Tektronix 2213 repair / leaked capacitor - Page 1
Forum section: electronics / repair

author: rkagerer
author_rank: Contributor
timestamp: September 13, 2019, 07:58:52 am
content: I have an old Tektronix 2213 that sat in a box on a shelf for a decade, then was flown across the country in a suitcase in checked luggage (but still in the box and well padded!). When I turned it on it hissed and there was no image. I took it apart and found a leaked capacitor (photos below, click them for full size). It's C971, which according to theService Manualfilters the +8.6V low voltage supply.I couldn't find an 840uF capacitor but I bought this 820uFELXY250ELL821MK20Sfrom Digikey. I replaced it, and the scope works!Questions:Should I be concerned about the 20uF mismatch? I figured it's only 2% off, and the original cap was a Sangamo301EN841U012B2which I believe had a 20% tolerance. Should I replace C975 (or any of the related filtering caps) to match?It took more force and (and heat) than expected to desolder the dead cap. They sure made things more durable back in those days! Any tips in case I find myself needing to replace other components? Note I was using solder braid since I don't have a pump.My scope was stored in a plastic bag, tied with a knot, inside a box. When I opened it up there was a strong smell, which I assume was a mix of "Eau Du Old Tandy computer" and leaked electrolyte. After removing the cap and letting it all air out for a day, the smell isn't as bad. Should I be worried about it (e.g. toxicity?) and do I need to do any other cleanup (e.g. wipe down all the surfaces, probe sheathing, knobs, etc. with isopropyl alcohol)?I measured the Probe Adjust point with my multimeter (nothing fancy, just a run of the mill Radio Shack 22-168A). It's labeled ~1KHz and ~500mV peak-to-peak, but my meter reported 883kHz and 273mV AC (not sure if that's RMS, or if it matters for a squarewave). Does the discrepancy indicate other problems?How's my trace focus and calibration look?I realize I shouldn't have a long wire(/antenna) hooked up on the end of my probe like that; just wanted to show the MM for reference.After using the scope for a while, I noticed the waveform occasionally jumps up or down by several volts, off the screen. After a while it goes back. Am I doing something wrong, or might this be a symptom of other issues?Has anyone found good quality scans of the schematics? Particularly #9, which shows the power supply.I haven't performed any measurements of the test point voltages inside the case. Given the high voltages inside, I'm a little hesitant to do too much with the cover removed, particularly with the high-voltage shield removed. Any advice for doing so safely? (I've read some people keep one hand behind their back to reduce the likelihood of shorting anything through their heart).Anything else I should check/do?

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2019, 08:31:21 am
content: Quote from: rkagerer on September 13, 2019, 07:58:52 amNope, but replace the other three too.Good job on the repair and it's all looking fine.

author: rkagerer
author_rank: Contributor
timestamp: September 13, 2019, 06:58:12 pm
content: Quote from: tautech on September 13, 2019, 08:31:21 amFor preventative maintenance, or did you notice something wrong with 'em? (Are you worried about them blowing and causing further damage to downstream components?)

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2019, 08:06:45 pm
content: Quote from: rkagerer on September 13, 2019, 06:58:12 pmYes and protective !Should they leak some and the scope continue to operate OK the cap electrolyte can do lots of damage until the scope fails.If we were to operate equipment with the covers removed we could monitor things like caps bulging or leaking but we do not. Hidden leaking caps could make the PCB BER then the instrument is lost for just the price and effort to have recapped it.

author: rkagerer
author_rank: Contributor
timestamp: September 13, 2019, 10:52:26 pm
content: Quote from: tautech on September 13, 2019, 08:06:45 pmThanks for the education! Is it the leaked fluid that does the damage, or the fact that the circuit is running with a now-out--of-spec cap (which I assume would affect downstream voltages)? Does that mean I should replace ALL the other caps on my scope? Or are these three a bigger risk factor? (and if so, how come).Hope you don't mind all the questions.

author: tautech
author_rank: Super Contributor
timestamp: September 13, 2019, 11:22:20 pm
content: Quote from: rkagerer on September 13, 2019, 10:52:26 pmPSU output voltages won't be affected only their ripple values if insufficient smoothing bulk capacitance is installed.The very little value variation of the cap you installed is of no concern in fact a modern cap may have better ripple specs than the original ones. The sanity check is to check that all power rails ripple still meet the service manual spec.That one of the 4 had failed is the main concern and hence the recommendation to replace them however I would carefully inspect all others too for signs of failure....hints of leakage and domed tops that indicated pressure build ups within that in time will lead to leakage.

author: m3vuv
author_rank: Super Contributor
timestamp: September 14, 2019, 03:29:13 am
content: check the remaining caps esr first !