Author Topic: Tektronix 468 repair/refurb (Read 71878 times)

Addicted2AnalogTek · « **Reply #125 on:** June 04, 2015, 02:28:28 pm »

even though the searches for the manufacturers part number turned up info stating that they are single layer type, they physically look like MLCCs. I found a deal on AVX mil-spec molded ones so I bought those - along with all the other MLCCs that I know I need so far. I'm about to place an order for all of the comps that are directly in the signal path from input to vert out and to trigger. That'll be enough for now, and if I need more for the horizontal, I will place an order for those then.

SeanB · « **Reply #126 on:** June 04, 2015, 07:26:00 pm »

Composite resistors will almost always drift higher with age, and as you have found out, you need low inductance film resistors ( the standard one is a spiral cut, so you need a more expensive straight cut one) or use SMD chip resistors ( the largest ones you can get in the value) and solder wire leads on them, as they are trimmed using a low inductance L cut. Only usable where power dissipation and voltage rating is not going to be a problem.

Addicted2AnalogTek · « **Reply #127 on:** June 04, 2015, 08:29:39 pm »

That's nice to know. I didn't realize that there was so much to choosing resistors. I did know that there are application specific types, but didn't really think too much into it. Thanks for the info.

Basically what I am getting is that for the critical circuits (most likely the ones I need to replace, of course) it will be easier to find the best type/build resistor in SMD - and probably less expensive than trying to find thru hole ones like that. Correct?

I've already spent quite a bit on parts and now realize that I could easily be spending over another $100 ( I always buy quite a bit extra of each for later ), so for now I think I am going to stick with manual spec. However I will definitely be researching a bit more into this so I make sure to get the best replacements for any future repairs. Plus I have already ordered a bunch of AB CC resistors. I'll be going thru my 465 after this and sharpening up all the circuits + calibrating, and for that I will DEFINITELY be getting either SMD or straight cut films for the signal path and CRT and any others in need of replacement.

Again - thanks for the info.

Addicted2AnalogTek · « **Reply #128 on:** June 04, 2015, 08:35:23 pm »

Found this:

http://www.eetimes.com/document.asp?doc_id=1278251

and this:

http://www.electronics-tutorials.ws/resistor/res_1.html

Addicted2AnalogTek · « **Reply #129 on:** June 04, 2015, 11:00:46 pm »

every single bad axial mlcc appears to have just turned into a crumbly clay type material around the lead that goes to ground. every one thus far has had a lead come off, and every time it's got a lead to ground that is the one that comes off. I'll bet that it's the lead on the negative voltage rail side or lower voltage side on all the other ones.

Addicted2AnalogTek · « **Reply #130 on:** June 04, 2015, 11:33:23 pm »

I found this listing which might be very useful for not just myself but anyone doing repairs and needing high quality oddball value resistors. The delivery time isn't great but it is compared to other things from hong kong.

http://www.ebay.com/itm/30x-Vishay-Dale-Resistor-1-4W-0-25W-1-Non-inductance-Customerized-Values-/221010258491?pt=LH_DefaultDomain_0&hash=item33753cea3b

Addicted2AnalogTek · « **Reply #131 on:** June 05, 2015, 02:48:06 am »

I'm just going to replace every one of these axial ceramic caps. too many are bad for me to sit here and poke around. I'll be here for a year and I'll end up replacing 90% of them anyway. Fortunately it seems like just about all of the transistors and ICs are functional, so that's a plus. There are some diodes and obviously resistors that are bad as well, but I don't even think that q109 is bad. I'm fairly certain that the noise around it is being caused by bad stressed out off value resistors and bad capacitors. More specifically that 47uf 35V 30D type that I haven't replaced yet....

Addicted2AnalogTek · « **Reply #132 on:** June 05, 2015, 07:27:21 pm »

Quote from: SeanB on June 04, 2015, 07:26:00 pm

Composite resistors will almost always drift higher with age, and as you have found out, you need low inductance film resistors ( the standard one is a spiral cut, so you need a more expensive straight cut one) or use SMD chip resistors ( the largest ones you can get in the value) and solder wire leads on them, as they are trimmed using a low inductance L cut. Only usable where power dissipation and voltage rating is not going to be a problem.

Well since I am replacing all of the caps - and just bought probably 1500 or more NP0 and X7R MLCC's, I might as well buy the good kind of resistors for at least the first stages of the input signal-- where it will really count. What type would likely be best? Maybe the Ohmite non-inductive type? for the first 4 or so that aren't board mounted, I don't want to go with chip resistors and have all that extra lead and solder joints like that. for the rest, I am going to end up getting a big assortment of the biggest SMD resistors I can find.

