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JW JW is offline
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Default Tektronix TDS-744A SPC fails

On Tue, 18 Oct 2011 11:18:58 -0500 Andy Cuffe wrote in
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On Mon, 17 Oct 2011 05:57:44 -0400, JW wrote:

On Sat, 15 Oct 2011 10:18:11 -0500 Andy Cuffe wrote in
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Hi Andy,

While I've done a few successful transplants, I would not recommend trying
this. It's a very delicate and difficult operation. The spots you are
seeing on the shutter are due to air bubbles in the optical gel. Usually,
this occurs between the shutter and the CRT itself, but I have also seen
it happen between the glass plates of the shutter. If between the CRT and
the shutter, it is repairable. you'll have to separate the shutter with an
X-acto knife, remove all the gel, clean the surfaces and RTV the shutter
back onto the CRT. If between plates, you'll probably break the glass
trying to separate them. If you do attempt this, be very careful when
removing the shutter that you don't cut the wires that are embedded in the

There's a guy who sells LCD replacement kits if your operation fails.
Expensive, but a well made kit. I've installed a number of these.

As for your signal path failures, I'll bet that one or more relays in the
attenuators have excessive contact resistance. I have found that you can
test them in-circuit in their NC state with a four wire capable DVM.
Anything higher than .1 ohms - replace. You'll likely find some in the 5+
ohm range. I've repaired more than a dozen TDS500, 600 and 700 series
scopes with this failure. Note that you'll need a soldering iron with a
*very* long tip to solder the pins as you have to work under the relays.
You'll see what I mean when you take a look!

For replacement relays, I use Panasonic DS1E-M-DC12V. There are four of
them in each attenuator.

Digikey has them.

Or save some money he

Lastly, if you are running the scope with it's cover off, be sure to run a
fan over the ACQ board as some components get pretty hot.

BTW - I think you were the guy who emailed me the TDS cal software - did
you ever get that to work? (I never tried...)

Let me know how you make out with a follow-up or email is jayw_comark AT

Thanks for the advice. I've been working on the problem on my own,
and I've come to the same conclusion about the attenuator relays. I
checked one channel, and found that two were .3-.5 ohms (measurable
with just a regular DMM).

I never trust a 2 wire DMM when reading resistances less than an ohm or
so. Contact and test lead resistance always seem to be an issue. I would
expect that you'll find some around 5 ohms or more, though.

The other two seemed fine, but I think I'll
replace them all so I don't have to go in again.

Don't bother with the thin blue one, I've yet to see one of those go

The bad shutter has an area the size of a quarter which appears green
when the scope is off (as opposed to the normal dark magenta). When
it's operating, the color in this area appears much less saturated. I
would say it's either between the LCD layers, or a failure of one of
the LCDs.

Once again, I'd bet that it's the optical gel.

It sounds like it's a delicate operation. I might attempt swapping
the good shutter onto a new CRT if I find one, but otherwise I'll
leave it alone for now. I don't want to risk destroying both CRTs.
At least the weak one responded well to my CRT rejuvenator. How hard
does the optical gel make it to remove the shutter?

It's not the gel that makes it difficult, it's the RTV that seals the
shutter to the CRT.

Does you have to
cut it away from the face of the CRT, or can you just pull it off
after separating the RTV form the sides of the CRT? Does it still
look good without the gel?

You have to cut the RTV off from the side of the CRT, then cut inward
between the face of the CRT and the shutter. It still takes quite a bit of
strength to pull the shutter free. If it doesn't come free then cut some
more, lather, rinse and repeat. It looks fine without the gel, but it's a
gooey mess cleaning it all off the CRT and shutter. I use a soft plastic
scraper to scoop most of it up, then a lot of rags and isopropyl alcohol
to finish the job. Wish I knew of something that would attack the stuff

I am using a fan when running it with the cover off. You're right
that it gets hot quickly. I'm surprised the service manual doesn't
have a warning about that.

I did email the TDS500 software to several people, so you probably got
it from me. Originally, I was looking for a way to back up the NVRAM,
but in the end I was able to back it up using my EPROM programmer, so
I never had to use it.

Now I would like to find the cal software for the 744. It's nice to
have, and it would answer my question about the TV trigger. I'll let
you know if I find a copy.

There's a guy named Hakan on the Yahoo Tektronix group that probably has
the TDS744 cal software. He also has all the schematics for TDS500, 600
and 700 series of scopes, but is unwilling to scan them for some reason...

I will post back with the results.

Looking forward to it.



You're welcome - good luck!