The Analyzer’s New Clothes

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This Tektronix 1705A L-band spectrum analyzer has temporarily borrowed a garment from an old waveform monitor.

Why?

Because as I had it on my bench powered up naked after running through a calibration on the CRT power supply, an operator comes walking in and rests his arm riiiiight here. Luckily I got him to move before any BANG occurred.

How do you even guard against awful human error like this?!

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Small naked wire and HV diode are on the input side of a voltage multiplier that puts out FU Kilovolts.

Now if you’ll excuse me— I need to make a sign for him. 😉

Bonus: The resistor the other tech never bothered to replace. Amazingly it still works, but I wasn’t gonna leave it like this!

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while we're on the subject of near crispy critters

That was the most polite defacement ever

But alas, the fact someone got in here means I have to clean everything out and ensure there are no curious backdoors now. Still though, I am absolutely loving the fact that whoever posted that used my lulz category and didn’t do any bizarre damage or remove my content.

Kudos to… whoever you are 😉

I’m leaving the post in place because I consider website defacements to be a really interesting sort of digital art, worthy of preservation (in the same way that vintage graffiti is).

On that note, see also: miamigraffiti.com

Tilt-A-Whirl Video Heads.

I was digging around in the… uh… museum… here at work and found a whole box of worn out video head drums. Some of them had a very interesting feature to them— piezoelectric tracking.

The head can be shifted up and down on a piezo bender.

Why is this here, you may ask? Well, here’s the reason—

Normally, when you play a tape in a helical scan transport like this, the video heads trace an arc across the tape as the drum spins. This arc more or less perfectly matches the way the video frames/fields are recorded across the tape *as it moves* at normal operating speed.

Purpose of the fixed heads and other gribbles on this drum will be read out of a dusty old service manual some day when I’m not fighting the migraine from hell.

But what about when you are NOT at normal operating speed? The tracking angle will not be correct, and the picture “tears” as the head runs across the boundaries between fields.

Enter the piezoelectric tracking mechanism. By applying a sawtooth waveform synchronized with the head drum’s rotation, Sony was able to cause the head to perfectly track a video field beyond angle differences caused by different tape speeds. Thus, when you grab the jog/shuttle dial on one of the decks employing this system and start moving around, or settle down on a still frame (don’t do this too long!), the picture remains clear.

The Sony J-1 Betacam/SX compact player I use at my desk doesn’t have this, and the picture tears when you mess with the speed or pause on Betacam SP (analog) tapes. On BetaSeX tapes, as my coworker calls them, the digitized frame data seems to land in a RAM buffer somewhere and you can still frame or slow down. The tape transport speed and drum rotation speed in Betacam SX mode are much different, and the angle error doesn’t cause as much of an impairment.

I recall seeing a high voltage warning on or near the head drum inside these decks. Not just for show. It’s about 200 volts!
A much more thoughtful description of the dynamic tracking system and better view of the heads and benders may be found here:

http://dexterslab2013.blogspot.com/2016/05/sony-betacam-dynamic-tracking.html

Just Big and Lousy

Today’s— uh, victim— JBL LSR2325P active studio monitor. It’s a nice sounding biamplified monitor with an active crossover system and suspicious “Imagine” brand capacitors. Hmmmmm. 😉

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Our music producer came to me with this loudspeaker he uses to play his creations for our news director, among other things, because it was crackling and popping ferociously when the input gain knob was touched. I found the input gain knob loose on the rear panel and guessed I’d also find cracked solder joints. But where?

Input gain control is below the inverted plastic bathtub under that board. So how do you remove this plastic bathtub? Desolder the shitty thermoplastic power switch—- which will melt and eject its metal parts. WTF??!!

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You can see the switch on the panel here – it’s a snap in flange mount – the only way to get around doing this would be to cut away the plastic flange and back it out, I guess. The tub it’s in is sealed so this wouldn’t create an air leak. But still— AARGH!!! Also, WHY THERMOPLASTIC? I have a problem with this. See, if the switch starts warming up, the plastic will soften, removing pressure from the contacts, creating more heat. Eventually the fault will only clear when the switch either totally loses contact or the thermoplastic erupts into flames.

Proper electronic assemblies use thermoSETTING resins. Glass reinforced polyesters/epoxies are nice. These are resins that set either when two parts (a resin and a hardener) are mixed, or enough heat+pressure are applied to kick over a curing reaction. This reaction is a one way process and the resulting product WILL NOT MELT and soften. It may eventually be flammable, but most thermosetting resins, especially glass fibre filled ones, have a very good track record of self extinguishing.

Phenolics are very common in solderable connectors. You can always tell when you’re dealing with a phenolic resin because it will not soften and allow the connector to deform with extended heating during soldering. These resins are often colored teal blue/green, or a tan color on Amphenol products. Ever wondered what the name “Amphenol” is about? 😉

Wikipedia article on thermosetting polymers

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I don’t even want to think too hard about what that AC power inlet fixture is made of, all things considered.

The header pins leading to both the boards inside this tub were also graced with total shit-grade soldering and I reworked them. There’s one board below with the three jacks and one board above with the amplitude pad and the HF/LF trim filters. I resoldered the input pot and tightened the nut around it with some Loctite purple on it. In theory, I probably should have used blue, but I can’t find the blue, and red is right out of the question. Whatever works, right? I’ve had just as good luck with things like this using nail polish on them.

 

That’s the fate that befalls any nail polish I buy that looked GREAT in the store but when I put it upon my claws it turned out all watery looking or otherwise unsatisfying. (“NYC Color”, this means you. Well– some of their shades. Some of their newer ones are actually formulated with, well, color, in them.)

After this– I can’t wait for my assistant here to show up so I can run my fingers through his thermal insulation and hear him make silly happy squeaky meows.

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How to achieve silly smiles… 🐱

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