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.
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.
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:
http://www.diyaudio.com/forums/inst...r-musical-instruments-here-3.html#post2633788
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.
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:
http://www.diyaudio.com/forums/inst...r-musical-instruments-here-3.html#post2633788
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.
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...
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...
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.
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.
The case of the missing L-R.
Near as I could estimate, this story began sometime in the late ‘70’s. There was a switch lubricant called “Rycon” 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 ‘80’s. 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 Rycon’s 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 didn’t 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 hasn’t been actuated in years. But when I traced the missing L-R signal to the switch it didn’t 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 wordy…. You enjoyed the story, didn’t you?)
Doc
Near as I could estimate, this story began sometime in the late ‘70’s. There was a switch lubricant called “Rycon” 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 ‘80’s. 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 Rycon’s 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 didn’t 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 hasn’t been actuated in years. But when I traced the missing L-R signal to the switch it didn’t 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 wordy…. You enjoyed the story, didn’t you?)
Doc
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.
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.
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
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
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.
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
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
In the theme- strangest electronic repairs. some more.
In my previous working life at my consumer electronic repair workshop.
1) reception came to get me, did i think i could repair this. What is this. A large metal object about the size of a mans arm, Hmmm. what does it do. Hmmm. tame receptionist described (as she had recieved the information form the customer. " Well its like this, this is inserted in the Bull's rectum and its turned on, then the agricutlural assistant collect the semen then is deposited out of his .... "
At least it was a only a connecting wire repair. when it came to testing time there was an absence of technicians.....
2) Reception told me, thing looks like an oscilloscope needs repair. Ok what does it do? Its a sheep pregnancy tester. Ok. Repaired power supply. tested on different input signals ok. My wife was pregnant at the time but would not allow any after repair testing!
3) Pulrple smoke fromt he television, Dont be stupid my receptionist. It might be brown, black or grey. No way is purple smoke coming out of a television! I will show you. Plug in TV, turn on. Some sort of crackle noise but no sound or picture. In 30 seconds BRIGHT PURPLE smoke rising from the back of the tv in a spiral like a snake. Ok dont breath the fumes and turn it off quick smart.. Traced to FBT must have contained fibreglass resin and it was burning from arcing windings.
So now i say, customer is allways right until proven otherwise.
Just a few stories from 30+ years servicing as tv,video, audio, and communications serviceman in small country town in South Australia.
minsik.
In my previous working life at my consumer electronic repair workshop.
1) reception came to get me, did i think i could repair this. What is this. A large metal object about the size of a mans arm, Hmmm. what does it do. Hmmm. tame receptionist described (as she had recieved the information form the customer. " Well its like this, this is inserted in the Bull's rectum and its turned on, then the agricutlural assistant collect the semen then is deposited out of his .... "
At least it was a only a connecting wire repair. when it came to testing time there was an absence of technicians.....
2) Reception told me, thing looks like an oscilloscope needs repair. Ok what does it do? Its a sheep pregnancy tester. Ok. Repaired power supply. tested on different input signals ok. My wife was pregnant at the time but would not allow any after repair testing!
3) Pulrple smoke fromt he television, Dont be stupid my receptionist. It might be brown, black or grey. No way is purple smoke coming out of a television! I will show you. Plug in TV, turn on. Some sort of crackle noise but no sound or picture. In 30 seconds BRIGHT PURPLE smoke rising from the back of the tv in a spiral like a snake. Ok dont breath the fumes and turn it off quick smart.. Traced to FBT must have contained fibreglass resin and it was burning from arcing windings.
So now i say, customer is allways right until proven otherwise.
Just a few stories from 30+ years servicing as tv,video, audio, and communications serviceman in small country town in South Australia.
minsik.
