Tektronix 7854 Oscilloscope  Blurry CRT and plugins - Page 1
Forum section: electronics / repair

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: August 10, 2018, 02:50:20 pm
content: I’ve got this 7854 with completely unreadable screen. I can see the traces but everything else is so out of focus I can’t really recognize any letters or numbers.Also, I have few plugins but wanted to ask if there was some kind of default set of plugins that was shipped with this scope and what would be optimal set of plugins for everyday use.Plugins I have are:7b53a dual time base x 27b85 delaying time base7b80 time base7b92a dual time base7a13 differential comparator x27a26 dual trace amplifierAre all those plugins usable in this scope?I’ve plugged the plugins in the A and B slots on the right so I have two time bases there.I can see the traces on the pictures I will try to include.When I plug something in one of the horizontal slots on the left the frame hangs and doesn’t pass the self test.How to test all the plugins and see what’s working and what’s not.

author: particleman
author_rank: Regular Contributor
timestamp: August 10, 2018, 04:48:49 pm
content: The last 7a26 plugin I got needed tantalum caps changed. Several were shorted, plugging it in caused the scope to not power on correctly. Changed caps and all works well now. 7k extender will make things easier.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: August 10, 2018, 05:41:42 pm
content: Here are some screen shoots with unreadable and out of focus numbers on crt

author: ignator
author_rank: Regular Contributor
timestamp: August 10, 2018, 05:45:19 pm
content: The good thing is that it mostly appears to be working.Open the case, and measure all the power supply voltages per the schematic. It's possible the previous owner messed with one of the focus magnets or grid voltages, and such. The manual should give an alignment procedure to follow.

author: tautech
author_rank: Super Contributor
timestamp: August 10, 2018, 07:29:27 pm
content: Quote from: Ordinaryman1971 on August 10, 2018, 02:50:20 pmMainframe:http://w140.com/tekwiki/wiki/7854Plugin list:http://w140.com/tekwiki/wiki/7000-series_plug-ins

author: Johnny10
author_rank: Frequent Contributor
timestamp: August 10, 2018, 07:36:37 pm
content: You have tried the focus knob on the far right?Takes a while to get used to setting up these scopes.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: August 11, 2018, 12:35:17 am
content: Correct, i've tried all the knobs and everything that is accessible on the front of the unit.There are some pictures with focus knob turned one way and the other...

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: August 11, 2018, 01:20:43 am
content: I've connected the signal gen and the trace is pretty nice and focused, it's the readout that's really bad.I still can't quiet trigger on this trace, it's running through the screen but it looks good.I've looked inside of 7A26 the dual channel trace amp and there is a bunch of tantalum caps, when I plug it in the frame it doesn't pass the self test...

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2018, 01:31:59 am
content: That trace looks pretty ugly to me, like there might be something wrong with one of the deflection amplifiers and the mangled readout is just a symptom. Look at how the gaps where the beam shoots off to draw the readout look all wonky.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: August 11, 2018, 06:04:40 am
content: I would like the readout to look that ugly and I would be very happy... just don't have an idea why would readout be that bad and the trace be... less ugly.Will check the voltages tomorrow.... very nice piece of gear...

author: james_s
author_rank: Super Contributor
timestamp: August 11, 2018, 07:24:25 am
content: Look at the way the readout is drawn, it's made of a bunch of small vectors while the main trace has much longer straight sections. Also it may just be the photo but the display looks extremely bright in some of those pictures. Something is clearly wrong, and I suspect it's the same problem making the trace and the readout look ugly.

author: floobydust
author_rank: Super Contributor
timestamp: August 11, 2018, 05:36:03 pm
content: You have to narrow down what is not working - readout focus, readout character size, banding on the waveform display, crappy blanking etc.The sine wave looks not so great, you can see two intermodulations causing banding and some sloppy blanking, presumably during the time slice the readout is displayed.I would first ensure all power supply rails are correct and their capacitors are not bad.The readout intensity and focus look way off. I can't tell where you have readout intensity or stored waveform intensity set on the front panel.If the power supply is OK, you could try the focus switch/amplifier trimpots sheet 13, Focus gain R63, Focus output level R70.Again, nothing should be that far off unless a component has failed, so trying to fix it by adjusting a trimpot has limits.W140 Tektronix 7854 wiki

author: Johnny10
author_rank: Frequent Contributor
timestamp: August 11, 2018, 07:57:45 pm
content: What does the readout say?Can't make out the characters.Should be "Self Test Complete"This pic from an Iphone is of my scope screen from 7854 1K sine wave.

author: Ordinaryman1971
author_rank: Frequent Contributor
timestamp: August 11, 2018, 09:07:25 pm
content: Can't tell you, regardless of the settings I can't read it.Will measure all the rails and report the findings...