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LeCroy 7200A Digital Scope Service Attempt

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  • LeCroy 7200A Digital Scope Service Attempt

    Towards the end of working in the lab at BGW Systems, I had bought a used LeCroy 7200A Color Digital Scope, with both a 4-trace and 2-trace plug-in. 2GHz BW instrument. A heavy boat-anchor, to say the least. I did have a wide Tektronix scope cart, so that's where it lived. The cart's front drawer became a nice platform for the computer keyboard that plugged into the back of the scope...any standard keyboard worked for that. And, for hard-copy, I had an HP LaserJet II Printer connected to it. At some point, the CRT image collapsed to about 1/3 the size of the display, and not long after, it stopped powering up. That was close to 20 years ago. It's been sitting in my storage locker on the scope cart since I moved out of Gardena to Glendale, CA. At the beginning of the Pandemic, fearing I was about to loose my lockers and all of their contents, I moved a lot of gear and a good portion of my technical library home. I also brought back the LeCroy 7200A, just in case I got brave.

    I had posted the following image of this in my post https://music-electronics-forum.com/...r/52073-/page3 Covid Shutdown may cause loss of my lab assets.

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    Having received some replies to that thread by Johnathan9192 on 9/13/2020, who also has one of these scopes, his instrument had also failed. He had removed the power supply from the top rear end of this instrument, pulled that apart and replaced the three main buss caps, other main supply caps and a large host of other electrolytic's on the mating PCB....I forget if he had also replaced the large horde of Dip tantalum caps. I mentioned having a decent service manual and all of the documentation I was able to acquire back in the late 1990's and reproduced it all on BGW's nice Xerox color copier, which made gorgeous B & W as well as Color reproductions for producing manuals. I still have that project of reproducing the service manual for him, though haven't forgotten...just procrastinating, sad to say.

    So, I pulled out the power supply from this instrument and brought it into the shop, and took photos of it during the disassembly for my records.

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    Once I got the power supply apart, and noticed the middle Input Buss Cap C24 was cocked to one side, suspecting there was electrolyte spilled out of the cap, I carefully began desoldering that cap. Now, the bottom side of this board, as can be seen in the end view of the assembly, the insulated cover plate slides into the groove of the heat sink, and sits very close to the surface of the PCB, so all of the component leads are trimmed very close to the surface. And, to make life absolutely miserable, the solder pad size on these component leads are woefully too small!! Same problem I find working on Gallien Kruger amplifiers! It took using both my Pace SX70 with a robust Desolder tip along with the edge of a wide chiesl tip in my Pace PS80 Soldering Iron to get enough heat onto the back side of the solder joints to start melting the solder. It also appeared to be the case of the plated-thru hole size was too small relative to the round leads of the buss caps, so I had to rock the cap a bit while desoldering.

    To my horror, when I got the middle cap free and removed it, along came the plate-thru of the Ground terminal still attached to the component lead of the buss cap!! I was a bit more successful on the other two caps C23 and C25, but after removing those, I found what i was dealing with was a multilayer PCB, and the Ground Leads connect to an inside layer!! The plate-thru is gone on the Ground lead all together on C24. I do have continuity on the Ground lead of C23 and C25, and, can get contact to the inside layer of the Ground circuit for C24, but, I don't see a direct path for that during replacement.

    That makes me start considering connecting either an Teflon insulated wire jumper wrapped around the terminal of C24, and connecting the other end to C25. The problem here is....buss cap terminals are extremely short, and, while having measured the ESR and capacitance of these buss caps, not finding any issues with them yet (haven't yet checked leakage current), their exposed lead length is next to nothing. AND, the space between the leads and the insulated cover at peak line voltage.....is very small....looks like 5mm clearance. I gotta think this thru.

    C25's solder pads on both sides are intact. C23 lost the (+) solder pad on the component side of the board, though that may not be a problem...contact is on the large copper plane on the back side of the board. I have mylar insulating tape, 3M adhesive copper tape, Teflon sleeving, Super Corona Dope and some other excellent double-stick Permecell insulation tape on hand. One of these buss caps had a dent in the upper side. And, an odd characteristic I'd never seen before. While I was desoldering the round stud leads of the caps, when it rolled, the insides seem to be grouped toward one side of the can, so it won't just roll, it will come to a stop and rock back and forth due to offset mass inside! Never had noticed that before on other caps....for whatever that's worth.

    So, comments and suggestions on how to recover from the loss of internal layer connection on the middle cap C24's ground circuit would be appreciated. I gather Jonathan9192 survived the hazards of the multilayer power supply boards, as his 7200A Scope was a successful operation.











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    Logic is an organized way of going wrong with confidence

  • #2
    I'm thinking you may need to add jumper(s). I've had to do that in the past with multilayer boards when a battery for memory leaked and damaged through holes. If you have a service manual that shows board layers, that might show where jumpers need to be run. If not, I've found circuit paths using a bright light shining through the board to determine where middle layer traces are. It may be difficult to use that method here because of that large + cap trace that covers most of the board.
    "Yeah, well, you know, that's just, like, your opinion, man."

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