Date   

Re: TDS794D, alleged shorted power supply?

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.


Re: TDS794D, alleged shorted power supply?

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?




Re: TDS794D, alleged shorted power supply?

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?


Re: TDS794D, alleged shorted power supply?

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.



Re: Just got a 475, nothing showing on display

Michael W. Lynch
 

On Wed, Jul 29, 2020 at 04:47 AM, donald collie wrote:

Is the fse on the main PCB at the back left blown? If so, it is almost
certain that the Tantalum capacitor associated with this circuit is
shorted......................Donald Collie
Check F1318 as Donald mentioned. Is it open? You should have 15V Unregulated at that fuse. If F1318 is open, check C1318, which is a tantalum cap in the primary of the HV transformer, this can short and stop the HV circuit from working. Check the power transistor Q1318, this is a common failure point as well on the same circuit.

+5: 4.99v, 48.5 ohm to ground, 0mv ripple voltage<
-8: -7.96v, 40.8 ohm, 0mv ripple<
+15: 14.98v, 68 ohm, 4mv ripple<
+50: 50.00v, 2.78k ohm, 36v* ripple<
+50 unregulated: 67v, OL, 31.9v* ripple<
+105/160: 139.4v, OL, 13.8v* ripple<
+110: 111.1v, OL, 20.9v* ripple<
Your Low Voltage power supplies seem to be working, the voltages are very close to ideal. Worry about ripple after later. You really need a scope to accurately measure ripple.

Concentrate on getting a spot or a trace. Shotgun replacement of parts which are not known or obviously bad is not the best way to proceed, replace only OBVIOUSLY burned or defective parts at this time.

Clean that beam finder switch with DE-OXIT or other cleaning method and exercise the switch, could be as simple as a dirty switch.

But it sounds like the same problem I have with a 465. I have a dead HV Multiplier<
Regarding the HV Multiplier that Stephen mentioned, this is also a very common failure which will open F1318. On most 465/475 Series scopes there is an isolation jumper on the ground or common side of the HV Multiplier, this can be a simple wire jumper or wire with a plain white ceramic insulator. If present on your scope, this jumper will be located under that HV shield of the bottom board near to and in front of the two nylon nuts that secure the HV Multiplier to the circuit board. This jumper is often not clearly identified in the manual. If you have this jumper, carefully unsolder and lift one end. This will take the HV multiplier out of circuit and allow the HV Transformer to generate the -3kV required for the CRT to partially function . Once the Multiplier is out of circuit, you should be able to observe some sort of visual reaction from the CRT, such as a short trace, a dot or a faint glow (depending on other failures).

WARNING: Keep in mind that you may now have potentials in excess of 3kV present IF you do this step and the HV Circuit comes back to life. Extreme care should be used when working on any component located under that HV Shield as High voltages may remain, even after the power is switched off.

Make sure that you have the correct version of the Service Manual for your scope. Study the Theory of Operation, Schematics and the Circuit descriptions. Do not attempt to proceed without the proper manual, either paper or digital.

A last recommendation (from someone who has been there recently): Focus on one problem at a time, do not try to diagnose/fix everything at once. Systematic diagnosis and repair yields the best results, As I stated previously, "wholesale" or "shotgun" replacement of components is not the most effective way to fix the problem. One lesson that I learned, REGARDLESS of the brand or type of equipment is that you must have the correct power supply voltages at all test points of the supply before proceeding to repair other faults. On this point, you seem to be on track.

Good Luck!

--
Michael Lynch
Dardanelle, AR


Re: TDS794D, alleged shorted power supply?

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





Re: Just got a 475, nothing showing on display

donald collie
 

Is the fse on the main PCB at the back left blown? If so, it is almost
certain that the Tantalum capacitor associated with this circuit is
shorted......................Donald Collie

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On Wed, Jul 29, 2020 at 9:24 PM Roger Evans via groups.io <very_fuzzy_logic=
yahoo.com@groups.io> wrote:

The high ripple you measure on the 50V and 100+ Volt supplies is usually
due to the large 'bulk' smoothing capacitors on those supplies drying out
with age and simultaneously losing their capacitance and developing high
series resistance. Both these changes render them ineffective at removing
the ripple at the output of the rectifiers. C1414 is almost certainly the
main culprit, C1412 is suspect but might be OK (it is a somewhat unusual
smoothing arrangement). You can make a temporary fix by using a modern
electrolytic capacitor and attaching it in parallel to the existing
capacitor, if necessary using some short leads to position the new
capacitor out of harm's way. The new cap needs to the same or higher
voltage rating and the same or slightly larger capacitance.

