7B92 with no repeating sweep


 

After freeing the EXT÷10 trigger source switch, replacing the broken rear plate (with one from a poorly treated 7B53A), cleaning the trigger, readout, and sweep boards with IPA to try to remove some kind of white residue (with limited success), and generally giving the entire instrument a close visual inspection, I decided that it looked good enough to try it in my 7633. I did verify that the white residue was not shorting anything out, though I have not figured out what the residue is or where it came from.

The good news is that it powers up and lights most of the lights. The bad news is that I can't get a repetitive sweep or trigger. I can get one sweep to happen when I switch the TIME/DIV knob between different positions, but that's it. I've tried in AUTO, NORM, HF SYNC and SINGLE SWEEP modes, with AC and DC coupling, using INT trigger source as well as LINE. Nothing gives me a repeating sweep. In SINGLE SWEEP mode the RESET button has no apparent effect.

I've read through the circuit description in the service manual, and did not find it particularly enlightening. I'm in the process of reading the Circuit Description Supplement, which is much more detailed (if not more enlightening for me). I'm tempted to just start testing circuit continuity and diode drops across diodes and transistors, in the hope of stumbling on the culprit (or eliminating a bunch of possible culprits), but I'd like to know if anyone has any wisdom or experience that might guide me. My plan is just to work my way through the internal trigger path and hope that I find a dead transistor or diode (regular diode, not a tunnel diode, which I don't understand how to check with a multimeter, though from reading old posts in this group, it sounds like I basically can't check tunnel diodes with a multimeter).

I would be very happy (and unsurprised) to discover that I have been setting the controls incorrectly (it's certainly happened before), but I have a feeling that's not what's going on here.

I'd really like to be able to get this 7B92 working, both because I'd like to have ALT horizontal mode on my 7603 and 7633, and because I just like the lighted delayed trigger controls.

-- Jeff Dutky

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