EMRFD Message Archive 13597
Message Date From Subject 13597 2017-02-17 00:40:30 sohosources My Oscilloscope Okay?
I have a question about my 'scope...
It's an Elenco S-1360, 60-MHz dual-trace unit. Same as B&K 2160 (not the 2160A...just the 2160). Mine's 20-30 years old and has seen very light use. The original owner gave it to me about 17-18 years ago when he discovered that he hadn't ever really used it and didn't expect to use it going forward!
Anyway, it worked fine about 8 years ago when I used it last. Now that I have the room and the desire to use it, I was faced with a dilemma about powering it up. In the end I brought it up slowly by using a multi-tap isolation transformer.
25 V for 15 mins; 50 V for 30 mins; 75 V for 30 mins, 90 V for 30 mins...and then 120 VAC.
The scope seems to work pretty well. The built-in calibrator (2 V P-P) squares up nicely and measures right on. Measurements seem to scale well between 1X and 10X on the probes. Etc.
My question is, on each subsequent power-up (after the power supply caps have drained), I get two quick audible "snaps" from some kind of high-voltage arc. (No snaps at all if I cycle the power right away, or even after 3-5 minutes, presumably before all of the HV caps have drained?)
I removed the top cover and used some canned air to remove the minimal amount of dust that was present, and I carefully inspected the innards for dirt, bulged caps, burnt spots, etc, finding none. It's as clean as a whistle inside, and looks factory fresh. I carefully wiggled a few connectors for good measure. :)
On the next go-round I carefully powered the scope up in a darkened room, looking for arcs and fireworks. There was nothing on the PCBs or the power supply section (potted HV circuits, too), but there was a slight flash (along with the two audible ticks) where the rubber-covered HV thingy (anode connector?) attaches to the forward body of the CRT. The "back" of the CRT, where the rest of the wires and the tube socket connect, is fine.
The HV thingy connects to the bottom of the CRT and is not accessible without removing the scope's bottom cover, which I haven't yet done. I did snake the canned air "nozzle" in close proximity, giving the rubber gasket and surrounding areas a few good blasts.
Subsequent power-ups (darkened room) no longer have any visible flashing, but the twin HV ticks can still be heard just like before. These are louder than the typical static ticks that I remember hearing from TV picture tubes as they're powered up or down, but not overly loud (like a bad switch or a small arc welder!)
After power-up, the scope sometimes shows an erratic trace for 30-90 seconds, until everything warms up, but then it settles down and seems to work just fine. Sometimes the trace isn't jumpy at all. Some of the POTS and rotary switched could use a drop of D5 or fader lube (slightly noisy), but that's to be expected.
So, I'm wondering whether I need to remove the bottom cover and do something to the HV thingy...or just leave it alone and let it "tick" on start-up? I don't really relish the thought of messing with the HV (or the HV connector)...and I don't have a service manual or even a schematic.
I'm not sure how to disengage the "HV hooking" connector under the rubber thingy...and I'm not completely sure how to bleed the HV with a screwdriver, etc.
Thoughts?
As always -- thanks.
--Kirk, NT0Z
Rochester, MN
13598 2017-02-17 01:37:18 Sandeep Lohia Re: My Oscilloscope Okay? 13599 2017-02-17 03:21:38 sohosources Re: My Oscilloscope Okay? 13601 2017-02-17 19:21:08 ac2gl Re: My Oscilloscope Okay? 13610 2017-02-19 22:49:00 sohosources Re: My Oscilloscope Okay? 13637 2017-02-28 13:03:17 sohosources Re: My Oscilloscope Okay?
Okay. Found the issue...and it was weird!
To see exactly where the scope was arcing, I had to remove the unit's bottom cover (where the anode cap is connected).
I discharged the CRT and removed the cap, cleaning under the rubber boot, etc. Even after 30 years, it looked brand new. Perhaps unsurprisingly, when I gave it the arc test once again, it still arced...
After several rounds of discharging the CRT and restarting the unit in total darkness (only arced on cold start), I finally saw that the thing wasn't arcing anywhere near the anode cap!
The CRT mounts (front) with four insulated "cradles" at the front, and there's an "adhesive" band of aluminum foil(?) that circles the front of the CRT. This foil is "under" each mounting cradle at the four rounded corners. And it must be "at HV," cuz it was arcing...
A TEENY piece of that foil had lifted, and an even TEENIER corner of the lifted foil (could only be seen with magnification) was pointing at a grounded mounting post that retained the upper right CRT cradle. This little "corona point" was double-arcing to the grounded mounting stud nearly a quarter of an inch away on each cold-start!
I pushed it flat with a chopstick (yep) -- and the arcing stopped completely! It's still good after a dozen or so cold-start tests.
How weird is that? :)
Lessons learned: How to safely discharge CRT anodes and not die! Tool? Clip leads and an insulated putty knife.
Ironies appreciated: It's the only remaining CRT in my entire life! Everything else is flat panel...
Super ironic: I own a B+K CRT tester/ rejuveinator!
As always, thanks for your suggestions. They are appreciated,
--Kirk, NT0Z
Rochester, MN