Failed tantalum capacitors usually fail short circuit and pull the suply
rail to almost zero volts. They are a perennial problem with old equipment.

Regards,

Roger




Re: Just got a 475, nothing showing on display

Roger Evans
 

The high ripple you measure on the 50V and 100+ Volt supplies is usually due to the large 'bulk' smoothing capacitors on those supplies drying out with age and simultaneously losing their capacitance and developing high series resistance. Both these changes render them ineffective at removing the ripple at the output of the rectifiers. C1414 is almost certainly the main culprit, C1412 is suspect but might be OK (it is a somewhat unusual smoothing arrangement). You can make a temporary fix by using a modern electrolytic capacitor and attaching it in parallel to the existing capacitor, if necessary using some short leads to position the new capacitor out of harm's way. The new cap needs to the same or higher voltage rating and the same or slightly larger capacitance.

Failed tantalum capacitors usually fail short circuit and pull the suply rail to almost zero volts. They are a perennial problem with old equipment.

Regards,

Roger


Re: Just got a 475, nothing showing on display

Stephen
 

On Tue, Jul 28, 2020 at 07:48 PM, Alexandre Souza wrote:


Change ALL tantalum caps. Probably it will work =)
Just a quick question that I’ve had for a while. Do you have to put back those little blue beads on the tantalum caps? They are not ferrite beads, I don’t think. They break like glass.


Re: Just got a 475, nothing showing on display

Alexandre Souza
 

Change ALL tantalum caps. Probably it will work =)

---8<---Corte aqui---8<---
http://www.tabajara-labs.blogspot.com
http://www.tabalabs.com.br
---8<---Corte aqui---8<---


Em ter., 28 de jul. de 2020 às 18:24, <jakenicholasward@...> escreveu:

Hey guys,

I just got a Tektronix 475 from ebay. The fan spins, lights come on, but
there’s no trace and no dot when I hit “beam finder”. I measured the power
supply test points and got these results:

+5: 4.99v, 48.5 ohm to ground, 0mv ripple voltage
-8: -7.96v, 40.8 ohm, 0mv ripple
+15: 14.98v, 68 ohm, 4mv ripple
+50: 50.00v, 2.78k ohm, 36v* ripple
+50 unregulated: 67v, OL, 31.9v* ripple
+105/160: 139.4v, OL, 13.8v* ripple
+110: 111.1v, OL, 20.9v* ripple

* where I measured high ripple voltages, my multimeter switched back and
forth from That voltage to zero in about 1 second intervals. Not sure what
that means.

The resistance on the +5, -8, and +15 volt rails seems very low, is that
normal? I didn’t see any expected values in the service manual (although I
haven’t read too deep).

I also noticed some kind of corrosion around where the giant caps(?) are.

It might be worth noting that the beam finder seemed to work on the ebay
listing, but I haven’t been able to make it work.

I’m new here and fairly new to electronics in general so I’d appreciate
any help. Thanks!




Re: Just got a 475, nothing showing on display

Brian Cockburn
 

Hi,

Jake's point about having an issue or uncertainty about measuring ripple with a multimeter reminded me about a similar question that's been in my mind for a while.

In the service manual of the 2245A (the 'scope I have), Table 4-1 calls for a 'DMM DC Volts range: 0 to 140 V. DC voltage accuracy: ±0.15%. 4½ digit display. e.g. Tektronix DM 501A'. It makes no mention of being able to measure ripple values with a DMM, suggesting instead that a 'scope is used. So is measuring ripple with a DMM valid? Is it useful given that maximum allowable ripple is given for two distinct bands: 60-150 Hz and 20-40 kHz (which I take to mean 20 kHz to 40 kHz, rather than 20 Hz to 40 kHz). Do I need a DMM that 'goes' to at least 40 kHz on ACV to make a useful measurement here?