Addicted2AnalogTek · « **Reply #133 on:** June 06, 2015, 06:38:44 pm »

A bit off topic, but check out this bad capacitor in this Tek Type 184 Time-Mark Gen I got for cheap...

Addicted2AnalogTek · « **Reply #134 on:** June 06, 2015, 08:09:50 pm »

I'm a bit confused here........

On several of these MLCC's the schematic says one value but the parts list says another. For example: A11 Diagram 3 Section 2-D, the schematic says ".01uf" and the parts list says "0.1uf". The installed part is a .01uf, so then that makes me believe that the schematics are correct and the parts list is filled with errors (since there are at least 8 0.01uf caps that the parts list says are 0.1uf caps).

My thoughts are to replace any/all parts that are going to be replaced with the values that were previously installed even if the parts list - or possibly even the schematic - says otherwise. Unless there is some obvious discrepancy where I can tell that someone just "made do" with a close value or something, and in that scenario I will use what the schematic specifies.

Does anyone disagree with this?

PaulAm · « **Reply #135 on:** June 06, 2015, 08:19:37 pm »

I have a 184 with the same bad capacitor. It is not obvious at all from the schematic what would fry it like that.

tautech · « **Reply #136 on:** June 06, 2015, 08:29:24 pm »

Have a hunt for other SM versions, just in case those errors have been corrected.

I'd also assess where/how the 103/104 MLCC's are used, coupling/de-coupling etc......just for a sanity check.
When you think you have it sorted, make a list of changes(if any) for future reference.

Addicted2AnalogTek · « **Reply #137 on:** June 07, 2015, 02:15:08 am »

If I assume that the parts on the boards are all correct, then there are errors in both the schematics and the parts list. Some are just incorrect in the parts list, some are just incorrect on the schematics, and some on both.

However, on the trigger board (where I am working now), C134, C330, and C333 have 0.01uf caps installed. My scope is an Option 5, and on the schematic it has 0.1uf marked for those, and the parts list also has 0.1uf marked. They are all decoupling caps. My guess is that whoever did the last change-out of all of these caps (probably sometime back in the 80's) they used a manual that was not for the Option 5 model, and installed 0.01uf instead of 0.1uf. Since both the parts list and manual have 0.1uf specified, I am going to switch them to that value. I think it's highly unlikely that both the parts list and schematics are both going to be wrong.

For any such future discrepancies like this, if both the list and schematic have the same value marked and it is different from the installed part, I am going to change the value. If I cannot decide, I will refer to other revisions of the manual. As far as I know, the schematics I am using are the most recent. They are a "MAR 1981 Revision" set.

Addicted2AnalogTek · « **Reply #138 on:** June 07, 2015, 02:49:43 am »

Quote from: PaulAm on June 06, 2015, 08:19:37 pm

I have a 184 with the same bad capacitor. It is not obvious at all from the schematic what would fry it like that.

If you repair yours before I repair mine, please let me know if there are any other bad parts on the 1Sec trigger selector circuit. I always thoroughly clean any piece of gear before I do anything else, so I am in the process of doing that right now. (during breaks from replacing capacitors)

I don't think mine has had any repairs done to it for the last 3 decades, or even used for that matter. The oxidation on the switch contacts didn't even have any scratches in it from being pressed, so I'm a bit scared to power it on without checking everything first. I'd like a variac for this but a dim bulb tester will have to do.

Does yours also have the Bulova 10MHZ ocxo in it?

Addicted2AnalogTek · « **Reply #139 on:** June 07, 2015, 03:16:16 am »

It seems like all the manuals I can find online are the have the same revision schematics in them. I think what I will do is just take note of the ones I have changed the values of in case I need to change them back.
I'm really not sure why they are different values from what is stated in the schematics and parts list. maybe it was a bit of a tweak by Tek service guys? I can't see it being a "I didn't have the right part" fix since it's more than just one or two on a board, and it seems to be specific sections that have been changed. for instance, the ones on the trigger board that I mentioned before - those are specifically on the "Option 5 trigger switches and trigger pickoff" schematic that specifies the changes/additions to the circuits. Only those had different values installed.

Maybe I'll just stick with the installed values.

I can't decide. If it was a tweak by Tek, I want to keep it that way. If some doof did it on accident, I want to change it back. .... who the heck knows.

Oh yeah, I want to add that 80% of the MLCCs that I've pulled thus far have fallen apart at the lead on one end, so I think I've made the right decision. I'm sinking way more money into this scope than I could ever hope to get back, but it's worth it for the knowledge - and I'll have a perfect 468 for decades to come!!!

tautech · « **Reply #140 on:** June 07, 2015, 03:24:17 am »

If they're decoupling just install 104's and be done with it.

