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Tektronix TDS350 PSU

  • Thread starter Payman Habibelahi
  • Start date
P

Payman Habibelahi

Jan 1, 1970
0
Hi Group

My Tektronix TDS350 Scope will not respond to the front panel power switch,
it clicks but the scope does not power up. The scope was working fine less
than a year ago when last switched on. I suspect the front panel switch is
not really a power switch but a start-up switch like on a PC i.e. mains
power is permanently connected. I have not attempted to diagnose the problem
beyond checking externally accessible fuses.

Is this a known failure mode? Can someone point me at what to check next
please, a particular problem with these scopes is no circuit diagrams in
the service manuals.


thanks

Payman
 
R

Rich Grise

Jan 1, 1970
0
Hi Group

My Tektronix TDS350 Scope will not respond to the front panel power
switch, it clicks but the scope does not power up. The scope was working
fine less than a year ago when last switched on. I suspect the front panel
switch is not really a power switch but a start-up switch like on a PC
i.e. mains power is permanently connected. I have not attempted to
diagnose the problem beyond checking externally accessible fuses.

Is this a known failure mode? Can someone point me at what to check next
please, a particular problem with these scopes is no circuit diagrams in
the service manuals.


thanks
Look for a little tiny rocker switch on the back near the fuse(s). And
reseat the power cord if it's the detachable kind.

Good Luck!
Rich
 
J

Jim Yanik

Jan 1, 1970
0
Hi Group

My Tektronix TDS350 Scope will not respond to the front panel power
switch, it clicks but the scope does not power up. The scope was
working fine less than a year ago when last switched on. I suspect the
front panel switch is not really a power switch but a start-up switch
like on a PC i.e. mains power is permanently connected. I have not
attempted to diagnose the problem beyond checking externally
accessible fuses.

Is this a known failure mode? Can someone point me at what to check
next please, a particular problem with these scopes is no circuit
diagrams in the service manuals.


thanks

Payman

Bad electrolytics(ESR)? Then shorted rectifier diodes.
The TDS series uses purchased power supplies,and even TEK had no schematics
for them.Strictly replace if bad.
It could also be a dirty front panel switch(membrane sw,the whole front
panel is one big one,IIRC)

You could find the PS control IC(switchmode regulator IC),and download the
datasheet for it from the IC manufacturer,that would give you a good start.
 
T

tekamn

Jan 1, 1970
0
Payman said:
Hi Group

My Tektronix TDS350 Scope will not respond to the front panel power switch,
it clicks but the scope does not power up. The scope was working fine less
than a year ago when last switched on. I suspect the front panel switch is
not really a power switch but a start-up switch like on a PC i.e. mains
power is permanently connected. I have not attempted to diagnose the problem
beyond checking externally accessible fuses.

Is this a known failure mode? Can someone point me at what to check next
please, a particular problem with these scopes is no circuit diagrams in
the service manuals.


thanks

Payman

Hello Payman,

yes, seems to be common: I have seen this now 18 times in a TDS3xx
series scope.

The front power switch switche a small DC voltage, very similarto the
ATX pc spower supplies.

So try this (simply on my repair experiences, others might propose
different):
1. download the service manual of the TDS340A/360/380 series scopes fro
mthe Tektronix website.
2. Don't expect anything spctacular, it's only down to module level.
3. Search for the fault-find diagrams. Look for the voltage levels
mentioned there. check in your scoep with DMM.
4. check for loose connection (The interconnection cables are causing
often trouble.
5. Still the problem exists?

So most propably the PSU has a fault, check the power resitor on the
promary (it's the startup circuitry), fails often. Replace by 15k /5W
type.
And check the rectifier diodes on the secondary.
Ok, just my 0.02 $

hth,
Andreas
 
Many thanks to all who replied particularly Andreas aka "tekaman" who
appears to be "right on the money", unfortunately his post hadn't
propagated to my news server so I am responding on Google.

Andreas on your advice I investigated the PSU the power resistor on the
primary that you refered to I identified as a 3K9 5W (R17) on the Zytec
22917300 PSU. This was indeed open circuit the track on the PCB had got
overheated and the solder joint had failed. Unfortunately correcting
this does not solve the problem.

The symptoms remain the same the only voltage available on the output
of the PSU is 1.3V, the Standby Voltage, on pin 13 and the only
activity in the scope is the switching of the PSU relay in response to
the front panel switch, still nothing else powers up.

You also mention the rectifier diodes on the secondary, I am finding it
harder identifying these without a circuit diagram, any clues that
might point me in the right direction?

Also does R17 going open circuit cause any other components to fail?

Jim Yanik mentioned ESR on capacitors, is there a way of detecting this
or does one simply have to replace electolytics that one suspects?
Many thanks again


Payman
 
Hello Andreas

Many Thanks for taking the time to write, unfortunately your post
hadn't propagated to my news server so I am posting on Google. I think
your analysis is correct, however the problem is still not solved.

On your advice I found the resistor 3K9 5W (R17 on the Zytec 22917300).
This had overheated the PCB track, which had failed and gone open
circuit. I resoldered the contacts but the problem remains.

The symptom is the same. I see 1.3V, the standby state voltage on pin
13 of J1 the output of the PSU and no other voltages on the output. I
hear the PSU relay clicking but the standby voltage does not change
(the service manual implies that it should).