For instance, my 5½ digit 220,000 count bench DMM claims to have a 3 dB bandwidth of 300 kHz. On the 200 mVAC range it clearly doesn't meet the required ±0.15% accuracy figure, but it's what I've got. (Accuracy is given as %rdg + counts: 20 Hz to 50 Hz => 1.00 + 100, 50 Hz to 10 kHz => 0.35 + 100, 10 kHz to 20 kHz => 0.6 + 200, 20 kHz to 50 kHz => 1.5 + 250, 50 kHz to 100 kHz => 5 + 400.) Do I just need to make the calculations about my DMM's worst case error in each (of its) bands and make sure that my measured value including the error is below the required (specified) level?

Thanks, Brian.


Re: TDS794D, alleged shorted power supply?

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


Re: TDS794D, alleged shorted power supply?

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


Re: TDS794D, alleged shorted power supply?

 

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.


Housings to make custom TM500 plug ins, plus some good salvage

 

I have had many requests from stuff day visitors about junk TM500 plug ins to use to make custom test items, extenders, and so on. I am now out of free TM500 giveaways, and scrap metalwork to make TM500 items, but I went on a deep search and found something just as good, maybe better!

I have a big pile of mint looking Argo Systems plug ins (complete with pull tabs and covers), they are made from Tek provided TM500 parts, so they are prefect for making your own TM500 widget. Just keep in mind these two things:
1. they ARE NOT electrically compatible with a TM500 frame, even though the connector is the same.
2. The connector has no slots, so if you are going to try and use it after stripping the other stuff out, be aware it has NO SLOTS, you will have to file those in yourself.

These are FULL of great salvage parts, one even has a Trilithic 1-32dB step attenuator (wow), the other type is loaded with socketed logic dips. In any case, it's the best solution I could come up with, and they are just $25 each. you can see them all here on the stuff day page:
https://www.sphere.bc.ca/test/stuffday.html#tekfans

There are even some double-wide ones in the Argo Systems listing below these plug ins, I can offer them for $25 as well.

scroll down almost to the end of the Tek items, and you will find the Argo as210 stuff.
all the best,
stay safe,
walter (walter2 -at- sphere.bc.ca)
sphere research corp.
https://www.sphere.bc.ca/test/index.html


Re: Just got a 475, nothing showing on display

Stephen
 

I’m almost as new as you are to this, and here. But it sounds like the same problem I have with a 465. I have a dead HV Multiplier. Experts here will guide you and tell you more. I hope I’m wrong.


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.)


Re: DC503 Not Working

Stephen
 

I’ve been quite busy today and I haven’t had the chance to check what you guys suggested yet.
However, I did notice something that may or may not be worth noting: When I input a signal to input A,
let’s say 1Khz, and “if” And only if the only digit lit is the one in the middle (the 3rd from the left I think), when I’m in the Totalize A position, and press the start button, it counts from 0 to 9 repetitively. And this is all this unit does.


Just got a 475, nothing showing on display

Jake W
 

Hey guys,

I just got a Tektronix 475 from ebay. The fan spins, lights come on, but there’s no trace and no dot when I hit “beam finder”. I measured the power supply test points and got these results:

+5: 4.99v, 48.5 ohm to ground, 0mv ripple voltage
-8: -7.96v, 40.8 ohm, 0mv ripple
+15: 14.98v, 68 ohm, 4mv ripple
+50: 50.00v, 2.78k ohm, 36v* ripple
+50 unregulated: 67v, OL, 31.9v* ripple
+105/160: 139.4v, OL, 13.8v* ripple
+110: 111.1v, OL, 20.9v* ripple

* where I measured high ripple voltages, my multimeter switched back and forth from That voltage to zero in about 1 second intervals. Not sure what that means.

The resistance on the +5, -8, and +15 volt rails seems very low, is that normal? I didn’t see any expected values in the service manual (although I haven’t read too deep).

I also noticed some kind of corrosion around where the giant caps(?) are.

It might be worth noting that the beam finder seemed to work on the ebay listing, but I haven’t been able to make it work.

I’m new here and fairly new to electronics in general so I’d appreciate any help. Thanks!


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