Tektronix 468 repair/refurb

[JacquesBBB]

Speaking of  C162, what is this third lead for ?
Are these two caps ? Or one of the lead is doubled ?

[Addicted2AnalogTek]

Speaking of  C162, what is this third lead for ?
Are these two caps ? Or one of the lead is doubled ?

It's one single cap, and it appears to be just a radial cap with a 3rd lead soldered to the top of the can.
I'm not really sure what the 3rd lead is for, but I do know that the can of a capacitor can also act as a plate and store charge.  On the schematic, it only shows a standard capacitor.  I haven't taken the board off to look at the traces, but my guess is that it's just extra grounding for added stability.
I just know that these caps are very hard to find replacements for unless specially ordered, and are very expensive.  I'm hoping that I can replace it with a regular radial cap with a 3rd lead soldered to the top of the can.
Here's a link to the VPR capacitor catalog info. 

http://www.cde.com/resources/catalogs/VPR.pdf

[JacquesBBB]

I have seen the catalog, but could not understand really what was the meaning of this pin.
I am asking that because I  might be very soon confronted to the same problem.
I just got a tek 468 from my favorite dumpster and it is right now in my trunk.
I have not done anything on it, and not powered it yet. All that I should start to do this week end.

[Addicted2AnalogTek]

When I get everything working in the non storage and start poking around replacing caps in the storage area, I will post some info about those caps and their connections, and also a link to the replacements.

[Addicted2AnalogTek]

The third lead on those capacitors doesn't appear to connect to anything.  It's a 4 layer board so it's a bit hard to tell, but I stuck a high-powered light to the board and there doesn't appear to be any traces connecting to it on any of the layers.  I checked around a bit with an ohm-meter as well and just got the typical 5Mohm countdown from the capacitor charging to one of it's other leads.  So it really doesn't seem all that necessary to even have the 3rd lead, although I expect it was used for a reason..... If I can't find out for certain what it's purpose is and if I can omit it, I suppose I will try and find a suitable 3-lead replacement, or tack a lead to the can of a radial, since that seems to be exactly what it is.   I actually think that is what was done when this capacitor was previously replaced, since it's a 672D and not a Mallory VPR like the 540uf on the board.

I think I'll just avoid using storage mode until I replace it since its reading over 43 ohms ESR.   I'm about done replacing all the tantalums and smaller electros now, so hopefully it has made a dramatic difference in the current consumption. I figure that the enamel on the transformer wires would have been fried off if it would have gotten any hotter...

[Addicted2AnalogTek]

I'm finally getting somewhere with this now... I think I may have tracked down the source of the CH2 vertical position instability issue.
There are several problems with this scope, so it's not an ideal first troubleshoot/repair piece of gear, but it is one heck of a learning experience!!

Here is a short video that shows a sample of the signal I get from pin 1, 2, and 3 of U341A and the B and E of Q350.  It gets a whole lot worse than what my scope screen shows.
I traced it all the way back to here from the Vertical Output Amp, and this is where it is the worst.  I don't see any difference between CH1 and CH2 beyond this point, but the signal back farther is so small that I can hardly view it at 5mv/div.



Anyone want to take a guess for which one is the rotten egg? Is it U341A, Q350, or neither??

[tautech]

Is the jitter on the base of Q350, that will confirm Opamp is the culprit.

[Addicted2AnalogTek]

Is the jitter on the base of Q350, that will confirm Opamp is the culprit.

yeah the jitter is on the base and the emitter - and both inputs and the output of U341A.   I really thought that I had it nailed, but I replaced the opamp and it's still there.  It seemed a tiny bit less pronounced, but it's definitely not by any means gone. 
 There is a lot of HF noise throughout all of the vertical section on both channels, so if it is a small oscillation in the signal that is being amplified by Q350/U341A, it's going to be nearly impossible to track unless I can solve the noise problem that covers all the vertical.
After failing to fix it by replacing the opamp, I went back directly to the input directly behind the attenuators to see if I could find ANYTHING different between CH1 and CH2, and there is a bit of a voltage difference there as well as throughout all the vertical boards. But as for AC signal, there isn't anything noticeably different from CH1 to CH2.  The voltage difference starts directly after the 6.2V Zener at VR388, so I put new ones on both channels - and no difference again.
All of the vertical stuff has HF oscillations, the chop clock has oscillations, the CRT circuit has issues (maybe Q109?) and who knows what else is wrong.  I'm starting to think I should just keep this as a parts scope and move on...

