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TDS784C Signal Path Compensation Error


Steve Hendrix
 

A year or so ago I bought a TDS784C that seemed to be in pristine condition except for a missing left rear foot. It powered up and worked perfectly on every signal I tried it on. I found that I rarely if ever had an actual need for its high bandwidth (1GHz) and I had other scopes that met my needs with less bench space, so I sold it on eBay.

The buyer has been very polite and cooperative, but requests help with finding a replacement foot, and also reports that he ran a Signal Path Compensation which reported an error:

"A compensation error has occurred. Examine the error log and note the message. Then refer this oscilloscope to a qualified service technician for repair. It is possible to proceed by pressing Clear Menu. The oscilloscope, however, may not perform according to specifications."

According to the error log, this error has been present since 2007, over a decade before i got hold of it.

Can anyone here offer suggestions?

Steve Hendrix


Siggi
 

Hey Steve,

The classic fault that causes SPC on these scopes is that the signal relays
on the input attenuator hybrids develop high resistance. They are used to
shunt in the internally generated calibration signal that's used for the
signal path compensation.
I replaced a few of those relays on my TDS784D, which cleared the problem.
The relays were readily available at the time, and I could readily measure
which relays were bad by a simple resistance measurement over the NC
terminals. Clipping the old relays out and soldering on the ceramic hybrid
was a little bit nerve wracking, but definitely very doable.

Siggi

On Sun, Feb 28, 2021 at 9:16 AM Steve Hendrix <SteveHx@hxengineering.com>
wrote:

A year or so ago I bought a TDS784C that seemed to be in pristine
condition except for a missing left rear foot. It powered up and
worked perfectly on every signal I tried it on. I found that I rarely
if ever had an actual need for its high bandwidth (1GHz) and I had
other scopes that met my needs with less bench space, so I sold it on eBay.

The buyer has been very polite and cooperative, but requests help
with finding a replacement foot, and also reports that he ran a
Signal Path Compensation which reported an error:

"A compensation error has occurred. Examine the error log and note
the message. Then refer this oscilloscope to a qualified service
technician for repair. It is possible to proceed by pressing Clear
Menu. The oscilloscope, however, may not perform according to
specifications."

According to the error log, this error has been present since 2007,
over a decade before i got hold of it.

Can anyone here offer suggestions?

Steve Hendrix







EricJ
 

I will agree with Siggi on this one as far as one or more of those relays
being the likely cause. One of my TDS scopes had the same problem.
Replacing the relays he mentioned solved it. And yes he'll need to be
careful, the hybrid boards are made of pretty thin ceramic.

--Eric

On Sun, Feb 28, 2021, 9:19 AM Siggi <siggi@undo.com> wrote:

Hey Steve,

The classic fault that causes SPC on these scopes is that the signal relays
on the input attenuator hybrids develop high resistance. They are used to
shunt in the internally generated calibration signal that's used for the
signal path compensation.
I replaced a few of those relays on my TDS784D, which cleared the problem.
The relays were readily available at the time, and I could readily measure
which relays were bad by a simple resistance measurement over the NC
terminals. Clipping the old relays out and soldering on the ceramic hybrid
was a little bit nerve wracking, but definitely very doable.

Siggi

On Sun, Feb 28, 2021 at 9:16 AM Steve Hendrix <SteveHx@hxengineering.com>
wrote:

A year or so ago I bought a TDS784C that seemed to be in pristine
condition except for a missing left rear foot. It powered up and
worked perfectly on every signal I tried it on. I found that I rarely
if ever had an actual need for its high bandwidth (1GHz) and I had
other scopes that met my needs with less bench space, so I sold it on
eBay.

The buyer has been very polite and cooperative, but requests help
with finding a replacement foot, and also reports that he ran a
Signal Path Compensation which reported an error:

"A compensation error has occurred. Examine the error log and note
the message. Then refer this oscilloscope to a qualified service
technician for repair. It is possible to proceed by pressing Clear
Menu. The oscilloscope, however, may not perform according to
specifications."

According to the error log, this error has been present since 2007,
over a decade before i got hold of it.

Can anyone here offer suggestions?

Steve Hendrix











Siggi
 

On Sun, Feb 28, 2021 at 1:07 PM EricJ via groups.io <wyzkydd2358=
yahoo.com@groups.io> wrote:

Replacing the relays he mentioned solved it. And yes he'll need to be
careful, the hybrid boards are made of pretty thin ceramic.
I snipped the pins of the relays individually with a good pair of flush
cutters, then de-soldered them individually. Even so it felt a little iffy,
and one of the relay pins pulled out of its solder joint on the ceramic
substrate. Maybe it was a cold joint, but you might consider cutting the
pins with a dremel or some such - some method that doesn't put a lot of
force on the pin or relay.


Steve Hendrix
 

Well, the buyer has turned uncooperative, said something about just needing a scope to make some measurements on a current project, and has formally requested a return thru eBay. So it looks like I'm out about $120 worth of shipping, and will have a go at fixing the relays when it gets back here. Can anyone guide me as to what type relays, and how many, I need to be looking at? I have the service manual, but they don't seem to drill down to that level of detail. Also, is this a typical contact oxidation problem, that I might be able to clear by repeatedly cycling those relays? What's the best way to tackle this one?

Steve Hendrix

At 2021-02-28 01:06 PM, EricJ via groups.io wrote:
I will agree with Siggi on this one as far as one or more of those relays
being the likely cause. One of my TDS scopes had the same problem.
Replacing the relays he mentioned solved it. And yes he'll need to be
careful, the hybrid boards are made of pretty thin ceramic.

