Tektronix TDS350 PSU

switch,

less

switch is

mains

problem

next

diagrams in

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

"Payman Habibelahi" wrote in news: snipped-for-privacy@individual.net:

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.

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

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

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

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

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

snipped-for-privacy@payman.com wrote in news: snipped-for-privacy@f14g2000cwb.googlegroups.com:

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.

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

ciao, Andreas

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