[Addicted2AnalogTek]

Oh, and I forgot to mention - I started pulling up leads and checking the resistors around U341... R442 increases in resistance when I heat it up with the iron. Isn't that the opposite of what is supposed to happen?   I checked a couple other new ones I have, and they drop when I heat them.

[Addicted2AnalogTek]

well I fixed it.   The turd was the .1uf 50V MLCC cap on the non-inverting input of U341A. It fell apart when I was pulling the lead up to check it.  The lead fell off.  I stuck it back in to test it anyway and it was over 50%. (156nF) .    So one problem down, and who knows how many more to go!   

In hidsight :  When the opamp swap didn't fix it,  it had to be something between the position pot and the non inverting input, since there was that noise on it, and no way for it to get from the inverting input or output back to that pin.  So it had to be either the pot, those resistors, or that cap. I really didn't expect to have any ceramic caps that are bad, but now I know that when they go bad, they really go! 

[tautech]

well I fixed it.   The turd was .....

 
Yes with older gear expect any component, tests and faultfinding will point you to the suspect area then the rest is up to you.
Thanks for sharing.....everybody learns. 

[MarkL]

I'm finally getting somewhere with this now... I think I may have tracked down the source of the CH2 vertical position instability issue.
...

This op-amp area was the first suspect in post #8.

  https://www.eevblog.com/forum/repair/tektronix-468-repairrefurb/msg667054/#msg667054

[Addicted2AnalogTek]

I'm finally getting somewhere with this now... I think I may have tracked down the source of the CH2 vertical position instability issue.
...

This op-amp area was the first suspect in post #8.

  https://www.eevblog.com/forum/repair/tektronix-468-repairrefurb/msg667054/#msg667054

Yes it sure was - The reason why I missed it is because I was following the manuals testing procedures. I only had input into channel 1, I was looking mostly at the waveforms as shown in the diagrams, and looking for inconsistencies in those waveforms.   It took a little while before I realized that the manual and it's test procedures are only useful to a certain extent.  From there you are on your own to figure out (if you don't already know) what to do in order to track down the specific type of issue you are looking for.
I also was in my infant stage of knowledge in using a scope. Didn't even know how to use the delaying time base.

I'm a first timer on a steep learning curve who is also fairly new to circuitry in general. This whole scope repair experience is what I am using to learn how to do all this, and also as an aid in learning how circuits/components function.

This is by no means over, though.  There are still several more problems and PSU loading going on.  I'm fairly certain that Q109 and at least one of the .0012uf caps need to be replaced, and a 22Mohm resistor that measures 25Mohm. ( waiting for delivery ).  There is some issue with the signal going to the trigger gen also. When I did some checks on A and B trigger, the only way it triggers correctly ( from a - point to a + point on the level ) is if I have it on HF reject.  It mostly has issues when CHOP is selected, so I know there's something up with or interfering with the chopped clock.  Something on the vertical preamp board is dumping oscillations into the -8 rail.   Who knows what's related to what here, and only 1 way to find out...  and ONE AT A TIME this time!! Those are just the things that I've noticed so far. 

A portion of the oscillations I was seeing were from the Wavetek 145, so I switched to the Tek CFG253 for now.  I poked some capacitors in the 145 and picked a handful of questionable electro's to replace, and if it works - great. if not - I'll deal with that later.  I'm not spending more than an hour on it until I either finish or give up on the scope.  My 166 needs help, too. Positive going waveforms have a neg offset, so I'm not using that. Too bad I bought everything except for a 106!! oh well. At least I've learned a bit and am back on the right track   - and I definitely appreciate all of the help and pointers!

[Addicted2AnalogTek]

found the source of the additional noise on CH2.  There is also a little on CH1, and I'm betting that replacing the R-C behind the attenuators on both channels solves that.    C3042 is a bit rough and causing some oscillations. I'm going to replace the R3042's as well since the cap is soldered directly against the body - It might not last being heated up 4 times.