Do you know if R17 going open circuit causes any other components to
fail?
Also you mention the secondary rectifier diodes, I am finding it
difficult to track these down without a circuit diagram, any clues?
Best wishes and thanks again.


Payman
 
P

Payman Habibelahi

Jan 1, 1970
0
Thanks to all who replied particularly Andreas AKA Tekaman, unfortunately
his post hadn't propagated to my news server so I have posted a reply using
Google, which also is having problems today, so apologies if multiple copies
appear. I think Andreas is correct, however the problem is still not solved.

On Andreas' advice I found the resistor in the primary circuit 3K9 5W (R17
on the Zytec 22917300). This had overheated the PCB track, which had failed
and gone open circuit. I resoldered the contacts but the problem remains.

The symptom is the same. I see 1.3V, the standby state voltage on pin 13 of
J1 (the output of the PSU) and no other voltages on the output. I hear the
PSU relay clicking but the standby voltage does not change (the service
manual implies that it should).

Is possible that R17 going open circuit causes any other components to fail?

Andreas Also mentions the secondary rectifier diodes, I am finding it
difficult to track these down without a circuit diagram, any clues?

Jim Yanik mentions Capacitor ESR is possible to test for this in circuit or
does one simply have to replace all suspect electrolytics?


Best wishes and thanks again


Payman
 
G

Glenn Davis

Jan 1, 1970
0
I recently repaired a TDS320 with a dead Zytec power supply. All of
the power supply voltages were off. The culprit was a shorted full
wave rectifier on the 5V supply. The 5V tap on the transformer is
rectified by the full wave rectifier and filtered with a capacitor and
inductor filter network. 5V is fed back to the primary circuit which
adjusts the transforer primary for variations in the 5V level. All of
the other secondary taps are rectified, filtered and regulated with 3
terminal regulators. If 5V is down, then all of the other voltages
will be down also.

During my analysis, I was impressed with the construction of the Zytec
power supply board because it was well layed out on quality two sided
FR4 PCB. The 5V output was shorted and I simply traced back to the
transformer and found the defective component. The part was easy to
remove from the heat sink and I found a substitute from an old PC
power supply. I do not remember the part number, but information was
available online.

I do not believe that a schematic is available, but the supply was
easier than most to trace without it. Just keep in mind that 5V must
work for the other supplies to work.

Good luck,

-- Glenn
 
J

Jim Yanik

Jan 1, 1970
0
[email protected] wrote in
Many thanks to all who replied particularly Andreas aka "tekaman" who
appears to be "right on the money", unfortunately his post hadn't
propagated to my news server so I am responding on Google.

Andreas on your advice I investigated the PSU the power resistor on the
primary that you refered to I identified as a 3K9 5W (R17) on the Zytec
22917300 PSU. This was indeed open circuit the track on the PCB had got
overheated and the solder joint had failed. Unfortunately correcting
this does not solve the problem.

The symptoms remain the same the only voltage available on the output
of the PSU is 1.3V, the Standby Voltage, on pin 13 and the only
activity in the scope is the switching of the PSU relay in response to
the front panel switch, still nothing else powers up.

You also mention the rectifier diodes on the secondary, I am finding it
harder identifying these without a circuit diagram, any clues that
might point me in the right direction?

Also does R17 going open circuit cause any other components to fail?

Jim Yanik mentioned ESR on capacitors, is there a way of detecting this
or does one simply have to replace electolytics that one suspects?
Many thanks again


Payman

Dick Smith Electronics makes an inexpensive ESR meter that is very
good.Mine cost me $45 USD.(kit form,easy and fun to build)

Anatek sells the kit version and assembled ones.
www.anatek.com
 
P

Payman Habibelahi

Jan 1, 1970
0
Good call Glenn!

The FEP16DT TO220 packaged dual rectifier diode pair have one that is
shorted I assume that these are used to full wave rectify the 5V secondary.
I will post an update once I have managed to locate a replacement.

Many thanks

Payman
 
T

TekMan

Jan 1, 1970
0
Payman Habibelahi said:
Good call Glenn!

The FEP16DT TO220 packaged dual rectifier diode pair have one that is
shorted I assume that these are used to full wave rectify the 5V secondary.
I will post an update once I have managed to locate a replacement.

Many thanks

Payman
...snipp..



Hi Payman,

semms you made it. Sorry, only postings accepted in the Google, not
as PM due to spamming my mail account (by others).

If you check my posting, you see that I mentioned the rectifiers too.

I've not seen two failures at same time in one Zytek (rectifier &
resistor) yet, but well, misery happens.

For the checking of the the rectifiers within the cirscuit, an
impeadance meter is perfect. If you don't have one: A Huntron Trcker
2000 (wihich is basically an I/V curve tracer with a test frequency of
up to 2 kHz) in the 2kHz mode works satisfactorily.

ciao,
Andreas
 
P

Payman Habibelahi

Jan 1, 1970
0
Problem solved!!

As pointed out initially by TekMan, JiM Yanik and finally Glenn Davis,
problem was the shorted diode (FEP16DT TO220 on a shared heatsink) used to
full wave rectify the 5V output. To complicate matters the Primary power
resistor (3K9 5W R17) had also overheated and lifted away from the PCB.

Many thanks to all who took time to write, I wouldn't have figured it out
without your help. Hope this thread will help out someone with the same
problem, judging by Tekman's response it's a common problem.

Best wishes


Payman
 
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