It's not always the capacitors: LeCroy LT342L Repair

[kc7gr-15]

In fact, in some cases, capacitors are the last thing to look at.

I picked up a 'tech special' O-scope last month, a LeCroy LT342L. Nice unit, 500MHz bandwidth, 2-channel, LCD display, lots of nice math and analysis features, etc. The 'tech special' part came in the form of it being (apparently) completely dead when plugged in and power-up attempted.

I was on my annual scrounging road trip at the time (just after the Labor Day weekend, September), staying with a fellow techie in San Jose (SF Bay Area). After we brought the beastie back to his place, we spent a good chunk of the evening troubleshooting the power supply. We both assumed, at first, the thing was plagued with the usual leaky electrolytic caps.

To paraphrase Johnny Carson, "Wrong, isopropanol-breath!" Every single electrolytic in that scope was completely intact, no signs of leakage or damage at all. Not only that, each and every one bore two manually-applied color ink dots on top (see the photos), which told us the caps had likely been hand-selected and burned in by the manufacturer (no surprise with LeCroy).

To add to the mystery, we could hear the upscaling whine of a switching supply firing up every time we plugged in the AC cord. And, when the front panel power switch was engaged, the fan would twitch in its proper running direction.

Thanks to the service manual (which I found online, my eternal thanks to whoever scanned it), we found the power supply was actually two isolated supplies on one board, and the primary one (which supplied 27VDC for control and startup purposes) was perfectly fine. It was the secondary supply (the one which provides power to the scope's guts and the fan) which wasn't running.

Further troubleshooting led us to suspect the PWM controller chip, a hard-to-find Mitsubishi part, had gone bad. I found some from a China-based source on Ebay and ordered them, then put everything aside for future work.

Fast forward to early October (yesterday, in fact). The chips finally arrived, and I replaced what I thought was the bad one. And, as you probably already guessed, I had the same symptoms. 

After some creative contributions to the world of invective, I had a brainstorm: Neither myself, nor my friend, had done a comprehensive check of ALL the power supply output points at any point during the initial troubleshooting run! 

I wasted no time in doing so -- And that was when I found everything was working EXCEPT for the +12V output which supplies both the fan and the +12_DIGITAL rail in the scope's guts.

The culprit turned out to be the component in the middle photo: A 7812 three-terminal regulator which had, for whatever reason, a dead output side. Three bucks and a bit of soldering rework later, I had the result in the final photo: A now-happy scope, displaying the output from a Tektronix quick-test board.

Lessons learned: ALWAYS check every single output from a suspect power supply FIRST. And don't automatically assume the problem is bad caps!

Happy tweaking.

[Old Don]

Some 40 years ago I was out of town visiting a friend and he wanted me to fix his tube stereo amp. One channel had 60 cycle hum. All we had for test equipment was his multimeter. So I swapped tubes and the problem stayed with the same side. I jumped upon the capacitor horse and rode it to death.  All the caps were replaced and there still was what sounded like 60 cycle hum coming from that side. About the only thing left was the plate and bias resistors and I changed out one or the other (lost the details in the years since) and problem solved. A %@#$%$#@% carbon resistor was breaking down under voltage. So yes, it's not always the cap's.

[eb4eqa]

Well done and thanks for sharing. That is a great scope, with a few annoying things against usability but a great scope.

Regards,
Roberto

[carl_lab]

Every single electrolytic in that scope was completely intact, no signs of leakage or damage at all. Not only that, each and every one bore two manually-applied color ink dots on top (see the photos), which told us the caps had likely been hand-selected and burned in by the manufacturer (no surprise with LeCroy).

Are you sure about that?
Those marks are often applied in production (visual polarity check).
Second color maybe check from service etc.

[voltz]

agreed. Its not always capacitors.

And this shows why its important to actually follow a procedure to locate faulty sections first rather than just dive in and knee-jerk start replacing caps. There seems to be a recurring trend on this forum for 'change all the capacitors'.. Thats not always the right thing to do. Never mind the cost of doing so, but the time involved is sometimes not cost effective.
I think its important to 'know' when to change a cap rather than 'just do it anyway'. Following a procedure (we call it fault finding you know) to isolate the fault first - then determine what parts, not just caps, need replacing (or not).

Obviously, if the cap is leaking fluid, bulged at the top or generally looking out of shape, yes replace it. I think you did the right thing here, those caps look absolutely fine with no actual visual problems, its safe to proceed with gentle powering up. A variac or simple mains lamp in series with the mains is perfect for slowly bringing up the voltage across old capacitors.

Nice scope btw!

[panoss]

In fact, in some cases, capacitors are the last thing to look at.

I picked up a 'tech special' O-scope last month, a LeCroy LT342L. Nice unit, 500MHz bandwidth, 2-channel, LCD display, lots of nice math and analysis features, etc. The 'tech special' part came in the form of it being (apparently) completely dead when plugged in and power-up attempted.

I was on my annual scrounging road trip

You got it for free?
Nice job, I wish you have fun with it.

What is a 'scrounging road trip'? You go to places where you can get electronics that others have thrown in recycle bins?

[kc7gr-15]

Oh, no, not a freebie. I got it for $200 because of its issues. Normally, that class of LeCroy 'scope would probably go for $700-$800.

