Tek 485 trigger

[particleman]

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.

[tautech]

with a 10V sine wave with a 10x probe

That 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.

If I decrease the voltage to 5V and use the 50ohm button and a 1X probe It will trigger on all settings.

The 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.pdf

Have 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

[particleman]

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.

[liquibyte]

Have 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

http://bama.edebris.com/download/tek/485/tek-485.pdf

[particleman]

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.

[tautech]

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.

Sounds like the Ch 1 input attenuators may have been damaged by an input overvoltage event.

[particleman]

I have swapped the attenuators from the working channel to the non working channel and the other way around and it made no difference.

[particleman]

Figured it out it was a broken contact on the hi-z board. the plastic had cracked allowing the contact to float around.

[tautech]

Figured it out it was a broken contact on the hi-z board. the plastic had cracked allowing the contact to float around.

Good find. 
Can you specify the connector number from the schematics so others with a similar problem know which one to suspect/check.

[particleman]

Tektronix part# 131-1031-00  on the back of the hi-z board.