Tektronix 468 repair/refurb

[tautech]

One would have to consider why Tek made the changes first. Reliability? Availability? Better specs?
Or general improvement in the scopes operation/stability?
Many part these days are better quality than those from back then, but wholesale changes especially in the signal path are to be avoided IMHO. Full and precise calibration of a scope is not a simple task, even less so due to the precision equipment required to do the job properly.
Have a good look at the chapter on calibration and see how much of the equipment required you have or have access to.
Sure the Cal will have drifted due to the scope's age and repairs you have made but there are good basic checks that can be done to ensure reasonable accuracy.
Dave did a good vid on basic checks of a Hameg, #502

[Addicted2AnalogTek]

I've been trying to figure out why those changes were made.  I don't really know where to look or who to ask specifically, but I'm sure I will find someone who knows or find the info at some point.   Considering the changes I am most concerned with - The zener change directly at the input - my guess is that it is a operation/stability type change.  It's definitely not availability though. 
The 6.2V zener off of the +15V to Q285 gets replaced with a 270ohm resistor, and an 8.2V zener gets installed parallel to C186 with cathode to Q285. Anode and one side of C186 go to ground.  It sounds to me like that type of a change would affect the performance & stability. Maybe they initially had not done long term testing on Q285 and decided a change in the input voltage would enhance performance?   I don't really know - I haven't gotten much into those aspects of circuit performance and semiconductor operating performance parameters yet.

The only reason I am really going at it with the signal path parts is because the waveforms and voltages in the signal path are/were way off.  at some points almost 25% off spec.  Otherwise I would have avoided replacing all of these capacitors and these few resistors. My thoughts are that if I know that the freshly installed parts are good, I will have a better basis to work from for fixing the issues it has. Every few minutes I was finding a bad cap, and it just seemed more logical to me to replace them all since they are all quite old, the scope has been used a heck of a lot, and they aren't all that expensive. (Plus I needed to get some for other projects anyway.)

I have been collecting the cal gear and have most of it so far. only things I lack are the 106 square wave gen, RC normalizer, and 10X attenuators. other than that I'm good to go. (aside from the couple caps in the time mark gen that need to be replaced).   I think my Wavetek 166 at about 5ns risetime will do for the time being.   I can also make a few circuits with fixed frequencies that have about <2ns risetime to really finish off the vertical checks/adjustments.  First I'm going to buy a $50 specific voltage standard box to get a general check on the cal gear and build up a quick board for my STP2145A OCXO so I have a second freq standard to check against the Bulova in the TMG.
But I'm not quite ready for the cal work just yet anyway. I still need to finish troubleshooting and fixing this 468 and then I will cal my Wavetek 166, check the cal gear against other gear at the college, and then get on to calibrating this 468 and my 465.

[Addicted2AnalogTek]

Changing out all of those caps made a HUGE difference in the PSU loading.  As expected, the waveforms are still off from something which I think may be in the trigger or vertical section involving the NORM trigger source, but other than that it was worth it to replace them all. The transfo is getting just mildly warm as it should now. Not so hot that I could fry an egg in 30 seconds!! ha

In NORM trigger mode, it still gets a trigger when I have NORM source selected. It also has what looks to be a 60 or possibly 120hz "wobble" to it in that source setting. This only happens with 1ms or longer selected in the timebase.   With any other trigger source setting selected I get a perfectly stable horizontal line when it should be there, and it's not present when it should not be - in any timebase selection. I think this might be my final issue to fix - all others seem to be fixed. 

[tautech]

You have to remember this equipment might be 30+ years old now and if it's spent a lot of it's time turned on, most components will be well past their MTBF.
Parts that were readily attainable back then are not so easy to source now, at least with Tek there is some NOS available, probably more so than any other manufacturer.
So one should be prepared to substitute after careful cross referencing.

For your zener a post or 2 back, these were the early days of zeners, noise or/and reliability might have been reasons for change.

[Addicted2AnalogTek]

Fortunately there are a few databases specifically for semiconductor parts cross referencing, so that isn't a problem for the most part.  Of course there are a few IC's and custom transistors that can't be substituted.

I'm attaching a pic of the area affected by the zener mod so you get a better idea of what it says to do.

It is saying to replace VR187 with a 270ohm resistor and install an 8.2V zener parallel to C186 with cathode to Q285.  As far as I understand, that will change the voltage level reaching Q285, thus adding some power to the input signal going back into the other circuits. I don't think it's a reliability thing, but I could be wrong of course.  Let me know if you think this is unnecessary or worth doing.  I guess I could always try it later on and see what affect it has..?

