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TDS 410A Power Supply Problems

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TDS 410A Power Supply Problems

Postby epilog on Wed May 11, 2011 6:41 am

After 2 years of faithful service the power supply on my 410A became defective some time after a power outage.
Its made by Power Systems Inc: PS-1725 (TEK #119-3939-01) and looks new without any discoloration.

I followed along with the service manual and identified "A25 low voltage power supply" as the culprit.
The PSU and power line filter was then removed from the case.
The service manual indicates to attached a 1A load to the 5V rail - I found 3 light-bulbs with a combined
current draw of 1.7A at 5V

The high side caps charged to 310V and I assumed the automatic voltage selector to be Ok.
On power up the light-bulbs briefly flash (75ms) every 2 to 3 seconds.
The rail voltages are so brief that a multimeter was of little use, so in came my old trusty analog scope.
Now it was easy to see the power rails exponentially tending towards their nominal values - again only for 75ms before collapsing.

Another subscriber here had success by replacing the LM393 dual comparator - in a few days I'll try to do the same.

Has anyone any suggestions for SMPS cyclic behavior?
Ideally a schematic would help.
Thanks in advance
epilog
 
Posts: 1
Joined: Wed May 11, 2011 6:10 am

Re: TDS 410A Power Supply Problems

Postby jdattilo on Sat May 21, 2011 3:49 pm

Light bulbs when cold have a very low resistance compared to their steady state operation so as a load they will draw alot of current at turn on until they get to operating temperature. The power supply may be shutting down due this overload. Do the test with a resistive load that is constant and see if it still acts the same.
jdattilo
 
Posts: 2
Joined: Thu May 19, 2011 6:57 pm

Re: TDS 410A Power Supply Problems

Postby e.m.c.2 on Mon Jun 25, 2012 4:07 pm

Hi all,
I also have a problem with this power of a Tek 410a,
were burned two resistors R10 and R11 on the primary section of the PSU,
if you can measure them to tell me their value?

http://img195.imageshack.us/img195/3935/img0050hwc.jpg
http://img443.imageshack.us/img443/5456/img0051rm.jpg
http://img35.imageshack.us/img35/1451/img0052zo.jpg

possibly if there was a scheme would be perfect

many thanks
e.m.c.2
 
Posts: 1
Joined: Mon Jun 25, 2012 3:44 pm

Re: TDS 410A Power Supply Problems

Postby drfonk on Tue Nov 06, 2012 1:04 am

This post is from quite some time ago but this may be of interest when fixing some older tek scopes. I have not managed to find any schematics for older tek scopes that I own but to my surprise, when I was fixing an old 500 Mhz Lecroy scope and a TDS460A 400 Mhz scope recently. I found the PSU PCB`s looked very similar, It turns out that both supplies were made by "power systems inc".

I needed to find out if a custom component was still available so I tried to contact the company behind the supply, after some research I found that they had been bought over in the 90`s by a large company and then liquidated in 2003 :( . After some further research I found that there was a few lecroy service manuals available which had very poor scans of the schematic on the power supply side. If found two different schematics which were very similar to the Tek supply but not exactly the same, although not the same, these schematics did help me figure out how the supply worked and what the problem was on both supplies I had.

best regards

John
drfonk
 
Posts: 8
Joined: Tue Nov 06, 2012 12:04 am

Re: TDS 410A Power Supply Problems

Postby w3kq on Sun Apr 13, 2014 1:58 am

John,

Thank you for sharing this information back in 2012. I found my TDS 420A LVPS dead tonight and decided to do a Google search before starting to try and reverse engineer this old supply. I'm so glad I did! I don't mind working on test equipment as long as I get to set the time frame.... but this time I need to fix another pressing item for a client and didn't need to find my digital scope dead. Thanks again!!

Rick
w3kq
 
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Joined: Sun Apr 13, 2014 1:52 am


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