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Tek TDS420 help?

D

DaveC

Jan 1, 1970
0
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
 
D

DaveC

Jan 1, 1970
0
All my experience is with the 540, but I think you're wasting your time
trying to troubleshoot ANYTHING

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.
until you replace ALL, AND I DO MEAN
EVERY SINGLE ONE on every board, the caps and CLEAN
CLEAN CLEAN and CLEAN again the circuit boards.

Yeah, I plan to do that. Making a complete list right now.

What techniques and chemicals did you use?

Thanks for your reply.
Dave
 
D

DaveC

Jan 1, 1970
0
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.
 
N

Nico Coesel

Jan 1, 1970
0
DaveC said:
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.

Tektronix has an excellent user forum covering these sort of repairs.
 
D

DaveC

Jan 1, 1970
0
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.
[...]
WB

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
 
D

DaveC

Jan 1, 1970
0
Tektronix has an excellent user forum covering these sort of repairs.

Thanks Nico. I'll check it out.

Dave
 
D

DaveC

Jan 1, 1970
0
Tektronix has an excellent user forum covering these sort of repairs.

Nothing I could find regarding the button switches.

Can you point me to a Tek forum topic about button contact repairs?

Thanks,
Dave
 
N

Nico Coesel

Jan 1, 1970
0
DaveC said:
Nothing I could find regarding the button switches.

Can you point me to a Tek forum topic about button contact repairs?

I'd register and post a question there.
 
J

James Sweet

Jan 1, 1970
0
DaveC said:
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.



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.
 
J

James Sweet

Jan 1, 1970
0
Wild_Bill said:
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.



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.
 
J

James Sweet

Jan 1, 1970
0
Wild_Bill said:
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.


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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