[tautech]

Being early in the signal path, it may change vertical amplitude, however a resistor/zener configuration is the common method used, additional stability should result. Note the DC +8.2V on pin 6. 
As your Cal is way off, definately do the mod and your new cal will null any changes the mod makes.

Make a note/list of mods you do.

[Addicted2AnalogTek]

gotcha. sounds like a plan.   Now to order some 8.2V 1/2w zeners (can't find 400mw unless I order from digi or mouser... NXP china junk - blehhhhh.   MOTOROLA for me!)

I believe I know where that "60-120hz like" wobble is coming from.. I looked on the timebase diagram for any setting that is 1ms and slower, and...well... it's the 1.5uf 10V 150D that I didn't replace. DOH!!!      Of course the one and only PITA-to-access tantalum that I forgot to order!!

The triggering issue.... the waveforms and voltages at the vertical outputs to trigger are quite a bit off and have some noise, so I think that is where the trouble may lie.  Of course CH1 is way worse than CH2 due to usage and all, so it's time to finally track that down now that I can get a clean signal into the circuit. YAY! 

One other thing I noticed is that in the parts list - for part 151-0341-00 (2N3565) it specifies "TO-106" and in my scope there are all crappy ol PN3565 TO-92 package transistors.   I have 100 or so TO-106 Signetics 2N3565's..... I feel inclined to replace them!  They may not change a whole lot, but at least they will be correct... 

I used this for the cross referencing.
http://www.reprise.com/host/tektronix/reference/transistor.asp

[tautech]

One other thing I noticed is that in the parts list - for part 151-0341-00 (2N3565) it specifies "TO-106" and in my scope there are all crappy ol PN3565 TO-92 package transistors.   I have 100 or so TO-106 Signetics 2N3565's..... I feel inclined to replace them!  They may not change a whole lot, but at least they will be correct... 

If they are used as pairs be sure to attempt to match Hfe, any old DMM with a transistor tester will give you indicitive gain readings for a "close enough" match.

[Addicted2AnalogTek]

it doesn't say anything about matching them and I'm checking their Hfe as I take them out to be sure. thus far their Hfe's are wildly different so I don't think it matters. for each channel there is a pair around the "position" op-amp and one used w/ voltage divider to put 15V into the signal. (I'm sure there's a term for this setup but I don't know what it is .. yet.)   
I'm going to match the pairs and match them all from channel to channel anyway.  Might as well since I can.

[tautech]

I'm going to match the pairs and match them all from channel to channel anyway.  Might as well since I can.

[Addicted2AnalogTek]

one of the delay line leads decided it was ready to fall off so I had to spend the last hour redoing that whole deal.  Now my delay line is 5mm shorter! AHHH.   hope that doesn't matter.... cuz I'm not replacing it!!

[Addicted2AnalogTek]

Anyone happen to know if this model/type of resistor is worthy of signal path?   I'm trying to stay thru hole but go more reliable than carbon comp.

http://www.ebay.com/itm/231374168010?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

Datasheet:

http://www.vishay.com/docs/31027/cmfmil.pdf

Doesn't say anything about being non inductive or low inductance, but considering that they are MIL spec Vishay/Dale one would think that they are going to be straight cut high end instead of the spiral cut type.     I know I can get 1000 for 100 instead of paying $1 each then sell some for my money back.  If I can find out if these are worthy for scope/gear repair, I may start ordering different values here and there as I need.

[SeanB]

That they do not tout inductance says they are spiral cut, so are inductive. If they were slot cut they would proudly advertise capacitance and inductance value maxima and typical values for them.

[Addicted2AnalogTek]

ah ok... so it would seem that I am more or less limited to carbon comp or SMD resistors unless I want to go into the 1W+ area to get the metal film type I would want.

Heh... interestingly enough changing out those 2N3565 transistors has almost completely eliminated that "60-120hz - like wobble" that I had. Glad I did it!!!   I wonder what other transistors I could benefit from changing out.....   I will be keeping a close eye for transistors that have been substituted in place of correct replacements so I can continue bringing this scope back into spec/original condition.    Another well-worth-the-effort swap!

[JoeO]

ah ok... so it would seem that I am more or less limited to carbon comp or SMD resistors unless I want to go into the 1W+ area to get the metal film type I would want.

