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Old 20th December 2011, 06:21 AM   #21
rjb is offline rjb  New Zealand
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I was installing a high powered aeronautical radio beacon on a tropical island. This included needing to adjust tappings on a large silver coated multi-turn coil, (3 ft in diameter, formed from 1 inch diameter tubing ) to match the aerial to the transmitter. This coil was in a box, open at the end, and the tapping was shifted by moving a large clamp from turn to turn. To do this you had to lie on the ground and look up into the darkish box, with a bright sky background shining in your eyes.

No matter what I did the damn thing would not tune.

Could not sleep that night, trying to figure what was wrong, so got up with a torch to try again. Then the problem became obvious. The coil was coated with a clear transparent insulating "varnish", except for a thin section on each turn. I could not see this in the daylight, the coil looked universally silver.

So simply moving the clamp to the uninsulated section on the appropriate turn fixed the problem.
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Old 20th December 2011, 06:53 AM   #22
benb is offline benb  United States
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I've heard of the Rhodes Chroma, and I even had its successor, the Chroma Polaris. It had a large main board with everything on it (80186 processor or embedded microcontroller variant, and all those wonderful analog synth chips they don't make anymore), and it developed an intermittent where it locked up. I never got it fixed.

Much more recently I got this beast that I've basically given up on. Its status is stuck in limbo with the description I've written, as it's now blocked up in a corner and pretty much inaccessible:
Post your Musical Instruments here.

I'm reminded of the original Apple ][, and surprised it was as reliable as it was, with as many chips it had on the board. Everything was chips in cheap sockets back then.
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Old 20th December 2011, 12:34 PM   #23
cliffyk is offline cliffyk  United States
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Then there's the human factor. Back in the days of $800 amber CRT monitors and character based applications a user reported she could only see some of the on-screen characters with the new monitor that had been installed overnight.

Two techs attempted to repair the problem, replacing the video card and cable and then requesting a replacement monitor--I went out to see what was going on, and turned up the contrast control...
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Old 20th December 2011, 01:02 PM   #24
djQUAN is offline djQUAN  Philippines
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I had an Audison mono car sub amp from a friend for repair. Took me a couple years to figure out what was wrong. It would work fine at times and not in others. I took the board out of the heatsink several times. The first time, I saw aluminum dust from machining the heatsink. Cleaned it off and put it back. Nothing else is wrong. No burned parts, tracks or bad solder joints. Worked fine on the bench most of the time (only goes into protect mode when the remote is switched rapidly on/off/on/off - I thought it was normal as it doesn't happen in normal use) and my friend said I keep it until someone else buys it.

A couple years later, another friend bought it and used it in his car. He and his installer turned it up full tilt for a few minutes till it went protect again. Called me and I said try turning it off/on slowly but didn't work. It went to permanent protect mode. Got it back in the bench and found out it had one out of 12 (IIRC) output mosfets shorted internally sending DC on the output and triggering the protection circuit. Replaced all mosfets and it worked fine ever since. I always thought semiconductors are digital. They either worked or not. Probably the one FET was already going bad and going intermittent causing the problem. I decided to replace all to make sure and keep everything matched.
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Old 23rd December 2011, 02:48 AM   #25
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The case of the missing L-R.

Near as I could estimate, this story began sometime in the late Ď70ís. There was a switch lubricant called ďRyconĒ that was in common usage in the electronics world. This was a blue-green jelly looking substance that had to be thinned to the proper consistency with Toluene. For whatever reason, Rycon pretty much disappeared from common usage by the early Ď80ís. I tried to obtain some at that time for use on a primary AC voltage standard and the manufacturer I was talking to (John Fluke Company) directed me to Cramolin.

Anyway, to return to our story; with time the possible reasons for Ryconís discontinuance were becoming clear; over time the Toluene thinning agent would outgas and evaporate leaving a thick waxy substance behind that no known solvent would cut. Finding it coating a wafer switch you were faced either replacing the switch or disassembling it and using an eraser to remove the gunk. (The waxy substance was not only non-conductive, but would hold contacts physically apart if it was between them.) I encountered it quite often working on older instrumentation. At Stabro I encountered it often enough that I had an electric eraser specifically for the purpose of removing it.