Never saw bright purple smoke, I must admit. Think that would have weirded me out as well. But I did make WHITE PLASMA once by forgetting to switch on the cooling pump on an RF dummy load and running a 22KW VHF Analog TV transmitter into what was then basically a 200Watt ceramic 50 ohm resistor. Quite pretty. Ate a 5mm slot about 30mm long in the aluminum housing before the SWR protect cut in and shut things down. I was heading that way anyway after seeing the plasma. A replacement load head was about $2000 at the time... but the previous engineer had done the same and the boss said one per engineer. Only reason load wasn't interlocked was because it was used on three different transmitters. I rebuilt both the one that I burned up and the one the previous engineer had. Both good for 50KW.
Doc
Doc
Im a Sony service engineer (well thats what they call us these days)
I remember a very strange fault with Sony VCR (H mechanism for those who know), The unit has come a simple repair just a faulty capstan motor (not starting up), but due to some miscommunicastion the client had thought the unit was repaired and decided to come into the store on a Sat morning to collect, fair enough but I hadn't finished the repair yet. So my manager asked me to quickly finish the mechanism repair urgently. Confidently I finished off the job even put the lid back on as I had done at 50 of these mechanisms before. So just before I sent the unit out I threw a cassette in and the unit ejected it straight away after lacing it up around the heads, very strange! Tried again, same thing just ejected the cassette, tried again, same thing!...no error codes just ejecting all the time....suspect the cassette, tried another, no still ejecting, decided to quickly remove mechanism and check timing, all good, gears all fine, mode switch correct, reinserted mechanism into unit...still ejecting!..mmm very strange...My manager came in and said whats up customer is waiting etc...I showed him, he looked as mystified as me...so pulled another complete unit out which i had and put its mechanism in my repair...still ejecting!...damn it...Now I had this other unit on another bench a few meters to the left of me...so i decided to put my rebuilt mechanism in it...it worked fine...mmm...okay quickly do a complete board swap...no still ejecting on my bench..but both units all working okay on the other bench..WTF...Turn around and look over my shoulder...We have a rack of completed repairs...there I see it...#@$@#$ me someone has put a competed VCR repair on the rack but with its remote tapped to the top of the unit...the unit has moved the remote has jammed and is transmitting the eject function...case solved finally...not happy
I remember a very strange fault with Sony VCR (H mechanism for those who know), The unit has come a simple repair just a faulty capstan motor (not starting up), but due to some miscommunicastion the client had thought the unit was repaired and decided to come into the store on a Sat morning to collect, fair enough but I hadn't finished the repair yet. So my manager asked me to quickly finish the mechanism repair urgently. Confidently I finished off the job even put the lid back on as I had done at 50 of these mechanisms before. So just before I sent the unit out I threw a cassette in and the unit ejected it straight away after lacing it up around the heads, very strange! Tried again, same thing just ejected the cassette, tried again, same thing!...no error codes just ejecting all the time....suspect the cassette, tried another, no still ejecting, decided to quickly remove mechanism and check timing, all good, gears all fine, mode switch correct, reinserted mechanism into unit...still ejecting!..mmm very strange...My manager came in and said whats up customer is waiting etc...I showed him, he looked as mystified as me...so pulled another complete unit out which i had and put its mechanism in my repair...still ejecting!...damn it...Now I had this other unit on another bench a few meters to the left of me...so i decided to put my rebuilt mechanism in it...it worked fine...mmm...okay quickly do a complete board swap...no still ejecting on my bench..but both units all working okay on the other bench..WTF...Turn around and look over my shoulder...We have a rack of completed repairs...there I see it...#@$@#$ me someone has put a competed VCR repair on the rack but with its remote tapped to the top of the unit...the unit has moved the remote has jammed and is transmitting the eject function...case solved finally...not happy
I had a power amp that kept blowing up while I was working on a mixer plugged into it.
It kept on smoking the class d output filter capacitor. Couldnt work out why.
Eventually it dawned on me that right above the mixer I had a mains lamp that was low energy and its oscillator was getting into the mixer and putting the very high frequency into the power amp in a big way.
It kept on smoking the class d output filter capacitor. Couldnt work out why.
Eventually it dawned on me that right above the mixer I had a mains lamp that was low energy and its oscillator was getting into the mixer and putting the very high frequency into the power amp in a big way.
back in the early '90's I was working for one of the more interesting Govt. departments.
