Copper Trace Repair (TDS420 Attenuator Board Restoration) - Page 1
Forum section: electronics / repair

author: Smoky
author_rank: Frequent Contributor
timestamp: February 25, 2020, 04:46:21 am
content: I just wanted to show how I repair a broken copper trace.My first electronic project was the repair of a Tektronix TDS420 oscilloscope about two years ago. Leaking capacitors caused extensive damage to many copper traces on several boards. Many of the broken traces were under voltage regulators and 1206 ceramic capacitors and resistors.Using what I had at-hand to fix the problem, I found a Cardas Golden Ratio capacitor that had very fine strands of copper that made up its leads. So I separated them. They measured .006" in diameter:Using a steel roller that I normally use to laminate countertops with and an autobody dolly, and after a couple rolls, it flattened the strands to ~.0027" or so:Since most of the breaks in the traces were right at the junction of the pads, I'll scrape the solder mask from beyond the repair area and pre-tinned the old traces. After dousing the area with flux, I lay the flattened strands over and beyond the breaks to avoid the repair strands from lifting when I solder the new component in place. I offset the new trace over the old to broaden the solder path but always crossing over where I found the loss of continuity.Here is a recent repair under a 14051B on a spare Tektronix attenuator board, part number 671-1680-05. The component sits nice and flat over the repair. Maybe this is a little nutsy but I'm new to this stuff and I like to try to find my own way, at first! Plus, I didn't want to see a bunch of wires flying all over the place:That little piece of trace is less than 1/16" long. I found 3 more traces broken under 1uf ceramic capacitors nearby. By the way, I read that a .006" wire has an ampacity rating between 340 and 540mA.

author: MrCreosote
author_rank: Contributor
timestamp: February 27, 2020, 06:35:52 pm
content: I see you're having fun with this too!I've done those max density parallel traces - common accident is removing CPU cooler bracket with screwdriver that slips and gouges across the traces.Also USB sticks that get broken/cracked when plugged in.A good stereo microscope is essential.I've used wire removed from solder wick (and anything else laying around)I never flattened, but that is nice however how well does it make a sharp bend on the board?I'll start with a long piece of wire taping in place so one end is where it needs to be for soldering.Then I cut the wire to length.With one end already soldered, it is easy to position the free end for soldering.

author: Smoky
author_rank: Frequent Contributor
timestamp: February 28, 2020, 02:00:12 am
content: In a previous project, I bent the wire before I flattened it. But in this Tektronix scope, the leaking electrolyte vaporizes the trace right where it necks-down at the pad. Not much copper trace in that zone! Usually, I just run a straight piece starting on the pad, over the break, and then onto a clean portion of the trace.In this situation, part of the corner of the pad was gone plus the break at the pad, and since I was installing a new chip, I made sure that the repair trace went beyond the foot of the chip, laid over the break at the pad, but then shot well beyond to a more distant spot on the trace. this gave me a "second" contact point making sure continuity was going to be made when it was finally said and done.The 14051B's came in with an order yesterday from Digi-Key and here it is installed:You can see the repair trace out in front of the foot of pin 7, plus, there is plenty of solder on the remaining portion of the pad underneath. The new chip passes all of its continuity tests tooLong story short around why I'm doing this repair. My restored TDS420 is coming back apart because I'm changing out the capacitors on the backplane board. In the process, I'm going to do a swap-test to see if this repaired attenuator board will pass its self-tests and SPC. If it does, it'll be a confidence booster and a board to stash.

author: Smoky
author_rank: Frequent Contributor
timestamp: March 01, 2020, 03:23:41 am
content: I love it when the weekend gets here. It means "electronics time"! Especially after I've been in a crawlspace all weekI might as well make this a TDS420 attenuator repair thread too. So today, I installed the leaded capacitors in place of the SMD type. I do it this way since the leaking electrolyte eats away at the edges of the solder pads and it makes it hard for an SMD part to be put back.Here is what I also discovered. The four ceramic capacitors nearby get harmed by the leakage too. The one by the red dot you need to pay close attention to. It gives a good continuity response when it's mounted, but when it's desoldered, the pad doesn't! Weird.I replaced all four with Murata 1uf X7R 1206 capacitors, part number GRM319R71A105KA01D (Digi-Key# 490-1800-1-ND). The old capacitors were measured on three capacitor testers to confirm the value.Also, two pass-throughs were damaged. I sucked them out and inserted a 34 awg copper strand and soldered it to the ceramic capacitor's pads.Here's how the finished job looks, and in about thirty minutes, we'll see how well it functions in my TDS420. Stay tuned

author: Smoky
author_rank: Frequent Contributor
timestamp: March 01, 2020, 03:43:29 am
content: Ok, here is the repaired attenuator board in the foreground and my working attenuator board in the back.Here she is installed

author: Smoky
author_rank: Frequent Contributor
timestamp: March 01, 2020, 03:54:33 am
content: Move over Einstein!!!Next is SPC...This scope was my first electronics project two years ago and it still brings great pleasure. One of the toughest and frustrating experiences. No way of knowing where the fault is, just keep doing continuity tests after continuity tests. And never knowing if the repair works until it's all buttoned back up. All due to damaged traces from leaking capacitors! I can get in and out of this scope now in seconds, blind-foldedAgain, the reason why I even attempted the repair on this second attenuator board was because I'm recapping the backplane board next. Those capacitors came in yesterday too.Anyway, I'm doing probe tests for each of the four channels now...Channel 1 through 4:Notice how the scope didn't know it's a Leap Year. Maybe it never dreamt it would still be aroundSo here's the recapped Backplane board too, part number 671-1676-02. I used long-life, low-ESR Nichicon capacitors for the replacements. And to let you know, I smelled that funky electrolyte odor when desoldering two of the old Nichicon capacitors. I didn't see any leakage on the board but, maybe, they were just starting to leak? I don't know, but she's back in tip-top shape now:I would see it wise to "reform" the new capacitors before you install them. That is, do leakage tests on them at rated voltage for about a minute each. This will break them in and make the fresh start-up of the oscilloscope go smoothly. Don't install them straight out of the package....and since the Centronics/RS-232 board used the same capacitors (220uf 35v), I refreshed them too.Plus, I replaced both batteries which attach to the CPU and DSP boards. I soldered leads to the back of the boards and fed 3VDC while the new Lithium cells were swapped-out so to not lose memory. One of the old batteries read 2.53VDC. The FFT math was retained.Bad to the bone!