Tektronix 7603 Oscilliscope Wide Beam Isssue

[twilightofthedogs]

Ok actually the military version of the 7603... an/usm-281c but close enough. Wondering if anyone knows what's going on with this... trace is showing up very wide. If I tap the housing I can get it to show up correctly but it will go back to wide if I tap again. Seems like a contact issue somewhere. When the trace is showing correct everything seems to work fine. Photos attached. Any thoughts?

[Enzo]

Um, does it still act that way if you disconnect the probe?

To my eye it doesn't look like an out of focus trace, it looks like a RF signal displayed at a low sweep rate.

Don't touch any control other than sweep rate, and turn the horizontal rate up click click click, turning nothing else. DO you get up to a higher range and find that wide band resolving into a sine wave?

I could be wrong.

[twilightofthedogs]

Doesn't seem to matter whether a probe is connected or not. Horizontal sweep doesn't do much to change the shape. Here's a couple shots of the scope with a sine wave applied in between a couple good thwacks

[loudthud]

Looking closely at the two pictures in the first post, it looks like you simply turned up the Intensity control. Any fault in the power supply, or the CRT circuit that could cause a trace to expand vertically would most likely also affect the trace horizontally. Press the BEAM FIND. The trace should shrink down, get slightly brighter and defocus. In the pictures you posted in post 3, I don't see the trace going off the edge of the screen like it should. The trace is way too bright, turn down the Intensity. In the second picture it looks like the trace is so bright that you can see the re-trace.

Observe the trace at very low sweep speed (0.2 seconds per division). Keep the Intensity (brightness) of the trace low. Do you have a well defined round dot, the Focus control should have control over the size and shape of the dot as it moves across the screen. Try this with both channels by changing the "VERT MODE" to the right of the screen from LEFT to RIGHT. Do the position controls on the plug-ins control the Vertical position of the trace when that plug-in is selected ?

[nevetslab]

I can't make out the time base settings in the second set of images, but see in the first two images the Time Base and Delayed Time Base controls are unlocked. Rotate the upper Delayed Time Base control CCW until it stops, now locked with the main time base control At that setting, when you first pull out the Delayed Time Base control, the trace is brightened. Rotating the Delayed Time Base control then reduces the span that would be expanded with additional control manipulations.

When you tap....WHAT are you tapping on? The handle structure that ties the rear to the front? The front panel housing? The CRT shield? Have you tried powering down, unplug each of the plug-ins, then reseat them. Does swapping vertical plug-ins with each other make any difference?

I had the OS/245 Military 7603 for years, before upgrading to a 7834 system. Had several 7000 series scopes, sold the 7704, 7603, gave away the OS/245 with the manual, but I do still have the 7603 Mainframe Service Manual. Do you have the manual for this scope? Very similar to the 7603, though has a different CRT, all of the CRT Readout system is not installed. Lots of additional shielding in the mainframe. All of the wiring is nicely bundled, and are tight-fitting gold square pins with mating harness wire sockets, so I wouldn't suspect them, though still possible you have solder joint fractures on header pins on the boards that are causing this.

I haven't looked lately to see what documentation is available on-line to be down-loaded. Nothing like having the original doc's, as Tektronix produced beautiful service manuals and very thorough maintenance and theory of operation sections. I could be talked into loaning out my 7603 manual if I had assurance I'd get it back. I no longer have access to the fine Xerox color copier from years ago, when I was reproducing and selling service manuals for test gear. I do at least have access to a copier/scanner that will do B-Size pages (11 x 17), though it would take some time to repro this manual.

[twilightofthedogs]

Woops. I realize now that I posted the wrong photo in my first post. Was supposed to be a fuzzy one and one with a nice tight trace. Sorry for the confusion. I got the old precision chopstick out this afternoon and with some precision poking I was able to at least narrow it down to the vertical amplification board. I took it off, cleaned up the contacts, renewed some heat sink paste I found in there and put it back together. That seemed to clear up the issue for now. Still a lot to figure out with this massive beast! Thanks for the generous manual offer nevetslab. I was able to locate the 7603 manual online so I should be in way over my head for a while now! I don't really expect a lot of trouble with this though, I mean it did JUST receive a "special calibration" in 1991 and all!