I figure I should work from the input signal back so I don't get any more false ideas about what parts are bad. I'll get my signal sorted so that my waveforms and voltages match the manual spec and work it like that. 

[Addicted2AnalogTek]

Both  R3042's were bad - wayyyy out of spec! 
Oddly enough, the caps are fine as far as I can tell.  They are 1nf +100-10%.  Replacing them anyway.
Chan 1:  R3042 = 731K ohm   C3042 = 1147pf
Chan 2:  R3042 = 736Kohm   C3042 = 1125pf
The resistors are supposed to be 470Kohm 5%, so that means they are both out over 30%!
I thought that maybe there was something I overlooked regarding the values (since it's a Heerenveen scope Opt 5), but no.... the color coding is correct.  Yellow Violet Yellow.

[tautech]

OLD metal film resistors?

[Addicted2AnalogTek]

actually no. they are 1/8W comps.  The 30ohm ones behind them are films but they are fine. the parts list says those are supposed to be comps as well, so I added that to the list.   For now I have 1/4w films and 102K 2kv caps in as temp replacements so I can keep moving. I used ones that are within 2% of spec.

[Addicted2AnalogTek]

I'm close to finding what is pulling down the rails on the vertical board.  it is something in between test point 7 and 9 on CH1.   it may not be the only thing, but it's definitely part of it.  there is 100mv extra on the TP9 waveform and 13mv DC noise where it shouldn't be. Also the -8V rail is 50mv lower there than anywhere else.

See any reason why I shouldn't use all comp resistors in my repairs except for where they are wire wound?

[tautech]

For those in the signal path I would probably keep to spec (  ) all others metal film other than WW of course.

Others might like to chip in with advice on this matter.....

[Addicted2AnalogTek]

Ive found quite a few metal film caps that must be replacements since the manual says they should be comp resistors.  I didn't know if comps would be more reliable or not. While looking up info on the CRT circuit I had read a comment someone left stating that said person should replace any bad caps in the "Focus" section of the CRT with composition resistors regardless of what the manual says - so I thought maybe that means they are considered more reliable in gear such as this and figured I'd ask.   One can never go wrong sticking exactly to spec, though, so if I don't find or get any definite feedback confirming my thought, I'll just stick with spec.

It seems that these axial MLCC caps are going to be a major culprit of many of my problems.  I've found 3 on channel 1 alone that are way out of spec.  I'm assuming their ESR is quite high as well (I can't get reasonable results using scope/FG below about 1uf).   C177 in the NORM trigger section (diagram 3, 1-L ) is the most recent.   0.1uf reading 0.13uf.    And I've got the hang of finding the ones directly off the rails (most are) using just a DMM.  Actually harder to find some of them with the scope since I can hardly see the noise riding the waveforms.  If I'm looking for a bad cap off the 5V rail, I just go thru and find where that rail is the lowest, then activate that circuit and find exactly what cap has the lowest voltage at it's 5V rail side. etc etc.   

Seems there's a trend of 0.1uf's that are bad.  Maybe my scope got lower quality batch.

[tautech]

Remember those in the Focus section are normally dividers subjected to HV, often high Ohmic values and IME fail or drift in value......they are subjected to some stress.

[Addicted2AnalogTek]

good point.   I did measure some comps that have drifted (such as the 22Mohm that is now 25Mohm).  I don't want to continue to pay multiple shipping charges since I now know that I am going to need a variety of parts (of which I will definitely be buying a bit extra!!!) so I am replacing what I have replacements for or that are bad enough for me to make do with what I've got. Noting them and adding them to the order list. 
I did notice that using metal film instead of comp for those 470K inputs isn't going to be working out. The signal is clean before those, but directly following them the peaks are slanted.  Maybe it's the higher voltage caps, the resistors, or the combo, but I can tell that the RC is causing overshoot on the rises and falls.  I just hope that the caps I find for replacements work well, otherwise I will be trying again since it's a critical part. If the input signal is wrong, everything is going to be wrong. 

It's incredible how many of those little MLCC's are bad..    I replaced all of the tantalums - dipped, 173Ds, 150Ds, and all of the small electros, so I won't have those to deal with.  Following all that, I noticed a tremendous drop in the amount of heat on the transfo and rectifiers - and also various components throughout. 

