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7L5 sweep problems


Ed Breya
 

This is a continuation of a serious 7L5 restoration started last year. Here's where I left off:

https://groups.io/g/TekScopes/topic/31248178?p=Created,,,20,2,0,0::recentpostdate%2Fsticky,,,20,2,0,31248178

Since the recent 7A42 battery issues discussion, I've been re-looking at my "smart" plug-ins to make sure there are no battery issues. The 7L5 does not even have a microprocessor, so no battery.

While looking, I figured maybe I'd take another shot at this repair project, and see how it is. I managed to find some of my old notes on this, but still some are missing. I fired it up to refresh my memory, and recalled some of the issues. I replaced a suspect part in the trigger section of the sweep board, and it now seems closer to proper operation. The problem where I left off was no video signal, but I think the basic problem is mostly in the sweep generator. I'm not getting proper horizontal sweeps with any settings. As I understand from what I've read in the manuals, if the sweep doesn't run right, neither does most of the system.

I've been doing tests, piecemeal, by taking all the ^%*#$^ apart, tacking wires onto various nodes, then putting all the ^&$#@% back together, and popping it in the mainframe for checkout and measurements. So far I've confirmed the seep reference voltage is present, and the time/div knob probably is working correctly, since the reference voltage changes from -2V to -4V at certain settings as it should. Also, the sweep out from the integrator is wrong - it's sort of a triangle wave pulse that seems too small (about 5-7 volts peak versus the proper 8 or more), and not often enough. Last time open, I wanted to tack onto the data lines for the timing resistor selector (HC4051) and sweep gate signal, but they weren't accessible without taking it apart deeper. These will be included next time. Also, the "dot" has disappeared, and doesn't show in any settings. Earlier on, it did show, but was stuck on, and flakey in position

As it is now, all the front panel functions seem to do something, and all the readouts seem to work. When A and B storage are off, there's the real time baseline sweep displayed at the bottom, but it's at a constant, slow rep rate (flickering) and velocity, unresponsive to the time/div setting (which is not read out on screen anyway, so can't tell if it's actually trying to do anything), except that it disappears completely at some time/div and span settings, where I think various modes are supposed to change. Manual scan does function, but it's dot flickers too, at the same rate. As I understand, in manual sweep the actual sweep runs at some higher rate to keep things working, and the horizontal signal is instead from the pot.

So, I'm pretty sure the sweep generator or something related is way out of kilter. It's tricky to figure out because the behavior of the sweep is modified by various operating settings too, so something elsewhere could be the real problem. I've read a lot in the various manuals, and getting better understanding of what's supposed to be going on. It is a complicated beast, with a lot crammed in there. Also, as I previously admired the way some of the plug-in is put together, I now hate the way other parts are. It's a real PITA to disassemble it below a certain level, or to access certain things, especially that sweep board, where most of my work is now focused. The other problem is that every time it has to be taken apart, it's more wear and tear on the innards, and risk of further damage - especially if I eventually have a tantrum and destroy it worse than I got it.

So anyway, if anyone has experience with the 7L5 and diagnosing these issues, please advise.

Ed