Topics

TDS794D, alleged shorted power supply?


Roy Thistle
 

I've got this TDS794D in skookum cosmetic shape... that is just daring me to plug it in, and boot it up. Only... the... assumed... "dumpster diver" I got it from claims... "It has a shorted power supply." Further query directed to the dd, did not elucidate significantly.
Fuse has continuity.
Line cord is good, under load.
No toasty smell to it.
I can't see as it has ever been opened (It still has an intact SIMCO, anti-tamper/cal sticker on it... and none of the usual clues a thing has been opened, since SIMCO had it.)
I am kinda chicken to try to power it up... because, I so want it to work... or be an easy fix.
The only explanations... for the "explanation" that it has a shorted power supply...that have crossed my mind... is that that is what SIMCO reported... or documented... if the 794D was indeed sent to them for cal/repair... or even came from them... maybe from an equipment auction?
Another thing that I was thinking it that if the scope boots, at least to error checking, would it report and error indicating a shorted (low?) power supply?
Any suggestions/recommendations, on how to proceed? Any "gotchas" to watch out for?
Anyone got a screen shot of one booting?
The service manual (from the Tektronix Website) has a reasonable service flow chart... so if I dare to plug it in... and it starts to boot... I can work from there. (I don't see anything about error reporting... or explanations/table of error codes.)


 

A likely candidate if the scope is cycling on and off is the CRT assy. Easy to test for - just disconnect the ribbon cable to it.


amirb
 

On Tue, Jul 28, 2020 at 08:48 PM, Jay Walling wrote:


A likely candidate if the scope is cycling on and off is the CRT assy. Easy to
test for - just disconnect the ribbon cable to it.
the OP has not even plugged it in yet so it is not clear if it will cycle on and off

if you have a auto transformer or isolation transformer, just start the scope at something like 50-60Vac
and increase it gradually and check the current draw. If not, just put a light bulb in series and plug it in

another option is to open the thing and unplug the power supply output connector (you need to lift the cpu board and perhaps some
option boards to reach it and then plug it in and check the power supply alone (even unloaded is OK)
this way you can visually check the psu as well. I would imagine you will see burned components right away


Richard Knoppow
 

Do you have or have access to a metered Variac? If you do connect it and start with low voltage and watch the current meter. Be ready on the Variac power switch. Current can come up pretty fast even if there is no short but a short will generally draw a lot of current right away. I would not be too concerned with the seals since this is a "reject".  How do you know the line cord is good under load if you haven't powered it up?
   Either put it on a metered Variac or dim bulb tester or stick it in a display case.
   Of course, I would open it up first and look for any signs of something shorting. Have someone else follow you up, its easy to miss even fairly obvious things.

On 7/28/2020 2:30 PM, Roy Thistle wrote:
I've got this TDS794D in skookum cosmetic shape... that is just daring me to plug it in, and boot it up. Only... the... assumed... "dumpster diver" I got it from claims... "It has a shorted power supply." Further query directed to the dd, did not elucidate significantly.
Fuse has continuity.
Line cord is good, under load.
No toasty smell to it.
I can't see as it has ever been opened (It still has an intact SIMCO, anti-tamper/cal sticker on it... and none of the usual clues a thing has been opened, since SIMCO had it.)
I am kinda chicken to try to power it up... because, I so want it to work... or be an easy fix.
The only explanations... for the "explanation" that it has a shorted power supply...that have crossed my mind... is that that is what SIMCO reported... or documented... if the 794D was indeed sent to them for cal/repair... or even came from them... maybe from an equipment auction?
Another thing that I was thinking it that if the scope boots, at least to error checking, would it report and error indicating a shorted (low?) power supply?
Any suggestions/recommendations, on how to proceed? Any "gotchas" to watch out for?
Anyone got a screen shot of one booting?
The service manual (from the Tektronix Website) has a reasonable service flow chart... so if I dare to plug it in... and it starts to boot... I can work from there. (I don't see anything about error reporting... or explanations/table of error codes.)



--
Richard Knoppow
dickburk@...
WB6KBL


Siggi
 

These power supplies have a power factor correction booster right up front.
This will start at around 90V DC, at which point the rest of the supply
will run full tilt. A variac is not advisable on these, as they will draw
the same power regardless of input voltage.
On the bench I ran the PSU under test with a lab supply, unloaded. These,
as most switchers will run fine on DC, so a current-limited lab supply is a
reasonable way to test for a short.

Þann þri., 28. júl. 2020 kl. 21:27 skrifaði Richard Knoppow <
dickburk@...>:

Do you have or have access to a metered Variac? If you do
connect it and start with low voltage and watch the current
meter. Be ready on the Variac power switch. Current can come up
pretty fast even if there is no short but a short will generally
draw a lot of current right away. I would not be too concerned
with the seals since this is a "reject". How do you know the
line cord is good under load if you haven't powered it up?
Either put it on a metered Variac or dim bulb tester or stick
it in a display case.
Of course, I would open it up first and look for any signs of
something shorting. Have someone else follow you up, its easy to
miss even fairly obvious things.

