Tektronix 7904 power supply - Page 1
Forum section: electronics / repair

author: Woodley
author_rank: Newbie
timestamp: September 01, 2015, 06:30:58 pm
content: 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 4.7KThese readings were between the test point and ground with all the plug-ins removed, Control Illum OFF and Grat Illum counter clockwiseI 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 InfiniteI 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!

author: PaulAm
author_rank: Frequent Contributor
timestamp: September 01, 2015, 08:09:37 pm
content: 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.

author: tautech
author_rank: Super Contributor
timestamp: September 01, 2015, 08:11:14 pm
content: 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.

author: Woodley
author_rank: Newbie
timestamp: September 02, 2015, 08:11:09 am
content: HiThanks 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

author: tautech
author_rank: Super Contributor
timestamp: September 02, 2015, 11:17:18 am
content: Quote from: Woodley on September 02, 2015, 08:11:09 amTek pdf in this thread, first post:https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/

author: Daxxin
author_rank: Regular Contributor
timestamp: September 02, 2015, 03:34:23 pm
content: check all psu diodes for shorted or low in reverse

author: PaulAm
author_rank: Frequent Contributor
timestamp: September 04, 2015, 05:24:01 pm
content: 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_-_7904AJoin the tekscopes group if you haven't already. Lots of good advice there.

author: TubeAlloy
author_rank: Newbie
timestamp: December 12, 2015, 05:01:28 am
content: 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.

author: siggi
author_rank: Regular Contributor
timestamp: December 13, 2015, 01:07:45 am
content: Hey TubeAlloy,Check the service manual. Probably the PSU needs a minimum load and is hitting overvoltage protection without it.SiggiSent from my iPad using Tapatalk

author: TubeAlloy
author_rank: Newbie
timestamp: December 13, 2015, 05:07:07 pm
content: 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.

author: dom0
author_rank: Super Contributor
timestamp: December 13, 2015, 05:25:04 pm
content: QuoteCheck 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.