Heh... interestingly enough changing out those 2N3565 transistors has almost completely eliminated that "60-120hz - like wobble" that I had. Glad I did it!!!   I wonder what other transistors I could benefit from changing out.....   I will be keeping a close eye for transistors that have been substituted in place of correct replacements so I can continue bringing this scope back into spec/original condition.    Another well-worth-the-effort swap!

Do you think that the problem was fixed by the new transistors or by the fact that you matched the gain of the new transistors.

[Addicted2AnalogTek]

I was actually misled by my setup.  I had them both in the GND input coupling setting, and it doesn't appear there at all.  It's only in AC or DC input coupling, and I'm fairly certain it's caused by the 1.5uf 10V 150D capacitor on the timebase board that I didn't replace yet.  As I mentioned before, it only happens with 1ms or slower on the timebase, and that cap is involved with all settings 1ms and lower.  So once I replace that it should go away.    I have to order them and play the wait for USPS game and then I will know for sure. 

Now that I've checked a bit more, though, it appears changing those transistors out and matching them up has improved the waveforms and voltages throughout the signal path. I assume that any new transistors in this old scope is going to make improvements anyway though. Unless I have to, or they are not the proper replacements, I won't be doing all that! haha   that would be a bit.... expensive.

[Addicted2AnalogTek]

Other than the hot rectifiers it seems like everything is almost wrapped up.  I have that one 1.5uf cap to deal with and some 110 unreg loading from the CRT and then I think everything else is just in the adjustments.
I just did the CRT grid bias adjustment and that cleared up the wide, fuzzy trace issue I was having.  That made me think.. once I change that cap and fix the 110 unreg loading, maybe it would be best to go thru the calibration procedures to tighten everything up.  Then I think it would be easier to find any remaining problems it might have since all of the waveforms and voltages in the schematics are based on a fully calibrated scope.  That should help avoid any more unnecessary part replacements.

[Addicted2AnalogTek]

I think that the booger in the CRT circuit is Q109.  I've checked all the components around it and even changed the all the transistors around it to mil spec.   That Q109 "buzzing" and CRT flickering I had when I touched Q530 is gone since I changed out both Q530 and Q533. However the voltages surrounding Q109 are still way off and unstable. 

The voltages surrounding Q109 (2N3053) are supposed to be: -0.6V on Collector,  -3.3V on Base, and -4.5V on Emitter.
My measurements:   -0.19V Unstable on Collector, -3.8V on Base, and -4.4 on Emitter.
I've changed CR104 and it helped a tiny bit. The only other thing I can do is change CR110, and I don't think that will make any difference but I may do it anyway. 
When I put my finger on the can of Q109, the voltage on the Collector climbs closer to normal, but it does not go past -0.35V. It also helps to stabilize it a little bit. 
I checked it with the diode tester and Hfe tester, and it appears to be OK, but that still doesn't tell me that it's definitely good obviously.

The high voltage side of the transformer is within spec and stable.  It reads -2414V.

Q1108 (2n3055) also may be on it's way out.  It's collector reads 3.4V low with a small amount of instability, and it's base is tied into TP112.  The Emitter to ground is reading 0.9mV while everything else in the vicinity that hits chassis ground is reading less. C108 seems to be good according to it's ESR and value, but I plan to replace it anyway.    I have some ST 2N3055's on hand if I decide to replace Q1108.  It's kind of a PITA to get out without taking the boards off again, though. 

What are your thoughts on all this?   Q109 & Q1108 to be replaced? 

[tautech]

Hmm.
I'd be suspicious of the high value resistors although they are not under the same stresses as those on the secondary side, they probably have drifted some from new values.
Q1108 is under the most stress in this part of the circuit, second will be the E caps associated. 

Zeners possibly. 

Have you setup as specified to check those DC voltages, they won't match unless you do.

BTW snippets of the schematic as you do make help easy. 

[Addicted2AnalogTek]

I"ve been checking all the resistors as I replace parts and also in circuit where I can, and it seems that most of them are close enough.  The 22Meg that I thought was way off is actually 22.5Meg, which is def in spec (31.1Meg max I believe). 

I've replaced all of the HV caps except for the .0047 6kv and the  .0012 4kv to ground at the focus pot. It focuses nicely and, since it's to ground and the voltages there are correct, I didn't feel the need to replace it.  I do have another one to do so if I decide I want to though.  I had previously stuck in one of the big fat ceramic 0047 6kv caps temporarily to check and see if the original was bad, and it's not.

