Tek 485 trigger - Page 1
Forum section: electronics / repair

author: particleman
author_rank: Regular Contributor
timestamp: September 23, 2014, 09:19:16 pm
content: I just got a good deal on a 485. one channel will only trigger on .1-.5 changing volts/div to 1-5V or 5-50mV it will not trigger. this is with a 10V sine wave with a 10x probe with the scope set at 1Mohm.If I decrease the voltage to 5V and use the 50ohm button and a 1X probe It will trigger on all settings. Any ideas.

author: tautech
author_rank: Super Contributor
timestamp: September 23, 2014, 11:03:00 pm
content: QuoteThat changes input to 1 V sine wave but a V/div setting of less than .1 will have the waveform much larger than the display.This is never a good idea with a CRO. The Vertical amp is overloaded.QuoteThe 50 ohm input is attenuating the signal to manageable levels.Do you have the same result with 5 V sine and 10 x with 1 M input?Get the Tek troubleshooting guide:http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdfHave you a Service manual?Plenty of links to find one here:https://www.eevblog.com/forum/repair/repair-documents-and-links-sticky-me-please-mods/msg477909/#msg477909

author: particleman
author_rank: Regular Contributor
timestamp: September 23, 2014, 11:26:07 pm
content: 5V sine ,10x probe setting, 1V per division, 1Mohm setting. No trigger. 5V sine, 10x probe setting, .5V per division setting, 1Mohm setting. Trigger and wave form. Channel 2 triggers with no issues on all settings.

author: liquibyte
author_rank: Frequent Contributor
timestamp: September 23, 2014, 11:27:15 pm
content: Quote from: tautech on September 23, 2014, 11:03:00 pmhttp://bama.edebris.com/download/tek/485/tek-485.pdf

author: particleman
author_rank: Regular Contributor
timestamp: September 24, 2014, 06:23:55 pm
content: I hooked an ohm meter to the output the working channel shows the correct 1Mohm through all divisions. Non working channel shows 1Mohm on divisions 5V-.1V then 0ohms in the 5-50mV. 50ohm setting shows 50ohm through all divisions on both channels and is working properly.

author: tautech
author_rank: Super Contributor
timestamp: September 24, 2014, 07:00:02 pm
content: Quote from: particleman on September 24, 2014, 06:23:55 pmSounds like the Ch 1 input attenuators may have been damaged by an input overvoltage event.

author: particleman
author_rank: Regular Contributor
timestamp: September 24, 2014, 07:47:26 pm
content: I have swapped the attenuators from the working channel to the non working channel and the other way around and it made no difference.

author: particleman
author_rank: Regular Contributor
timestamp: September 25, 2014, 07:32:04 pm
content: Figured it out it was a broken contact on the hi-z board. the plastic had cracked allowing the contact to float around.

author: tautech
author_rank: Super Contributor
timestamp: September 25, 2014, 07:59:30 pm
content: Quote from: particleman on September 25, 2014, 07:32:04 pmGood find.Can you specify the connector number from the schematics so others with a similar problem know which one to suspect/check.

author: particleman
author_rank: Regular Contributor
timestamp: September 25, 2014, 11:48:04 pm
content: Tektronix part# 131-1031-00 on the back of the hi-z board.