As for the road trip: I grew up in the Bay Area (Berkeley, to be exact), with easy access to lots of electronic surplus sources all over the place. When my then-girlfriend-now-wife (of 23 years) and I moved to Washington state in 1993, I quickly discovered a drawback I hadn't thought of: A severe shortage of said surplus sources (there I go, being alliterative again...)

Although the situation for used electronics has improved in WA state, over the last couple of decades, the Bay Area (silicon valley in particular) still has an excellent variety of electronic swap meets and electronic surplus places (I have a whole web page devoted to such things... See http://www.bluefeathertech.com/technoid/swapindex.htm) It also has a fellow techie-friend who I stay with, and who is very much like the brother I never had growing up (in a good way, thankfully). 

So: Once a year, I do a week-long road trip back to the Bay Area while my lady watches over things at the house. Been doing it since 1994, only missed a single year to date (long story), and I don't see myself stopping any time soon.

I hope this clarifies things for you. Keep the peace(es).

[Armadillo]

Its not always the capacitors but sometimes we do hope that they were.!.
In my case;
It was that infamous autorange circuit that brought a few components with it, and sadly also the Green potted A100004 module.
The picture showed the tunnelling works done just to get to the components. I would definitely say that this repair is not commercially viable because it too time consuming.
For those lecroy oscilloscopes that use the autorange circuit, I would advise to remove it as soon as possible especially those using AC230 power line.   

[scopeman]

I had exactly that failure in a LT344L that I picked up on ebay a few years ago.

I meant to go back and look at that power supply design and see what capacitive load is on that supply. It may need the diode from input to output to prevent the output capacitor from blowing the regulator.

Sam
 

[jaycee]

It was that infamous autorange circuit that brought a few components with it, and sadly also the Green potted A100004 module.
The picture showed the tunnelling works done just to get to the components. I would definitely say that this repair is not commercially viable because it too time consuming.
For those lecroy oscilloscopes that use the autorange circuit, I would advise to remove it as soon as possible especially those using AC230 power line.   

Makes me wonder, if they are depottable... has anyone reverse engineered one ?

[Armadillo]

It was that infamous autorange circuit that brought a few components with it, and sadly also the Green potted A100004 module.
The picture showed the tunnelling works done just to get to the components. I would definitely say that this repair is not commercially viable because it too time consuming.
For those lecroy oscilloscopes that use the autorange circuit, I would advise to remove it as soon as possible especially those using AC230 power line.   

Makes me wonder, if they are depottable... has anyone reverse engineered one ?

Yes, someone did. As I know, there are still few errors, despite but usable enough.

[jaycee]

I was thinking along the lines of, rather than depot and repair a faulty module.. produce a new one on PCB as a replacement
There are similar "hybrid" modules in some of the Commodore Amiga computers for example, in that case it's a simple video DAC. Those were made at the time when 1% resistor tolerance needed a special process - of course now easily doable with off the shelf SMD resistors. I and others made a small PCB for a modern equivalent module which drops into place

[Armadillo]

Most had this idea, until I routed the board did I realize that I cannot physically shrunken it to the size I wanted.
Just don't underestimate the board, the resistors are thin membranes hidden below the ICs.
You can see those darker streaks on the board from the photo I attached.
But, I didn't pursue further doesn't mean you cannot.
Do update us your good news.
 

[shakalnokturn]

Those modules are hell to de-pot...
It has been reversed engineered, substitutes have been made to scale (I have a 9354 scope running one).
I put many hours contribution into making it happen.

[Armadillo]

Photo to demonstrate?

[Edit: I read one post where you bricked the resistors while depotting  ]

[shakalnokturn]

I sure slightly altered some of the resistors de-poting but the guessing was good enough to get something working.
No one else has been arsed to de-pot another to get better measurements anyway....
I'm not at home for Christmas but I'll get a couple of photos out some time. The design uses mainly 0805's

[Armadillo]

So far I have not seen anyone posted a to-skill replica [or board] as yet.
Looking forward to your photo posting.   

[IanMacdonald]

The common faults seem to change over time. Which might be due to component age, or it might be due to poor quality products coming on the market which were not around a decade or two back. With electrolytics I would reckon that is definitely the case.

A different situation exists with paper capacitors, which were quite reliable in their heyday but seem to have a finite -if relatively long- lifespan, and which most have now reached.

[scopeman]

On the Green module please check the Yahoo! or Groups.io files section where you should find all the files and data needed to create a replica of the module in a DIY PCB assembly. Shakalnokturn and yours truly put many hours into this and I believe that his probably has many 10's of hours on it but I am open for correction on the hours.

Sam
W3OHM
LeCroy_Owners_Group on Yahoo! Groups
LeCroyOwnersGroup on Groups.io

[shakalnokturn]

Hello Sam!
Yes, 10's of hours put into the reverse engineering, the test scope also has 10's of hours running on the replica module by now. Thanks again for your contribution.

So here are the photos, just to show how close size is to the original, not quite to scale in fact, could be done though...

The one running my 9354 is wrapped in kapton tape for dust protection.

[scopeman]

Looks like one of my LPKF milled out prototypes!

Sam

[shakalnokturn]

It is one of yours Sam! I've not needed more PCBs made yet.

Paul