What's the likelihood of virtually all the caps being dead on a Tektronix 2235?

[requim]

Looks like I tracked it down.  Well credit goes to an earlier poster.  It appears the mosfet Q9070 is toast.  With any luck that's the only thing wrong with this.  I just placed an order w/ Digi-Key so with luck I'll have the parts by Tuesday or Wednesday.

I'd still like to know how to disconnect the mains for future reference.

Oh I do have one question.  So when I was putting all the caps back on the board I realized something was amiss.  In the schematic and the parts list it lists C956 as a 270uF Electrolytic Cap +100-10%, 40V.  However I didn't have that cap in my bucket of parts from when I stripped the caps off the power supply.  I skipped it and when I was finished with the rest of the caps I still had a single 840uF 10+1000%, 12V Electrolytic cap.  I rechecked the board and where I stored the caps several times and couldn't find any empty spots on the board so I assume the 840uF was in C956.  Since I did not have a 270uF cap I put a  330uF 50V cap in its place.  My question is this -- Can someone please look at the schematic and tell me whether the schematic is wrong or whether I should double/triple check the caps on the board again?  I googled around and didn't find anything on it.  I do know that there are several misprints on the schematic from previous searches but this doesn't appear to be one of them.

Thanks!

[SgtRock]

Dear Requim:

--Doing the math, any cap that ranges between 243 uF and 540 uF will meet the specifications. So if you measure your 330 uF cap, and it measures 541 uF, is does not qualify.

--If you cannot measure the 330 uF cap, you should by a 270 uF cap. I like to measure everything before installation, as I have, from time to time, had new components that were out of specification. Also, you might want to review my earlier remarks. I hope someone can verify the value from the schematic for you.

--I hope this helps. That is a great scope.

"My favorite things in life don't cost any money. It's really clear that the most precious resource we all have is time." Steve Jobs
 
Best Regards
Clear Ether

[requim]

@SgtRock - The Panasonic 330uF 50V cap I put in is spec'd +/- 20% so assuming Panasonic doesn't have any major issues with quality control the cap should be well within spec.  I'm just new to this whole game so I'm extra paranoid about substitutions. In the future I'll verify the values of the caps before soldering them.

[SgtRock]

Dear Requim:

--The Panasonic 330uF 50V cap +/- 20% should be just fine, barring the one in a thousand or so chance of a bad one.

--I am sure everyone would like to see some pictures, if you
have the time and a camera.

"I have had my results for a long time: but I do not yet now how I am to arrive at them."
Carl Friedrich Gauss 1777 1855
 
Best Regards
Clear Ether

[requim]

I have both and will oblige.  Didn't seem to be much interest in my posting of the Lambda LC-512 I tore down and restored so didn't see much point posting photos of the scope. 

I also tore down and fixed a Harman/Kardon receiver and HP e3610a DC Power Supply but didn't take photos because I didn't think anyone would be interested.  I am currently working on two HP 204C Sine Oscillators and will soon be working on a Agilent 6611C DC Power Supply and HP 34401a DMM.  If you want pictures posted of any of my current or future projects just let me know which ones and I will oblige.  Oh and I will probably be taking apart my old Yamaha receiver to figure out why the headphone jack suddenly went down a large number of decibels.

[FenderBender]

Yeah as others have said, some of the old style wet tantalums tend to go bad. I've seen nothing but Japanese or American caps in Tek stuff, so shouldn't be an issue of the electrolytics.

[requim]

Here are some pictures as requested:
















The culprit:




If you want to see them in full-resolution you can find them here:

http://www.flickr.com/photos/54406519@N08/sets/72157627955284523/

[hacklordsniper]

I have both and will oblige.  Didn't seem to be much interest in my posting of the Lambda LC-512 I tore down and restored so didn't see much point posting photos of the scope. 

I also tore down and fixed a Harman/Kardon receiver and HP e3610a DC Power Supply but didn't take photos because I didn't think anyone would be interested.  I am currently working on two HP 204C Sine Oscillators and will soon be working on a Agilent 6611C DC Power Supply and HP 34401a DMM.  If you want pictures posted of any of my current or future projects just let me know which ones and I will oblige.  Oh and I will probably be taking apart my old Yamaha receiver to figure out why the headphone jack suddenly went down a large number of decibels.

A teardown which will not be interesting does not exsist 

[SgtRock]

Dear Requim:

--I agree with Hacklordsniper, and uninteresting tear-down does not exist. And in the case of very popular pieces, like this one, people may even, go back some time later to view the pictures, so they can confirm things about their own equipment. Hence, it is always nice to have:

(1 Exact name of the unit in the title line, as you did here. So people can easily find it, sometimes much later, maybe even yourself.
 
(2 Lot of pictures. You could increase the resolution a little bit on yours, if you wish. DJ does not stint on bandwidth, and I think, he likes to look at the pictures himself. Also a bit more light, next time would be nice.