On 7/28/2020 2:30 PM, Roy Thistle wrote:
I've got this TDS794D in skookum cosmetic shape... that is just daring
me to plug it in, and boot it up. Only... the... assumed... "dumpster
diver" I got it from claims... "It has a shorted power supply." Further
query directed to the dd, did not elucidate significantly.
Fuse has continuity.
Line cord is good, under load.
No toasty smell to it.
I can't see as it has ever been opened (It still has an intact SIMCO,
anti-tamper/cal sticker on it... and none of the usual clues a thing has
been opened, since SIMCO had it.)
I am kinda chicken to try to power it up... because, I so want it to
work... or be an easy fix.
The only explanations... for the "explanation" that it has a shorted
power supply...that have crossed my mind... is that that is what SIMCO
reported... or documented... if the 794D was indeed sent to them for
cal/repair... or even came from them... maybe from an equipment auction?
Another thing that I was thinking it that if the scope boots, at least
to error checking, would it report and error indicating a shorted (low?)
power supply?
Any suggestions/recommendations, on how to proceed? Any "gotchas" to
watch out for?
Anyone got a screen shot of one booting?
The service manual (from the Tektronix Website) has a reasonable service
flow chart... so if I dare to plug it in... and it starts to boot... I can
work from there. (I don't see anything about error reporting... or
explanations/table of error codes.)



--
Richard Knoppow
dickburk@...
WB6KBL





Stephen Hanselman
 

You’ve probably checked this already but...

We have repaired many TDS540’s on all of them we found that the cage that the PS sits in was filthy. In particular the mesh wall between the PS and the HVPS. This will cut airflow and therefore cooling AND that the PS has a temp sensor, it can lead to PS cycling.

Hope that helps a bit good luck.

Regards,

Stephen Hanselman
Datagate Systems, LLC

On Jul 28, 2020, at 17:49, Jay Walling via groups.io <jayw_comark=yahoo.com@groups.io> wrote:

A likely candidate if the scope is cycling on and off is the CRT assy. Easy to test for - just disconnect the ribbon cable to it.



Roy Thistle
 

On Wed, Jul 29, 2020 at 04:14 AM, Siggi wrote:


These power supplies have a power factor correction booster
Hi Siggi: Yes! Thanks for that!
Most modern(circa 2015) SMPS have PFC for Energy Star et. al. compliance. I assume the 794D is using a non-Tek, O.E.M., off-shore SMPS? Did you see anything on the power supply about Energy Star?
So we are talking a Boost... then individual buck converters for the subsystems power supplies? (I haven't cracked open the 794D yet. [That would destroy the SIMCO cal. sticker.])
Do you know if the PFC Boost controller (PID probably?) uses a chip, or is it implemented mostly in discrete?


Siggi
 

Hey Roy,

The TDS5/6/7XX series all have similar power supplies, which I believe are
of Tek design. Some of them are fully documented, though there are multiple
variations of them. I bought the TDS520 service manual from Artek, which
was quite helpful for sorting my TDS684A's PSU. The schematic and component
layouts were not exactly right, but the differences were minute.
You can also look at the TDS520B component level service manual, which is
available from Tek, and there's the TDS540 manual on TekWiki:
http://w140.com/tekwiki/wiki/TDS540. There may be other schematics that are
nearer to your particular scope.

Siggi

On Wed, Jul 29, 2020 at 2:03 PM Roy Thistle <roy.thistle@...>
wrote:

On Wed, Jul 29, 2020 at 04:14 AM, Siggi wrote:


These power supplies have a power factor correction booster
Hi Siggi: Yes! Thanks for that!
Most modern(circa 2015) SMPS have PFC for Energy Star et. al. compliance.
I assume the 794D is using a non-Tek, O.E.M., off-shore SMPS? Did you see
anything on the power supply about Energy Star?
So we are talking a Boost... then individual buck converters for the
subsystems power supplies? (I haven't cracked open the 794D yet. [That
would destroy the SIMCO cal. sticker.])
Do you know if the PFC Boost controller (PID probably?) uses a chip, or is
it implemented mostly in discrete?




Roy Thistle
 

On Wed, Jul 29, 2020 at 04:14 AM, Siggi wrote:


A variac is not advisable
Hi Siggi:
Thanks for mentioning this.
I've used Variacs (with meters to for V, and I)... I've used Variacs on linear power supplies... and also 100W incandescent lamp (in-line with the live wire of the power cord) But, I've only done that if I know how the particular linear power supply is working.
From what I understand about the primary side of most SMPS, I can't see how Variacs, or light bulbs, as limiters are going to help... unless there is a effectively dead short in the primary side... but a blown fuse is telling you that. There are better methods for finding a short.
Anyway... myself... I wouldn't use Variacs, or light bulbs, for testing SMPS, unless and even if, I reasonably understood how the particular SMPS works.


Roy Thistle
 

On Wed, Jul 29, 2020 at 11:14 AM, Siggi wrote:


I believe are
of Tek design
Hi Siggi:
Thanks for letting me know.
I'm sort of surprised Tek was designing their power supplies, even back then! ... as many manufacturers were then turning to 3rd parties to provide power supplies.
Thanks for the recommended docs... I've got some of them (from TekWiki, and also from the Tek Website) I think the www..er Zenith (not the defunct manufacturer) has a TDS540 smps schematic for download? (which I got?... need to look!)
Thanks again for letting me know about the smps similarity between the various TDS5/7 series. Now that I know this I'll reference those docs for the smps.