Tektronix 7904 power supply

[Woodley]

Hi... I'm new to this forum so forgive me if I'm raising a familiar subject but my Tektronix 7904 is in 'tic tic' burst mode.
I've read the 'Servicing the 7904 high-efficiency power supply' paper and I need to get some comments on the following.....
The paper says to test the resistance of the power supply test points on the Z axis board. So I did.  The results were:

Test point_____  Resistance______ What I measured
+130V                6K                       8.05K
+50V                  2K                       3.7K
+15V                  90                       85
-15V                   100                     213
-50V                   250                     4.8K
+5v lamp            800                     4.7K

These readings were between the test point and ground with all the plug-ins removed, Control Illum OFF and Grat Illum counter clockwise

I then removed the power supply and tested the points again....

+130V                6.6K                 10.1K
+50V                  2K                    4.57K
+15V                  90                    85
+5V                    65                    57
-15V                   110                  226
-50V                   2K                    57K
+5V lamp            Infinite             Infinite

I thought that as none of these reading are lower than they should be, the mainframe can't be overloading the supply so I started investigating the PS. I found a tantalum cap (C1353 on the rectifier circuit board -17V line) to be short circuit and I thought I had found the problem. After checking pretty much all of the other caps and semiconductors on the PS, I reassembled it and fired it up.... tic tic tic.
Does any one have any suggestions? Should I just go and replace all the electrolytic caps?

Thanks for listening!

[PaulAm]

The tick problem can be a real bear.

The service manual has a flow chart to follow to help troubleshoot the power supply.  A number of people in the same boat have found it useful to build a dummy load so you can work on it out of the scope.

You might try looking into the tekscopes yahoo group archives; you'll find quite a bit on the problem there.

I solved the problem by buying another 7904 mainframe for $25.

[tautech]

Welcome to the forum.

While not intimately familiar with the 7904, Yes I'd suspect faulty PSU caps could trip the controller into an over-current mode.
Do you have the Tek faultfinding pdf from the links thread in this Repair board?

It's also useful if you provide a link to the Service manual so we can all be on the same page. 

[Woodley]

Hi
Thanks for your replies.... here's the link to the document I have been using.
Could you show me the way to find the fault finding PDF?
Also, I'll take a look at the yahoo group.
Does anyone know how to build the dummy load that you mentioned... it would be a big help.


http://www.ko4bb.com/manuals/92.22.85.72/Tek_7904_Power_Supply.pdf

[tautech]

Hi
Thanks for your replies.... here's the link to the document I have been using.
Could you show me the way to find the fault finding PDF?
Also, I'll take a look at the yahoo group.
Does anyone know how to build the dummy load that you mentioned... it would be a big help.


http://www.ko4bb.com/manuals/92.22.85.72/Tek_7904_Power_Supply.pdf

Tek pdf in this thread, first post:
https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/

[Daxxin]

check all psu diodes for shorted or low in reverse

[PaulAm]

There are also leaky transistors and out of tolerance zeners.  It could also be the inverter IC.  Unfortunately, there doesn't seem to be a single most common failure for this problem.

There's a pdf in this directory on fixing the power supply:

http://www.ko4bb.com/getsimple/index.php?id=manuals&dir=Tektronix/Tektronix_-_7904A

Join the tekscopes group if you haven't already.  Lots of good advice there.

[TubeAlloy]

Hi,

  I just got a Tektronics 7904 yesterday. When I turn on the power for the 1st time, I heard two pops. The scope turns on, showed characters on the CRT screen, but no trace. Nothing I did could get the trace to show up.  So I try to debug, starting from the power supply.

  A question about this "burst mode". If I completely disconnect the power supply from the main frame by removing all the connectors, do I suppose to get into the burst mode? That is what I got. Before I disconnect them, the PSU was quiet. After disconnecting, I got this tic tik tik sound. Is my PSU bad?

  Thanks for your help.

[siggi]

Hey TubeAlloy,

Check the service manual. Probably the PSU needs a minimum load and is hitting overvoltage protection without it.

Siggi


Sent from my iPad using Tapatalk

[TubeAlloy]

Thanks siggi. The service manual says the ticking sound happens when the psu has problems but not bad enough to blow the fuse.

After moving things around a little, an extremely bright trace appears. It runs for about a second then the psu does two rapid ticks, and the trace "dissolves" then reappears. This cycles round and round.

The semi rectified output from the +-50V supplies are asymmetric: one shows 30V the other 48V on a cheap multimeter. I will replace the electrolytic capacitors on both of them when the store opens.

There are a lot of plug in components, anyone of them can have a bad contact.

[dom0]

After moving things around a little, an extremely bright trace appears. It runs for about a second then the psu does two rapid ticks, and the trace "dissolves" then reappears. This cycles round and round.

Check if the Z axis logic and amplifier work correctly. Superextremeexceedingly bright trace and a fast power off could mean that the beam current (which is monitored by the supply) limit was tripped.