I was testing something with my trusty old Tektronix 2232 100 MHz digital storage scope and this happened:
My guess as to what I’m seeing: a pretty significant bit of the input to the DAC (digital to analog converter) that sets the beam’s horizontal position is stuck, causing the display to break up and overwrite itself in unreadable stripes.
This display is of the vector type. There is no linear, raster scanning like in television or computer monitors; it’s more like an electron beam Etch-A-Sketch. Two DACs driven by the microprocessor set the beam’s horizontal and vertical deflection and it excites the phosphor wherever it lands. A control grid in the cathode ray tube allows it to be blanked to be moved without lighting the phosphor it crosses.
When this skipped around the beam wasn’t blanking; you could see it smear right back.
I tried power cycling. It’d be okay a minute or so after a minute off then do that again.
I tried clearing all settings and memory.
I tried looking through the service manual.
I smacked it.
The problem immediately cleared and does not come back.
Why didn’t I try this first? Am I losing my mind here?!
The fault was likely a loose connection at a backplane connector, socketed IC, or ribbon cable down inside, or maybe even a cracked solder joint.
If it recurs I’ll investigate, but for now I’ll rest easy knowing I don’t have to replace this wonderful scope I’ve used for years with some soulless modern piece of Chinese plastic poo that can’t actually do X/Y plot mode right.
For a while I’ve wanted one of those Simpson multimeters like we have at work, with the big needle analog meter…
Today I found something a little neater. Someone set one of those Harbor Crack multimeters down on top of it and I laughed and groaned at them all at once.
The gunk on the faceplate cleans off easily. Now I just need to make up new test leads and it’ll be good to go.
The meter has a hilarious ballistic to it. It’s not entirely undamped, but it basically overshoots the reading once then drifts back down onto it. I’m guessing that’s the result of the armature coil being a bit heavier than usual from all the turns to make it… super sensitive.
On a side note I’m looking at the schematic and there’s no diode to rectify AC to DC for the meter… This means the meter must actually have a field winding for AC measurement and thus, by design, it’s true RMS! Not bad at all for a meter made in 1948.
Breaking news near Downtown Miami! Okay, we’re good here, just tell the live truck to send us a signal on one of our ENG channels to the downtown repeater aaaaaaand… BLITHERFART!!! WHAT IS THAT?! Our truck’s signal gets smashed, and there are No Excuses On ‘Da Bowl!
Someone else’s live truck is feeding a tape from hours earlier on the day. Spin the receiver around a bit and it’s clear that they’re aimed at the same receiver site or thereabouts. The station responsible has a receiver up there too, I believe.
Okay, so let’s see. How many people had to FAIL to accomplish this?
1. The studio ENG operator. The studio ENG operator would have been the one responsible for directing the truck to use this channel; or, they would have been able to tell the truck, hey, change channel, we ain’t down with O P C. (Other People’s Channels)
2. The truck’s crew. They should have also known better.
They continued to send the footage from tape for a couple minutes then just sent black for a while after that before *finally* coming down….. after any hope of us getting our shot waned. Fortunately, the story turned out to be a total non-event. But still, FAILURE.
The station responsible called us and apologized so I won’t yell at them specifically here, but come on— don’t just grab someone else’s channel, and at least, not without asking nicely first! News happens, man!
This is on a Shure M267 mixer. I’ve seen at least three different variations on this same mixer. The one I’ve got at home contains only one transformer, at the output. This one contains five.
“Vactrol” style lighted photocell in optical limiter circuit. The audio envelope is detected, amplified, passed through a slow filter and applied as a bias voltage to adjust the light brightness. The photocell half of the device is used as a variable attenuator to back the volume down as needed to prevent clipping.
These devices are also commonly used in DJ mixers.