Well, either CK Switch company was left with a stock of Rycon, or they didnít rotate their stock. Around 1990 (according to date codes on other parts) along comes Optimod and orders a bunch of right angle pc mount double pole double throw switches. At least one of what they received was treated internally with Rycon.

Now, thirteen years down the road from that time the waxy characteristics made themselves known. I guess it slowly oozed down between the contacts, as the switch itself probably hasnít been actuated in years. But when I traced the missing L-R signal to the switch it didnít come out the other side. Subsequent Ohms measurements showed the switch to intermittently show high resistance (>10Meg Ohm) down to less than an Ohm. When I removed the switch and opened it up, there was the familiar blue-green color of Rycon. The stuff inside was more like plastic than wax.

The purpose of the switch in the circuit was to switch the audio signal between either the DBX noise reduction circuitry or to bypass it. Having high resistance, the L-R signal dead ended. The switch now completely cleaned and lubricated with Cramolin has been replaced. Leaving some future unsuspecting engineer to deal with the unknown long term characteristics of that lubricant.

(So I was a little wordyÖ. You enjoyed the story, didnít you?)

Doc
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Old 23rd December 2011, 03:27 AM   #26
Pano is offline Pano  United States
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Yes, I enjoyed it. And it put a bit of gunk fear in me, while reminding me of a story.

Back in the early 70s I bought my first "Hi-Fi" cassette deck. I loved that thing, it was "My Precious". Knowing that the heads needed regular cleaning I bought an Apex head cleaning cassette. It contained a sort of scrubber tape that ran across the heads and cleaned them. I used it on a regular basis.

Fast forward a few years. The left channel stopped working. No sound. The right was OK, but nothing out of the left. So I pulled it apart, dug in and started tracing signals as best I could without a scope. Nothing made sense, all looked good but there was nothing coming out of the left channel. So I put it away for a few months in despair.
When I came back to try fixing it again, I happened to look at the heads. As brown and caked as the banks of the Mississippi river! There was a tremendous oxide build up like nothing I'd ever seen. A friend handed me a cotton swap with alcohol and away went the oxide. And the left channel was back in full glory. So was the frequency response.

I never thought to check the heads because I had been "cleaning" the regularly with that stupid Apex cassette. Never again.
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Old 23rd December 2011, 04:12 AM   #27
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Here's a collection of these types of stories at these links:

Design News "Made by Monkeys"
Design News - Made by Monkeys - Latest Content

and

EDN "Tales From the Cube"
Tales From The Cube | EDN

(You might have to click through a starting advertisement)

Cheers.

ZAP
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Old 23rd December 2011, 08:07 PM   #28
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We seem to be doing okay right here.
Doc
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Old 23rd December 2011, 08:42 PM   #29
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Long ago a friend had a car, a BMW I think, that ran fine going forward. Trying to go in reverse, the engine would immediately die. Took us a while, but finally we discovered that the motor mounts were a bit soft. When trying to go in reverse, the engine shifted ever so slightly, shorting out one of the coil terminals to the frame. Going forward the forces were in the other direction, so no problem. I don't remember how we fixed it, probably rotated the coil a bit or put some tape on it.
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Old 25th December 2011, 11:46 PM   #30
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Some of the Jeep Cherokee wagons were notorious for sudden stalling on quick left hand turns. Not fun. Never did find the problem on my brothers.

Keeping it going:

A Fluke 5215 High Voltage AC calibrator amplifier. Would work fine on lower frequencies but not higher than a Kiloherts or so. Racked my brain for almost two weeks looking for problem before I finally told boss I'd better bail out on it and send it to Fluke. Heard back from Fluke that they replaced every board and it still had problem. They finally found and replaced bad main transformer that would get and odd parasitic oscillation at higher frequencies. Didn't feel so bad after I heard Fluke had to virtually reassemble it from the ground up to fix it.

Doc
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