i am just a nothing compared to your skill level and knowledge..
just asking for curiocity..
what was the exact procedure.
1.matching the diagram with original machine.........really salute you
2.physical inspection of any damaged components
3.searching the exact components..same brand perhaps...again salute..it is ultra frustration when you dont get exact parts...being patient and sourcing is not easy
4.repairing..testing
now at stage 2 or after stage 2...any part which is non functional but appear okay physically.....what was the method to identify..
thanks
oh sorry if i missed any steps
Hey, its not as difficult as it seems. Usually technicians have his uncany knack of making things look extremely difficult which is a ploy to have other's glorify them and also to charge a lot of money. Actually, some basic knowledge of electronics, passion for audio electronics, some patience and time are all that's required.
Yes, basic knowledge of electronics is required, for identifying components, for understanding connectivity, for understanding circuit diagrams (symbolic representations, equivalents, colour codes, etc), basic knowledge of how amplifiers, etc are structured (preamp, poweramp, power supply, etc) and substructured (poweramp: pre-driver, driver, output, etc), logical trouble shooting exposure, knowledge of how to handle a multimeter, soldering station, availability of a bench powersupply, bench amplifier, speakers for testing, a junk box with lots of wires, connectors, screws, bolts and nuts and what not - which can be used for salvaged parts (use this for testing and replace with new components once you've finalized the replacements)
One must also have access to the internet (one must master the art of locating information), communication skills (to communicate with others, especially on ebay), the ability to research and analyze and finally must be patient. At times things may not work or may head towards a dead end. Also spares hunting is a difficult task. That is the reason why these restoration jobs take 5-6 months.
Before starting, photograph, key aspects (This is particularly important if dealing with mechanical devices like turntables)
The approach I use is:
1) Try to download a service manual, this is the very first and the most important step. Without a service manual, getting something back to its original state is virtually impossible unless one is an expert
2) Study the service manual carefully to get an understanding of circuit design. What is included in the circuit and what is the logic behind functionality. What are the key aspects of the circuit. Look up case studies on the internet, others may have restored the same piece of equipment, the same type of equipment may have exhibited typical similar problems which others encountered. Here is an example of a document which turned out to be the holy grail of this amplifier restoration:
http://www.uwedamm.de/V5000_Reparaturanleitung_V3.pdf. I can boldly say that if we did not find this document (which had to be translated from German into English), fixing this amp would have been extremely difficult.
3) Open up the equipment and visually examine (carefully). Look for burns, leaks, breaks, prospective tampering and missing components. Try to sniff out any odd odours from near the power-components and transformer (many dont agree but odour is a huge give away for electronics - one can tell if the equipment was stored, overheated, fried, recently soldered on, etc if one is familiar with the smells). Also look for soldering flux accumulation on the PCB, this signifies recent work on it.
4) Compare the component layout on the PCB to the component layout illustrated in the schematic, use a random sampling method to check if key components are of the right value and in their proper places
4) Depending on the diagnosis, disconnect the powersupply and check it separately, check semiconductors, don't stop when for find a faulty one, there are likely to be others so check them all, if with a solid state amp for example, start with the output power transistors (closest to the speaker rails) and move backwards to the pre-driver and driver, also look out for burnt resistors and broken/leaky capacitors, replace all and test using a low voltage PSU (so that you have time to turn off if anything starts to fry). Test the poweramp separately once you've replaced all faulty components. Disconnect any protection circuitry while testing, but use only low voltages if protection circuitry is disconnected. Use the same approach with the preamp, go backwards (from output to input). A useful tool is the good old signal injector. Its very useful while testing an amp with a working poweramp.
5) Once you're done with all replacements and you have a functional amp, make a note of the components changed (with date). If you've used equivalents or substitutes, make a not of these on the schematic. If the amp has been hot-rodded (modified), outline the modifications in the schematic (for future reference). Connect back the protection circuitry and the powersupply, run the amp on the test bench for atleast 10-12 hours, look out for over heating, distortion
6) then go for the bias check and calibration and other finer aspects
7) run through the pots, switches and connectors, replace faulty ones, clean existing ones
8) clean up the cabinet (before screwing everything back) and polish the exterior
Once you have the amp all screwed up, play, play and play it as it needs to burn in. Also re-check functionality and you're ready to sit back and enjoy the music.
To answer the second part of your question, yes components may test ok but may not work. This is a very difficult scenario to diagnose and also to fix. One must be a little more experienced to deal with this problems. The first step would be to try and locate the issue. It can be something simple like a PCB contact problem or dry solder or something major like a malfunctioning component. Again, I would break the amp down into sections and subsections and test individually till I zero in on the defective area. For example, many years ago, when I was in school, a friend gave me his transistor radio to fix. The issue was that it would not receive shortwave (he wanted this badly to listen to BBC test match special). We checked the radio inside out and could not locate any fault. Then after a few days, I was discussing the history of the radio with the owner when he said that the radio went dead sometime back and a local technician was able to fix only the medium wave section. This was a huge breakthrough, on further checking, I then discovered that the technician had replaced the original shortwave oscillator coil with a medium wave oscillator coil (which looked similar) and hence the shortwave bands did not work. This was fixed in no time. Later on when in college, my brother and I fixed a 14 inch black & white television's sound RF stage which tested ok but would not work. A coil replacement solved the problem.
Overall, its an exciting experience, every restoration or repair job has huge learning associated with it. My brother and I have been fiddling around with electronics since our early school days. We've built a lot of stuff, we've broken a lot of stuff and repaired a lot of stuff and we've learnt a few things on the way, still learning.