--Eric

On Sun, Feb 28, 2021, 9:19 AM Siggi <siggi@undo.com> wrote:

Hey Steve,

The classic fault that causes SPC on these scopes is that the signal relays
on the input attenuator hybrids develop high resistance. They are used to
shunt in the internally generated calibration signal that's used for the
signal path compensation.
I replaced a few of those relays on my TDS784D, which cleared the problem.
The relays were readily available at the time, and I could readily measure
which relays were bad by a simple resistance measurement over the NC
terminals. Clipping the old relays out and soldering on the ceramic hybrid
was a little bit nerve wracking, but definitely very doable.

Siggi

On Sun, Feb 28, 2021 at 9:16 AM Steve Hendrix <SteveHx@hxengineering.com>
wrote:

A year or so ago I bought a TDS784C that seemed to be in pristine
condition except for a missing left rear foot. It powered up and
worked perfectly on every signal I tried it on. I found that I rarely
if ever had an actual need for its high bandwidth (1GHz) and I had
other scopes that met my needs with less bench space, so I sold it on
eBay.

The buyer has been very polite and cooperative, but requests help
with finding a replacement foot, and also reports that he ran a
Signal Path Compensation which reported an error:

"A compensation error has occurred. Examine the error log and note
the message. Then refer this oscilloscope to a qualified service
technician for repair. It is possible to proceed by pressing Clear
Menu. The oscilloscope, however, may not perform according to
specifications."

According to the error log, this error has been present since 2007,
over a decade before i got hold of it.

Can anyone here offer suggestions?

Steve Hendrix












 

Hi Steve,

be aware of buyers that take out or swap parts they need for a defect unit of their own.
A friend once showed me a Tek scope that came back with the processor chip removed from its socket...

cheers
Martin

On 28. Feb 2021, at 23:33, Steve Hendrix <SteveHx@HxEngineering.com <mailto:SteveHx@HxEngineering.com>> wrote:

Well, the buyer has turned uncooperative, said something about just needing a scope to make some measurements on a current project, and has formally requested a return thru eBay. So it looks like I'm out about $120 worth of shipping, and will have a go at fixing the relays when it gets back here. Can anyone guide me as to what type relays, and how many, I need to be looking at? I have the service manual, but they don't seem to drill down to that level of detail. Also, is this a typical contact oxidation problem, that I might be able to clear by repeatedly cycling those relays? What's the best way to tackle this one?

Steve Hendrix


Steve Hendrix
 

At 2021-03-01 12:16 AM, Martin via groups.io wrote:
be aware of buyers that take out or swap parts they need for a defect unit of their own.
A friend once showed me a Tek scope that came back with the processor chip removed from its socket...
Thank you for the heads-up. Is there any way to guard against that?

Steve Hendrix


BRYAN SWAN
 

There is a unit on the usual site if you want to see the relays   item262497920634


 

Steve,

Some time ago I got hold of a 520 digital scope. To my surprise it passed all self tests and looked clean and pristine inside, so I decided to sell it as working OK, with a lot of pics including of the selftest.

The buyer got the item, said its OK but didn't pass selftest and asked for a partial refund. I knew the unit was very well packed, so I became suspicious. I refused and the seller did not really argue about.

As I did not offer to take back in first place, and didn't use paypal or anything similar, Ebay could not interfere. They're trying to change things in Germany these days, so they will be always in the loop. If this suceeds I might not offer anything else than local pickup in future.

cheers
Martin


Steve Hendrix
 

Please contact me offline at SteveHx@HxEngineering.com. Thank you in advance for any info you can provide.

Steve Hendrix

At 2021-03-01 07:32 AM, you wrote:
Steve,

Some time ago I got hold of a 520 digital scope. To my surprise it passed all self tests and looked clean and pristine inside, so I decided to sell it as working OK, with a lot of pics including of the selftest.

The buyer got the item, said its OK but didn't pass selftest and asked for a partial refund. I knew the unit was very well packed, so I became suspicious. I refused and the seller did not really argue about.

As I did not offer to take back in first place, and didn't use paypal or anything similar, Ebay could not interfere. They're trying to change things in Germany these days, so they will be always in the loop. If this suceeds I might not offer anything else than local pickup in future.

cheers
Martin




Siggi
 

On Sun, Feb 28, 2021 at 5:34 PM Steve Hendrix <SteveHx@hxengineering.com>
wrote:

Can anyone guide me as to what type relays,
and how many, I need to be looking at?

Here's an EEVBlog thread with the info:
https://www.eevblog.com/forum/testgear/tektronix-tds784d-successful-attenuator-repair/.
You may want to verify that this information is correct before you order
the relays.
IIRC there are three per attenuator.


Also, is
this a typical contact oxidation problem, that I might be able to
clear by repeatedly cycling those relays?
I took one of the bad relays from an attenuator and cycled it for thousands
of cycles out of the scope, but it didn't seem to help. I even drilled a
hole in the relay to inject some DeOxit then cycled it some more, which
still didn't seem to help.
Maybe if you could run a low current through the contacts as they're
cycled, that'd work, I don't know.


EricJ
 

Lucky for you I happened to have a spare hybrid input board sitting on my bench. Here's a better pic of the relays, you need 4 for each channel, so 16 total to replace them all. You could just check each individually for high resistance in the NC contact as already mentioned too, and replace only the bad ones... But might as well get them all while you're in there IMO.

https://groups.io/g/TekScopes/photo/261338/0?p=Created,,,20,2,0,0


Steve Hendrix
 

Thank you for the pic. That's a big help in knowing what I'm getting into.

Steve Hendrix