(3 Also, a complete list of all parts, including fasteners, along with multiple sources. Had you didn't I. Snork!

--I sincerely appreciate the trouble you have taken, and I look forward to seeing her up and running.

"You know that I write slowly. This is chiefly because I am never satisfied until I have said as much as possible in a few words, and writing briefly takes far more time than writing at length."
Carl Friedrich Gauss 1777 1855
 
Best Regards
Clear Ether

[requim]

@SgtRock - I posted the full resolution images to my flickr account.  I didn't think most people would like it if I linked in to 10 2 mb photos into the page.  Not everyone has uberfast bandwidth.  This site restricts images to 1 megabyte in size and 10 attached images per post.  So in some ways it was easier to just post it to flickr and link to it.

As for lighting, I can't do much about it for the time being based on where I'm staying/living.  But I agree.

[SgtRock]

Dear Requim:

--I stand corrected. Very satisfactory.

"Reality is merely an illusion, albeit a very persistent one."
Albert Einstein 1879 1936

Best Regards
Clear Ether

[lowimpedance]

Hi Requim
 When servicing any gear where removing a number of components or connectors /wires etc I either draw a rough pictorial style diagram to show where the connections/components go with colors or values etc, even when I have a service manual. Or these days a number of photo's is allways useful.
 One other thing I have one of these scopes and have on two occasions had the IEC inlet filter smokem up. The original spec'd tektronix component was not rated properly for the mains here in Australia, (240V, component rated at 220 or 230 from memory so marginal), and was a known problem. I have replaced it with my 'chosen' substitute with no further problems.
 The thing to note with service manuals even ones still on the manufactures websites for obsolete gear is changes may not have been reflected in those versions of the manual.
Case in point I have a HP1741 CRO for which I also have the manual that came with it when new and there are many changes to the circuits either in values or complete new designs which for some have been supplied with manual changes sheets however the description of one key cct that's been completely revised has not been changed to reflect the new cct design.
And yes downloading the manual of the Agilent website does not even show some of the component value changes let alone give any details to the newer designed cct.
 Sorry to ramble but the point is whats to say that the cap you had left over was not the correct one for the position you indicated in the supply.

Incidentally in my 2235a manual C956 is a 10uF cap, C957 is a 270uF cap on the schematic, both on the 30v line forming a PI filter with the 33uH L956.
(If I am looking at the right part of the cct that is!!!!)
I also looked through the manual change info at the back of my manual but did not see any reference to changes of this component number.

Anyway was it only the mosfet switch?? , nice CRO which you will enjoy using for years to come.
regards
john

[requim]

So I put the new mosfet in, braced myself for it, and had a big letdown when the unit, while it didn't short out anything, didn't turn on either.  No lights, nothing.  Could the mystery capacitor be to blame?  I probed a few points on the board and saw current is flowing.  I plan on looking in the service guide and trying to probe various points where they give known ranges/values and seeing if any of them match.

As I said, no apparent short circuits and no blown fuses.  One step at a time I guess. What I am surprised by is that before, when it shorted instantly a couple lights on the board light up ( I was able to capture this using high speed cameras) but now no lights on the board turn on.  I suppose I should check my solder joints for the caps I desoldered/resoldered.

[lowimpedance]

I have not read all the replies, but has anyone suggested to feed 40v dc from the connector T948-3.4 from the pre reg. output which goes to the main switch mode supply (that actually powers all the rails of the cro. That will tell you if the cro itself is working including the main supply and the problem is with the pre reg.
I repaired a 2215 which is similar and I was able to isolate the problem to the pre reg supply in this way (proving the cro worked). So if you have no 40v (or so) at tp940 or at R907, and you have a bench supply that can be adusted to simulate the output of the pre reg you should be able to test whether or not the main supply is okay and indeed the cro is working.
Have you measured that test point and have the 40 or so volts?.

The problem with the pre reg in the 2215 was the transformer t906.

I guess you know to be very careful when working around these area's

John
 

[spec]

I know it's been around 3 years since the last post on this topic but I am intrigued to know if the fault was located and fixed. It seems to me from the posts that the blown power FET may not have been the cause but a symptom.  By the way, Tekfan informed me that, "The difference between the [Tektronix 2235] A and non A versions is an improved power supply in the A version. There's a factory mod for the non A version which involves replacing a transistor with one with better specs and adding a few diodes and resistors. ". This may have some bearing on the matter.

[M0BSW]

Nicely said. However even out of circuit capitance measurment will not tell you if a cap is bad or not most of the time, you need an ESR meter, possibly one that can measure in circuit.

I use one from GME technology and it has proven itself very good

Although by the time you've taken it out you may as well replace it anyway...

Well said, If it was a  refurbish job I would anyway, at least that way you shouldn't have to do a revisit for a long time, if you replace with decent caps.