I guess there is a reason why almost every single resistor that the manual specifies as comp have been replaced with metal film?  It appears that someone has systematically went thru and replaced them all at once..  the soldering is professional, so I guess I should keep to that and where there is currently a film resistor, and it needs to be replaced, replace it with another film?

[tautech]

It's incredible how many of those little MLCC's are bad..    I replaced all of the tantalums - dipped, 173Ds, 150Ds, and all of the small electros, so I won't have those to deal with.  Following all that, I noticed a tremendous drop in the amount of heat on the transfo and rectifiers - and also various components throughout.

I guess there is a reason why almost every single resistor that the manual specifies as comp have been replaced with metal film?  It appears that someone has systematically went thru and replaced them all at once..  the soldering is professional, so I guess I should keep to that and where there is currently a film resistor, and it needs to be replaced, replace it with another film?

I'll consult my Dr. EE mate on this matter a little later tonight and report back.

[tautech]

I guess there is a reason why almost every single resistor that the manual specifies as comp have been replaced with metal film?  It appears that someone has systematically went thru and replaced them all at once..  the soldering is professional, so I guess I should keep to that and where there is currently a film resistor, and it needs to be replaced, replace it with another film?

I'll consult my Dr. EE mate on this matter a little later tonight and report back.

Well HOLD UP!
As I suspected and has been confirmed there is EXTREMELY good reason why composite resistors are to be used.
Inductance, or more correctly lack of.
For any in the signal path, inductance will impact on frequency response. 

So, to repair to spec......best you keep to spec. 

[Addicted2AnalogTek]

I guess there is a reason why almost every single resistor that the manual specifies as comp have been replaced with metal film?  It appears that someone has systematically went thru and replaced them all at once..  the soldering is professional, so I guess I should keep to that and where there is currently a film resistor, and it needs to be replaced, replace it with another film?

I'll consult my Dr. EE mate on this matter a little later tonight and report back.

Well HOLD UP!
As I suspected and has been confirmed there is EXTREMELY good reason why composite resistors are to be used.
Inductance, or more correctly lack of.
For any in the signal path, inductance will impact on frequency response. 

So, to repair to spec......best you keep to spec. 

Ahhh yess.... that definitely explains what I am seeing behind the R3042's!  INDUCTANCE! That's what is causing the "overshoot" on rise and falls!   Like I've said, I always prefer to stick to spec.... so now I'm going to need to buy a stockpile of comp resistors to fix what some dumdum did to this scope!!!!!   Oh well, at least I'll finally have some new comps, and 1/4w resistors in general. 

I'm having a very hard time locating exact replacements for those C3042's.  They are 0.001uf 200V +100-0% Ceramics.  I was thinking about using Orange Drops to replace it - but aren't those a type of stacked foil film/polypropelyne?  So I better stick to ceramics only for signal, I think anyway.   
I am not sure if film foil caps are all inductive or just some, but I found these as possible replacements.  Let me know what you think.

http://www.digikey.com/product-detail/en/WMC2D1K-F/338-3227-ND/1930379
Datasheet:  http://www.digikey.com/product-detail/en/WMC2D1K-F/338-3227-ND/1930379
---It says those are specifically for high pulse applications, which leads me to believe they might give me a sharp, clean signal. Non-inductive of course.

http://www.digikey.com/product-search/en?pv16=2&FV=fff40002%2Cfff8000b%2C340040%2C38000e%2C1140050&mnonly=0&newproducts=0&ColumnSort=0&page=1&stock=1&quantity=0&ptm=0&fid=0&pageSize=50
---The mil spec MLCC option

http://www.ebay.com/itm/LOT-24-NEW-SPRAGUE-192P102X9200-CAPACITOR-200V-0-001UF-D478252-/381077387116?pt=LH_DefaultDomain_0&hash=item58b9fb776c
---The Sprague 192P option
And of course there's the Orange drop option.

From the info I have found from part number searches, it appears that the originals are single layer ceramics, of which I cannot find.  I have found some for $18 a piece off ebay that are single layer.... No way I'm paying $36 for two ceramic caps!!!
I wouldn't be expending all this effort and asking all these questions about these if they weren't critical parts.....