Addicted2AnalogTek · « **Reply #141 on:** June 07, 2015, 01:45:50 pm »

Gotcha. will do. Thanks

Addicted2AnalogTek · « **Reply #142 on:** June 07, 2015, 08:25:17 pm »

Even this 184 appears to have some corrosion buildup on the neg side of that same capacitor. Obviously it's in much better condition than mine is, but I guess that one has been used a lot less?

http://www.ebay.com/itm/Tektronix-184-Time-Mark-Generator-Great-Condition-/251978155600?pt=LH_DefaultDomain_0&hash=item3aab118650

The third picture is the one I am referring to.

PaulAm · « **Reply #143 on:** June 07, 2015, 09:36:10 pm »

I'm inclined to think that damage is from aging/corrosion failure. There's always a time distribution of failures, so perhaps that's one of the lucky ones.

The inside of test equipment is more related to the environment it's been sitting in than how much it's used. If you have a lot of fine dust in the air, it will filter through any holes over time.

Addicted2AnalogTek · « **Reply #144 on:** June 08, 2015, 01:40:53 am »

some boards have those glass capacitors that look like SMD caps that are formed into a glass package. Anyone have any thoughts on whether I should replace or leave them?
Basically what I'm interested in finding out is if those are a lot more reliable than these MLCC caps I am installing.

Addicted2AnalogTek · « **Reply #145 on:** June 08, 2015, 03:05:36 am »

I think I may have figured out the trick to finding out what part is correct..... It appears that all of the actual part numbers in the list are correct, even though the descriptions and/or schematic may have the wrong values. Most of the time the schematic is correct, though. It's the descriptions of the parts in the list that are off.
For example, on A-29 (TV sync separator) C401 is .01uf on the schematic. That is the installed part value. The parts list description for that part is "..... 0.1UF, 20%, 50V" but the part number is "MA201C103KAA" - stating that the part is a 103, or 0.01uf.
I went thru and checked the same thing for caps on boards I've already done, and it's the same there as well. So for future reference when repairing an old Tek scope, check the part number to verify the value, not the description!

tautech · « **Reply #146 on:** June 08, 2015, 06:32:21 am »

Quote from: Addicted2AnalogTek on June 08, 2015, 03:05:36 am

For example, on A-29 (TV sync separator) C401 is .01uf on the schematic. That is the installed part value. The parts list description for that part is "..... 0.1UF, 20%, 50V" but the part number is "MA201C103KAA" - stating that the part is a 103, or 0.01uf.

That seems to confirm a SM error.
Not uncommon.

Addicted2AnalogTek · « **Reply #147 on:** June 09, 2015, 04:46:34 am »

well, I'm glad I found the "MANUAL CHANGE INFORMATION" In the back of SM Vol2!!!!! Especially since the parts lists and schematic haven't been revised with the changes!!! Things could have gotten a little ugly if I had changed the values back to the old and actually powered it up without knowing any better... I don't even know if my scopes been updated with those changes or not, though, so I guess I have some checking to do. It'll be easy since it's entirely in pieces....

Addicted2AnalogTek · « **Reply #148 on:** June 09, 2015, 05:00:21 am »

Well, I'm thoroughly confused. My 468 has a Heerenveen Holland part number, but it doesn't have the parts to back that up. The board model numbers don't match up, and some of them have part numbers that aren't even in the manual!!

What the hell is going on here?!? I think I picked the wrong scope as a first rebuild! hahaha Oh well, more to learn... I'll get it back to mint one way or another!!

Addicted2AnalogTek · « **Reply #149 on:** June 11, 2015, 02:53:03 am »

This scope has not been updated according to the Change Information that is at the end of the Volume 2 manual. Do you think that I should perform the changes/additions after I have gotten it running properly again? I mean those changes must have been implemented for a reason, right?
According to the model number of my scope and the board model numbers, most of the changes do apply to my scope.

I'll be getting the rest of the caps and resistors I need tomorrow so I will have it back together and running shortly after that so I can figure out what else I need to get. There are still a few caps I want to replace that I haven't ordered yet, but they are easily accessible ones (CRT and Horizontal amp circuit). In my mission to replace all of the small ceramics, I found quite a number of them that were cracked, and several that were very easily broken off. Most of the 50V ones have been replaced with 100V mil-spec ones, so they should last quite some time.


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Author Topic: Tektronix 468 repair/refurb (Read 71878 times)

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