Tektronix TDS-744A SPC fails

[Andy Cuffe]

I'm working on a TDS744A digital oscilloscope that fails SPC (signal
path compensation) most of the time. It usually passes when it's
cold, but after warm up, it almost always fails with the error "Cal
meas clipped at max dig o". The self test always passes.

I haven't been able to find any obvious performance, or accuracy
problems on any channel, or range, and it seems to be stable during
warm up. Are there any known common causes of this error on the
TDS744, or other similar model? It seems to be a common complaint
about this model, but no one has mentioned a fix.

So far, I've checked it over for obvious problems. Cooling U1250 on
the acquisition board seems to increase the chances of a successful
SPC, but it's hard to be sure. It might be a coincidence, or just
compensating for another problem. It doesn't have the leaking
electrolytic problem of earlier models.

I'm also looking for the TDS600B and TDS700A adjustment software (both
are needed for calibration according to the service manual).

[Reinhard Zwirner]

Andy Cuffe schrieb:

May I dare to direct you to the Tek Scopes Yahoo group? You'll find a
lot of Tek specialists there.

HTH

Reinhard

[Andy Cuffe]

That's worth a try. I'm waiting for them to approve me now.

As a side question, has anyone here ever removed the LCD color shutter
from the CRT? I have two similar scopes (a 544 and 744). The 544 has
a mint CRT, but there are some bad spots on the shutter. The 744 has
a lot of burn in, but a perfect shutter. I would like to swap the
good shutter onto the good CRT, but only if I can expect the results
to be good.

[JW]

On Sat, 15 Oct 2011 10:18:11 -0500 Andy Cuffe <acu...@gmail.com> wrote in
Message id: <or8j97ditoi54ssn3...@4ax.com>:

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
RTV!

There's a guy who sells LCD replacement kits if your operation fails.
http://km5tz.netfirms.com/TDS.htm
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.
http://search.digikey.com/us/en/products/DS1E-M-DC12V/255-1473-ND/365060

Or save some money here:
http://www.onlinecomponents.com/panasonic-ew-aromat_ds1emdc12v.html?p=10553789

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
yahoo.com

[Andy Cuffe]

On Mon, 17 Oct 2011 05:57:44 -0400, JW <no...@dev.null> wrote:

>On Sat, 15 Oct 2011 10:18:11 -0500 Andy Cuffe <acu...@gmail.com> wrote in
>Message id: <or8j97ditoi54ssn3...@4ax.com>:
>
>

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

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.

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? 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?

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.

I will post back with the results.

Thanks,

Andy

[JW]

On Tue, 18 Oct 2011 11:18:58 -0500 Andy Cuffe <acu...@gmail.com> wrote in
Message id: <4h9r9793ivn7c2fmb...@4ax.com>:

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
bad...

>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
better.

>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...

http://tech.groups.yahoo.com/group/TekScopes/

>I will post back with the results.

Looking forward to it.

>Thanks,
>
>Andy

You're welcome - good luck!

[Andy Cuffe]

On Wed, 19 Oct 2011 06:16:31 -0400, JW <no...@dev.null> wrote:

>
>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.
>

I wouldn't trust my exact measurements, but there was consistently
more resistance in two of the relays, which is obviously not good in a
high precision circuit.

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

I wasn't bothering with those for now, but it's good to know they
don't cause many problems. I suspect the blue ones are just for
switching between 50 ohm and 1M termination. They looked like they
would be tricky to replace due to their short pins, and they are a lot
more expensive.

>
>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
>better.
>

It sounds like a mess, but at least you don't have to cut all of the
RTV off.

I know replacing the CRT with an LCD would be more sensible, but I
like the look of those CRT + LCD shutter displays. If anyone reading
this has an old TDS5XX, 6XX, or 7XX color CRT that they want to get
rid of, please contact me. I'm interested in anything that has a
decent looking color shutter and/or CRT. I'll even take a known bad
one for practice if the price is right. I'm located in the US.

[Andy Cuffe]

Good news, replacing the relays appears to have cured the SPC problem.
Half of the old relays had an ohm or more of resistance, so it's no
surprise it was having problems.

Replacing the relays was fairly easy, but you'll need a good soldering
station with a long tip. The attenuators are ceramic hybrids, so I
used silver bearing solder just to be on the safe side.

I'm still looking for a CRT or two, if anyone has one left over from
an LCD upgrade, or parted out scope.

I will post back if I find the cal software.

One other question I have is about the TV trigger board (option 05).
My TDS544A has this, and I would like to move it to the TDS744A. I
already built new power ribbon cables and added the second connector
to the bus board. The TV trigger option shows up in the 744 and
passes the self tests, but the trigger level appears to be off. It
will only trigger when the trace is adjusted so that it's cut off at
the top of the screen. I suspect the board is either incompatible, or
that there is a calibration for the TV trigger level that needs to be
done.

Does anyone know if any calibration is required after adding TV
trigger? Did Tektronix even allow you to add option 05?