[nevetslab]

Apart from Tek's tendencies to have used multi-section cap-cans that over the years fail, and a PITA to replace/remove, they DO hold their adjustments and calibration for a LONG TIME. I've been buying top grade used gear like that for decades....and have never had the funds to send out for calibration. Their relative accuracy with all the other comparable gear in their class yields very accurate and predictable measurements.....certainly good enough for my needs working on pro audio & music gear. As 7000 series gear flooded the used market in the late 90's/early 2000's, I bought a lot of plugins and some extra mainframes. Also have the plug-in extender module, which allows active servicing of vertical plug-in amps as well as time bases on these instruments. I have a 7633 on my bench, plus a backup mainframe in storage, along with a back-up 7834. I tried to put my 7623A into service, only to find the mainframe is not working now, so another project for laterman.

Before the days of ebay, there were a couple equipment manual sellers that appeared at the TRW Radio Amateur Technical Swap Meet last Saturday of the month in Redondo Beach, CA, so I always made it a point to track down all the documentation of whatever gear I bought, along with many prime used test equipment sellers. All that went to ebay once that got solid footing.

[loudthud]

In general, there will be only one adjustment in the power supply and perhaps one more for the high Voltage. Don't adjust the power supply unless you are prepared to do a complete calibration. That one power supply acts as a reference for all the others. Usually it's the -50V on the 7000 series IIRC. The 7603 uses a linear power supply. The TO-3 power transistors use Tek made PCB mount sockets that will easily break if you over tighten the screws. Best if you don't touch them.

[twilightofthedogs]

In general, there will be only one adjustment in the power supply and perhaps one more for the high Voltage. Don't adjust the power supply unless you are prepared to do a complete calibration. That one power supply acts as a reference for all the others. Usually it's the -50V on the 7000 series IIRC. The 7603 uses a linear power supply. The TO-3 power transistors use Tek made PCB mount sockets that will easily break if you over tighten the screws. Best if you don't touch them.

Thanks. I was able to figure out the power rail check and fortunately everything was spot on. If this were a guitar amp I'd be eager to get in there and replace those 50 year old power supply filter cap cans, but I studied all what I'd have to do to accomplish and think I'll leave it alone for now!

[nevetslab]

I too have left the cap cans as is until they fail. I know at some point, my bench scope will have to be dealt with, as there's a tiny bit of power supply ripple that can be seen in the trace. When I had to move my shop over to a different building for a year, when I came back, I was concerned that I'd have to deal with that, but it powered up just fine after 15 months of being off, and still running just fine. Love it!

[nosaj]

I can't make out the time base settings in the second set of images, but see in the first two images the Time Base and Delayed Time Base controls are unlocked. Rotate the upper Delayed Time Base control CCW until it stops, now locked with the main time base control At that setting, when you first pull out the Delayed Time Base control, the trace is brightened. Rotating the Delayed Time Base control then reduces the span that would be expanded with additional control manipulations.

When you tap....WHAT are you tapping on? The handle structure that ties the rear to the front? The front panel housing? The CRT shield? Have you tried powering down, unplug each of the plug-ins, then reseat them. Does swapping vertical plug-ins with each other make any difference?

I had the OS/245 Military 7603 for years, before upgrading to a 7834 system. Had several 7000 series scopes, sold the 7704, 7603, gave away the OS/245 with the manual, but I do still have the 7603 Mainframe Service Manual. Do you have the manual for this scope? Very similar to the 7603, though has a different CRT, all of the CRT Readout system is not installed. Lots of additional shielding in the mainframe. All of the wiring is nicely bundled, and are tight-fitting gold square pins with mating harness wire sockets, so I wouldn't suspect them, though still possible you have solder joint fractures on header pins on the boards that are causing this.

I haven't looked lately to see what documentation is available on-line to be down-loaded. Nothing like having the original doc's, as Tektronix produced beautiful service manuals and very thorough maintenance and theory of operation sections. I could be talked into loaning out my 7603 manual if I had assurance I'd get it back. I no longer have access to the fine Xerox color copier from years ago, when I was reproducing and selling service manuals for test gear. I do at least have access to a copier/scanner that will do B-Size pages (11 x 17), though it would take some time to repro this manual.

I have the 7603/R7603 service manual, the operators manual and the dual time base 7b53a/5\7353an service manual. These are original. They are looking for a new home. Contact via pm for more info.
nosaj