I'd spent a solid couple of weeks prototyping this design I had come up with, loads of 74 series (about 30 IC's in total), some 4000 series a few opamps etc.. Powered it up - nothing. Spent two days checking every single bit of wirewrap at least three times. After the two days I was almost crying with frustration - nothing - no signals at all - PSU's all fine, but not a sausage. Then a senior chap suggested I go and have a cup of tea and do something else for a change. When I returned (refreshed) from the canteen I immediately spotted the problem - I hadn't put any IC's in the wirewrap holders....
Same dept. different month. Just fixing a CRT monitor for a nice female (and my senior) colleague - dry joint on a small electrolytic capacitor, decided to impress her by changing the electrolytic rather than just resoldering the leg. Replace the part, whilst trimming the leads on the capacitor, one of the trimmed leads is launched 6 feet across mine and my colleagues bench (over-enthusiastic chomp with the Lindstroms..) and into the back of another CRT monitor where it proceeds to go **bang** and dark.
Same dept. Dropped a brand new loaned £40K network analyser in-front of the rep (didn't realize it was so-heavy), very professional chap - his smile barely flickered (we did spend mega-bucks with them).
I'd spent a solid couple of weeks prototyping this design I had come up with, loads of 74 series (about 30 IC's in total), some 4000 series a few opamps etc.. Powered it up - nothing. Spent two days checking every single bit of wirewrap at least three times. After the two days I was almost crying with frustration - nothing - no signals at all - PSU's all fine, but not a sausage. Then a senior chap suggested I go and have a cup of tea and do something else for a change. When I returned (refreshed) from the canteen I immediately spotted the problem - I hadn't put any IC's in the wirewrap holders....
Same dept. different month. Just fixing a CRT monitor for a nice female (and my senior) colleague - dry joint on a small electrolytic capacitor, decided to impress her by changing the electrolytic rather than just resoldering the leg. Replace the part, whilst trimming the leads on the capacitor, one of the trimmed leads is launched 6 feet across mine and my colleagues bench (over-enthusiastic chomp with the Lindstroms..) and into the back of another CRT monitor where it proceeds to go **bang** and dark.
Same dept. Dropped a brand new loaned £40K network analyser in-front of the rep (didn't realize it was so-heavy), very professional chap - his smile barely flickered (we did spend mega-bucks with them).
Years ago I used to work on mainframe computers. At one customer site, they were experiencing magnetic tape drive problems but only during a certain time interval in the afternoon. On attending the site in the particular time period I observed that the windows in the computer room faced on to a heavily trafficked road and sunlight was reflecting off car windscreens directly into the room. This was playing havoc with the optical sensors in the tape drives that maintained the tape height in the vacuum buffer columns. Curtains solved the problem.
At another site in a ten story building, commisioning of the new system was delayed because of a disk drive read error problem. Diagnostic programs were of no help but a physical inspection revealed that the head drive carriage would oscillate rapidly back and forth for a second or so about every twenty seconds. The penny eventually dropped.
They had a direct line of site to the cities busiest airport and, every time the radar swept across them, the modulation caused the servo to oscillate. Shielding the wall on that side fixed the problem.
At another site in a ten story building, commisioning of the new system was delayed because of a disk drive read error problem. Diagnostic programs were of no help but a physical inspection revealed that the head drive carriage would oscillate rapidly back and forth for a second or so about every twenty seconds. The penny eventually dropped.
They had a direct line of site to the cities busiest airport and, every time the radar swept across them, the modulation caused the servo to oscillate. Shielding the wall on that side fixed the problem.
One day at a Litton div. in Silicon Valley I found our lab expediter trying to move about $680,000 worth of HP8566 spectrum analyzers (10ea) on two overloaded carts. I suggested it would be okay to make several trips.
Inside story floating around Silcon valley about when the military was making selection of 100Mhz scopes. They had all the candidates on a conveyor belt and sumeone switched it on and dumped the lot on a cement floor. They picked the two that lived (HP1740A, Tek 465B).
Doc
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