Tek TDS420 help?

This scope has the famous "FAIL ++ Acq" error which points to the infamous failed (and leaking) SMD caps on the acquisition PCB.
I'm moving on to troubleshoot another (unrelated, I think -- and hope) issue.
During the 25 second "on" period, relays click on & off, the monitor goes through it's boot display (all bright, then dark, then the clock icon), then the graticule is drawn, which is quickly replaced with the "Tek Running Autoing" display listing the "FAIL ++ Acq" error.
During the power-up period, the front panel LEDs all turn on for about 25 seconds, then off for 5 seconds, then only the CH 1 LED lights.
The service manual says (in the Front Panel troubleshooting chart):
Do all of the front panel LEDs turn on and then turn off a short time later?
(Then:)
Does one front panel LED turn on and then turn off, followed by the next LED until all LEDs have turned on and then turned off?
The LEDs do not go through this one-at-a-time sequence. The manual's flow chart points to a failed CPU PCB if the LEDs don't blink this sequence.
Can someone with a TDS4xx confirm power-on LED behavior?
I'm trying to determine whether, because the LEDs aren't blinking "properly" and most of the front panel buttons are not working (the buttons bordering the monitor, plus all the CH input buttons), does this mean a replacement CPU PCB is in my future, or if it's possibly related to cap failure (ie, other cap failures beyond the acquisition PCB)?
Has anyone else here have a similar problem (non-functional buttons) in the past? I'd appreciate hearing your experiences.
Thanks, Dave
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OK, I suspected that, but non-functional buttons seemed a symptom not related to caps issue. But I'll take your advice to heart and do *all* the caps.

Yeah, I plan to do that. Making a complete list right now.
What techniques and chemicals did you use?
Thanks for your reply. Dave
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A little more info on my non-functioning button problem:
I disassembled the front of the scope -- bezel, buttons, front panel assembly.
The elastomer button strip (the "rubber' buttons) have a conductive backing which presses down onto the menu flex circuit (flex pcb).
The back of the button looks good (very light wear indications). The flex circuit traces are not metal, which surprises me. It's made from pretty fragile carbon-like traces.
When I measure the continuity of the closed contacts of a button that works, I see approximately 200 ohms. Closed contacts of a button that doesn't work, I see 400 ohms (sometimes more).
What resistance should I see at the contacts when the button is pressed?
Can I clean these contacts with alcohol? Or...?
Can the traces on the flex circuit be renewed (ie, silver-bearing paint)?
Thanks.
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The method I use for cleaning those conductive pads on button switches (keyboards, IR remote controls, etc) is very lightly scuffing them with a pen-type tool that has strands of glass fibers in it.
It can be difficult to determine the "normal" resistance of these types of contacts, because typically, the resistance decreases as more pressure is applied to the button.
The button pads that are shiney, somewhat glossy, are the ones that need scuffing, IME. I lightly scuff the conducive pads just enough to remove the shine, sometimes just a few passes across the pads.
I can never be certain how long the scuffing treatment will last. Most times the problem is corrected for at least a couple of months to about a year, maybe longer.
The tool I use was found in a camera department of a retail store years ago, as a battery contact cleaning tool. I've read numerous comments that the same type of tool is available at autobody shop supply places, as a scuffing tool for small chips in in painted surfaces on car bodies.
I would also like to know if any conductive paint-type products work effectively, and are somewhat permanent.
-- Cheers, WB .............

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Thanks for that input Bill. I'll see if scrubbing lightly helps.
The flex pcb switch contacts are more what I suspect than the button. And those are seriously fragile. It looks like a tiny bead of carbon-filled rubber cement. I dented a trace (actually chipped off a *tiny* piece) with a DMM test probe. Yikes!
Thanks again, Dave
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Wild_Bill wrote:

Don't scuff them! You don't want to remove material, the problem is skin oils or oils from the rubber that leach out and reduce the conductivity. All you have to do is clean it thoroughly and it will work like new and be fine for years.
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I used to think that cleaning the conducive pads would solve key or button problems, but the provblems returned quickly, several days to a week. I tried using some of he milder solvens like 90% isopropyl alcohol and denatured alcohol on swab tips, and may have even tried lacquer thinner, as that would generally be my next choice. The problems kept returning.
For items like the rubbery button material in IR remote controls, the stuff does get dirty and ooze oils from the plastic, but that was easy enough to clean off. Then I noticed all the faulty buttons conductive pads had a glossy appearance identical to the pattern they're pressed against, so I gently scuffed them just enuough to remove the shiney pattern, and they worked perfectly for much longer periods.
They get shiney again after hundreds of presses, and when the faulty ones were examined, they looked exactly the same as they did before they were scuffed. Buttons that are rarely used on keyboards, for example, keep working while the most frequently used key conductive pads develop the glossy pattern. After a very light scuffing, they work fine.
As I mentioned before, the very light scuffing procedure may involve just a few passes across the conducive pad area, nothing any more severe than a few brush strokes.
I'd still like to know of any recommendations for an effective conductive paint-type product, for some of my older gear that I'd like to keep using for many more years.
The best key switching design I've seen was on a vintage (maybe mid-1960s) deskop calculator with Nixie tubes. The keyboard had a reed swich under each key, and the hard keys had a small magnet in them.. very nice assembly, and I expect that it was very reliable.
-- Cheers, WB .............

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Wild_Bill wrote:

I suppose the surface could wear over time, I've just never seen it myself. Certainly try just cleaning before doing anything more drastic though, liquid dish soap and warm water works as well as anything I've tried.
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DaveC wrote:

I've fixed a lot of similar buttons in TV remotes. What I do is wash the rubber membrane with warm soapy water, liquid hand soap works fine. Scrub it gently with your fingers, rinse, and lay it out on a cloth to dry. The PCB can be cleaned with a rag dipped in the same soapy water, or alcohol would probably work too.
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Tektronix has an excellent user forum covering these sort of repairs.
--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
  Click to see the full signature.
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Thanks Nico. I'll check it out.
Dave
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Nothing I could find regarding the button switches.
Can you point me to a Tek forum topic about button contact repairs?
Thanks, Dave
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I'd register and post a question there.
--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
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