I want to replace Q1108 anyway even if it isn't bad, but all I have on hand are the ST 2N3055's from China which I don't feel too confident about using.  I don't know if they are genuine (don't say "MALAYSIA" under "ST" logo) but I know they worked fine in some other circuits.  Can't decide if I want to risk it temporarily or not......  I'm going to order some MOTO or RCA ones.

The 75V and 160V zeners are on the other side of the transfo near the CRT bias, and it adjusted perfectly. the voltages around there are good, except the waveform at TP86) is "doubled". The waveform at TP86 with CRT blank is always displayed in addition to the Unblank waveform (which is the correct waveform).  (I will add pic of this scopeshot shortly) 
--The scopeshot below is different and is what it looked like before I replaced C318, and the larger amplitude WF shown with the correct WF is the waveform on the other side of C210 (which I ordered a replacement for).   I figured I would mention this possible issue as well in case you thought it might be related to the issues on the primary side of the HV transformer.  I  don't think it is, but as always I could be wrong. Also could be wrong about the cause as well, and maybe it's CR323 by C318. I hope not though, because those diodes are very hard to find and expensive!

[Addicted2AnalogTek]

Oh, and the waveform and voltages at TP84 are correct.  Actually, all of the waveforms except for TP86 are perfect.  I am using the waveforms and voltages from the 465 manual and 468 manual.

As one might guess, there are no waveforms specified in either manual for the primary side of the transformer, which frustrates me. Either way I believe that problem may be narrowed down to either (or both) of those two transistors ( Q109 or Q1108) unless there is something I am missing entirely here. 

[Addicted2AnalogTek]

I need to start getting my cal gear into working order.. the calibration procedure jumps directly into the time mark gen with the CRT y axis and geometry adjustments.   Just a quick off topic question...

Does anyone have a Tektronix 067-0502-01 or -00 model amplitude calibrator?   Mine's Ne2 Neon on the underside is nearly black, and I would like to know if this is to be expected over the life of these units, or if there is likely something in there in need of repair. My unit works, but I'm just not sure if that means there is some funkyness going on in the power supply.
I also need to find the value of the dead cap in my 184, but I can manage that from the manual.

But anyway, back to the 485 - I ordered the 2n3055 and 2n3053. Both RCA.     Even if one of them is still good, I want new in there since they are constantly under a good bit of strain.  After replacing those and the 47uf cap, the entire primary side of the HV transformer will have new semiconductors.

[tautech]

TP86 WF would worry me, it's not jitter, there are 2 distinctly different waveforms. 

Waveform on the primary around Q1108 are generally a sinewave @ 20-40 KHz and amplitude close to the supply voltage of 15 V.

Have you seen Dave's vid of cal  he did on a Hameg:
http://www.eevblog.com/2013/08/03/eevblog-502-19-hameg-analog-oscilloscope/

[Addicted2AnalogTek]

Yep I saw Dave's Hameg cal video and also W2AEW's Heath cal vid.  Dave's is more of a quick adjustment. W2AEW's is an actual cal but on a much, much less complex scope.   Both are/will be quite helpful, and it's probably a good idea to just go thru and do some quick adjustments to this scope once this CRT issue is solved and get it a little bit tighter to spec to aid in the rest of the checking/troubleshooting.  I still don't have a ceramic screwdriver yet.... 

yeah, everything in the HV area is and should be running at 50khz.   First, before replacing C318, that second waveform was as pictured..   take a look at what it is now... 
Sorry bout the somewhat fuzzyness.  the camera didn't want to focus.   
The 2nd waveform is different now. It's the same as the correct waveform, but with the unblank amplitude.  The correct waveform changes with the intensity setting as it should, but the second one stays the same no matter what I adjust.  Before, in the other pictures, that second waveform acted the same way, but it looked exactly like the waveform where "300V p-p" is on the schematic.   That is what lead me to believe it was the 47pf cap, but I have no way of knowing without just replacing it since I don't have a 100x scope probe to check around in the HV side. All I can do is poke around with the 80K-6, and I'm not about to hook that into my scope! ha.

[Addicted2AnalogTek]

Ok so there's a little more going on than I thought.  I'm fairly certain that Q1108 is loading the 15 unreg and pulling down the emitter of Q109, and Q109 is pulling down the 110 unreg.   Q1108 probably aided in killing Q109. 

I decided to start checking supply foldover again, and all of the supplies are folded.  Disconnecting the vertical preamp board eliminates ALL of the over-current issues. The rectifiers run a bit warm still, but nowhere near as hot.   So now I also have that to track down and fix. From what I remember about the waveforms when I first started, it's going to be in the switching logic.