Help request on Tek 465 oscilloscope - Page 1
Forum section: electronics / repair

author: vangheli
author_rank: Contributor
timestamp: May 07, 2019, 07:08:37 pm
content: Hi everybody.I have a problem that I am not able to trace on my recently acquired old Tek 465 scope.Initially ,operating on CH2 , pressing the INV button , the trace disappeared. I adjusted , as per service manual section 6 page 15 subsection 12 ,the Invert Balance trimmer R215, and after the trimming ,actually the trace only moved not more than half division operating the CH2 INV button.But I observe that the trace is not inverted. The only way to see it inverted ,after pressing the INV button is to use both channel ,on the alternate mode, trigger on CH1 (with the two signals frequency related).Is there anybody who can explain this behavior of the scope , and show me the direction to follow to fix the problem?Thank you for your help

author: David Hess
author_rank: Super Contributor
timestamp: May 07, 2019, 10:17:53 pm
content: The invert circuit occurs before the trigger pick-off so it should not affect triggering in any other way. This indicates a significant problem. Does triggering on channel 2 work when invert is not used?The way the invert circuit works is that the switch swaps bias between two pairs of transistors so the mechanical switch itself is not part of the high frequency circuits. If there was something wrong, then likely the amplitude when invert is active would differ significantly which fits with the invert balance needed to be adjusted.So check the vertical amplitude with and without invert.Also reseat transistors Q222, Q224, Q226, and Q228 (right side of schematic 2) in their sockets. Do not remove or swap them around because they are matched pairs. If a transistor connection was bad, then after doing this you will need to readjust the invert balance.I suspect what is happening is that either transistor Q226 or Q228 has a bad connection in its socket so when invert is active, only half of the differential signal is going through and the common mode level is incorrect causing an invert balance problem and preventing proper triggering.

author: barry14
author_rank: Regular Contributor
timestamp: May 08, 2019, 12:29:43 am
content: Your scope is working normally. If you are triggering on Channel 2, inverting the signal will not show on the screen since the triggering settings have not changed. Thus if the trigger is set for positive going, then the scope will still display the positive going portion of the channel 2 signal and the inversion does not change that. As you observed, however, if you set the scope to trigger on channel 1, and the signal on channel 2 is time related to that signal, then the inversion will show since channel 2 is not the trigger source.

author: David Hess
author_rank: Super Contributor
timestamp: May 08, 2019, 03:19:37 am
content: He said the trace disappeared when triggering on channel 2 when inverted.

author: vangheli
author_rank: Contributor
timestamp: May 08, 2019, 08:07:51 am
content: thank you David for your advice .As soon I have time I will recheck everything and come back.

author: vangheli
author_rank: Contributor
timestamp: May 08, 2019, 08:10:20 am
content: thank you Barry14 for your help.I will be back with other information as soon as possible