never used it (I don't have the option on any of my scopes!) but I think it is supposed to be useful if you are looking at a complicated waveform, maybe using also the delayed sweep capability, or triggering from an external trigger input. In this case none of that is relevant, simply that some of its circuitry is shared with the display of the stored digital signal. If you press the button you should see the same square wave but its amplitude might well change. It would be nice if someone who has a 465 (which is very similar) could confirm this, I am just working from the manual.

Also you implied that the LED display might well be showing the correct voltages for the unseen digital waveform. A nice check would be to see what happens to the cursor display of Volts when you switch from DC to AC coupling on the input signal. With DC coupling you would expect the LED to go from zero to the calibrator voltage (assuming that the calibrator does switch from zero to 1 volt or whatever, which is nearly always the case) whereas with AC coupling the LED should go from (minus half the voltage) to (plus half the voltage). That would show fairly conclusively that the processor, the input A2D circuitry and the memory were all OK.

Roger