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Old 7th January 2012, 11:39 PM   #71
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Quote:
Originally Posted by nigelwright7557 View Post
If that doesnt work I pull out all semiconductors and test them. While they are out I check all resistors for value and capacitors for shorts. If I still cant find the problem I put an LC meter on the capacitors.
I had an odd one when I was repairing consumer audio. One channel of a receiver - I think it was a Harman Kardon but it was long ago. The amp had a very strange distortion that made no sense. Asymmetrical clipping but all the power devices and supplies were fine and made no difference whether it had a load on it. I pulled the voltage amp device after the differential pair and put it on a curve tracer. The curve trace showed a transistor that had an apparent resistor in parallel with the E and C terminals. Replacing the faulty transistor fixed it but it was the only one like it in over 35 years.

G
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Old 7th January 2012, 11:44 PM   #72
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The problem with amplifiers is they are a loop so a fault anywhere can pass symptoms around the whole loop.
I had an amplifier not working right but the voltages looked OK. In the end I took out all the transistors and of course it was the last one that had failed with a gain of 1.

I spent one happy half hour trying to work out why an amplifier would not power up. Kept plugging the amp into my extension lead and it was just dead. Checked the transformer for open windings etc and it seemed ok. Of course i hadnt plugged my extension into the mains ! And thats with 30+ years experience.
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Old 8th January 2012, 12:03 PM   #73
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Quote:
Originally Posted by nigelwright7557 View Post
The problem with amplifiers is they are a loop so a fault anywhere can pass symptoms around the whole loop.

Yes, can get tricky. Same principle applies to everything from precision HV AC calibrator amplifiers (Fluke 5215), exotic high voltage high current Sorrensen Research power supplies and even more exotic Television transmitters with bandwidths from 4.5 to 20MHz opperating on UHF.

The new digital transmitters run either 25-35 KW tubes, or mountains of 1KW solid state modules that get combined in multiple stages to reach desired output power. Even the modules are made up of smaller pallet sections assembled in same manner. Power gets split coming in and runs in phase to individual pallet amps then recombined at output. Outputs are combined in banks then the banks recombined in cabinets. Cabinets combined to feed main out. Now all digital controlled down to individual pallets.

But point being that broadcast transmitters are just big amplifiers. The same principles apply from audio up to UHF, as do the same type problems.
Try recapping two transmitters with 32 drawers of 16 pallets with two big SMT caps per pallet.
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Old 8th January 2012, 03:37 PM   #74
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Interesting on reading all the countless problems with electronic equipment and how some drove the repairer person nut's to solve the nagging fault!...
I recall one time at the local tv/video repairs centre, where we had a Hitachi a7 set that refused to stay on for more than 2 hours and then shut down to standby! these set's did a power on followed by a check of it's voltages then if all's well power up...mm! not this one...so time to get stuck in..ht set to 152v and so going over all set up's...in the end it turned out to be a 80 pin smd chip in the middle of the underside pcb that needed to be changed out and solved the fault... these day's you open a lcd and think wow where do you start!

Still I'am glad to say, I'm back to servicing/repairing audio equipment/valve amplifiers as this is where it all started way back in the 80's... you just can't beat that feeling of bring a old vintage amplifier back to full working order...
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Old 8th January 2012, 04:10 PM   #75
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these day's you open a lcd and think wow where do you start!
Probably with the backlight power supply. Highest failure section on LCD's. Oddity is that it's a transformer failure as often as a driver FET. (I know question was redundant, just indicating that even in an LCD there are places to start.)

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Old 8th January 2012, 04:51 PM   #76
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Probably with the backlight power supply. Highest failure section on LCD's. Oddity is that it's a transformer failure as often as a driver FET. (I know question was redundant, just indicating that even in an LCD there are places to start.)

I would agree there.. Doc

Service places charge a high price to repair lcd's set's and one tech said 'we just change over board's 'that sort's hour's of fault finding and keeps cost down for us! what happen to good old tracking down the faulty part and getting things back to working order..

That's the part we and new diyer's enjoy learning how/why it work's and above all know how to break the circuit in the stages to get it working...
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Old 8th January 2012, 08:38 PM   #77
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I understand both perspectives. As DIYers we have the luxury of dogging something out. MFG's are looking at economics of a repair. Both are valid. If you are a repair facility, you need to turn a profit to stay in business. Good generalists are hard to find. Working as senior repair person in a calibration lab I used to go through stacks of employment applications from newbies fresh out of 18 month tech school who all thought they knew it all. Very few are capable of trouble shooting complex instrumentaion. Someone with good generalist experience (DC, Audio, Video, RF, digital) can sometimes keep three other techs busy just swapping parts they tag.

I've said to others before, given enough time, information and resources, there is nothing that I can't fix. There are many things that are not ecconomical or practicle to fix, but there is nothing that I'm incapable of fixing. It's a trade off. Sometimes you just have to deal with ecconomic realities.
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Old 8th January 2012, 11:10 PM   #78
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That's one of the neat things in broadcast electronics. Some of the old machines HAVE to be kept running just to transfer to countless hours of old shows. Some of them you might even like. I try to come at it as a 'purist' to not only get it to work but restore it to its original state but I had an Amoex AVR-1 that would be more trouble and cost than its worth.

http://www.lionlamb.us/quad/avr1_3.jpg
http://www.akdart.com/vtr/jpeg/21-01a.jpg

The sync change over relays were just ridiculous trouble to find and repair. I soldered jumpers on the sockets and used a retired Grass Valley sync change over switch with an NTSC and PAL sync generator to get the pulses to run the machine. The machine does NOT take a color black but requires sync, blanking, subcarrier, burst gate and some I forget. The sync generators CAN lock to color black. That combined with the analog to serial digital adapter on the back of the machine brings it into the digital 21st century. FYI I soldered those jumpers so that it's still 'restorable' 'just in case'.

G

Last edited by stratus46; 8th January 2012 at 11:20 PM.
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Old 8th January 2012, 11:28 PM   #79
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The machine does NOT take a color black but requires sync, blanking, subcarrier, burst gate and some I forget.
Ouch! What a dinosaur. Is it 2"? I only go back as far as 1" tape.

We just bought a 1/2 dozen video recorders that are smaller than a cigar box, record onto SSD and and do just about any video format you want. Built in monitor, too. Amazing how far things have come.
http://www.sounddevices.com/products/pix.htm
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Old 8th January 2012, 11:37 PM   #80
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Ouch! What a dinosaur. Is it 2"? I only go back as far as 1" tape.

We just bought a 1/2 dozen video recorders that are smaller than a cigar box, record onto SSD and and do just about any video format you want. Built in monitor, too. Amazing how far things have come.
PIX Video Recorders | Sound Devices, LLC
Yup, it's a 2" quad, weighs 2200 lbs, requires a 208 volt power supply and while it has an internal air compressor, it's connected to house air at 55 PSI. Standard def video and mono audio. I see it every day at work. Next week they're sending me to Sony SRW school which is the other end of the VTR spectrum.
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