Tek 7B92 Intermittant Trigger - Page 1
Forum section: electronics / repair

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 23, 2018, 11:48:26 pm
content: It mostly has no trigger, no matter the setting. However, it does sometimes have a flickering trigger in the correct settings. I once had a stable trigger on AC LF or HF filter only, but that went out as well. I'm suspecting (but still hoping it's not), the K600 relay. This seemed to be confirmed by switching the relays around making it totally dead.Assuming it's the stupid proprietary relay...now what?The guy I got it from has another one, but chances are it could be dead too. What's the best option? I did find thishttp://w140.com/tekwiki/wiki/Miniature_relayswhich has references for dealing with dead relays. Should I try repair, modern replacement, or salvaging a relay from an unwanted module (if I can get one off the guy I got the scope from)? The articles did recommend this as a replacementhttps://www.mouser.com/ProductDetail/TE-Connectivity-Axicom/V23079D2003B301?qs=bmiCE96t7eTU3yBKytJE%252bQ%3D%3Dbut with me it will likely be a dodgey repair (I hate small awkward parts). But I'm also afraid original parts will just fail again.NOTE: turning the relay around had no effect, still dead.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2018, 11:24:54 am
content: The two relays are only involved with the external trigger input. If the 7B92 is not working correctly when set to internal triggering, then something else is wrong.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 24, 2018, 01:30:09 pm
content: Quote from: David Hess on October 24, 2018, 11:24:54 amWell at least it's not that.But it'a still acting like a bad connection. I'll have a look at the switches, but other that nothing else looks visably corroded (could be invisibly tarnished though).EDIT: not the chassis either, another sweep unit works fine in that slot.

author: David Hess
author_rank: Super Contributor
timestamp: October 24, 2018, 04:36:02 pm
content: I only have 7B92As so I am not that familiar with the tunnel diode based 7B92. In order I would:1. Make sure the +/-15 and +/-50 volt supplies are present in the plug-in. A solid tantalum may have shorted and burned out an LCR decoupling network at the power inputs.2. Test the external trigger input to see if it works any better than the internal trigger.3. Go through the trigger calibration procedures in the service manual. Tunnel diode trigger circuits can drift.The people over on the TekScopes@groups.io and TekScopes2@groups.io email lists may have better suggestions.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 25, 2018, 12:25:55 am
content: I have no plugin extender, but I could just check for shorts. I'll also test external trigger. I doubt it's a calibration issue, as that would still be a steady no trigger which this is not, it's rapidly and randomly going in and out of trigger lock, then it goes out for a while before flashing again. Unless something is right on the very brink.

author: Cyberdragon
author_rank: Super Contributor
timestamp: October 25, 2018, 08:02:16 pm
content: There are no shorts. However, there was obvious damage where a pin of U660 had been totally crushed and was not in the socket. Unfortunately, that was not the only problem.It has no trigger at all now, no internal, external, delayed, or even single sweep (which just gets stuck in ready). However, the trigger does fire once each time you mess around with switches, so it's not totally fried. I have also checked the diodes and transistors on the trigger board (roughly, as in in-circuit).I'll see if trying to calibrate it does anything or reveals something.

author: David Hess
author_rank: Super Contributor
timestamp: November 01, 2018, 01:32:52 am
content: You may need to start tracing circuits although it is a bad sign if automatic mode does not produce a steady sweep with no trigger.The side covers on the mainframe can be removed to give some access to the plug-in without an extender.

author: Cyberdragon
author_rank: Super Contributor
timestamp: November 01, 2018, 06:08:37 am
content: It sweeps in auto, just not in normal or single.On a side note, the slot where this module was found (A) seems to malfunction even with a known working module (fine in slot B). Could this be related to a cascade failure or is it a seperate issue? Also note, it was installed next to a 7A42 that suffered a battery leak (thread coming soon), which may be related, though I see no signs of corrosion on the 7B92.

author: Johnny10
author_rank: Frequent Contributor
timestamp: November 01, 2018, 06:50:17 pm
content: I had plug-ins not working correctly or not recognized caused by the internal connectors being loose and dirty.Think it is common problem on these old mainframes.

author: David Hess
author_rank: Super Contributor
timestamp: November 06, 2018, 09:49:02 am
content: Quote from: Cyberdragon on November 01, 2018, 06:08:37 amIt is probably a separate issue. Check the interface connector inside the mainframe for physical damage.