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http://www.ozvalveamps.org/howtofix.html | Last update: 19:57 24/09/05
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Contains:First and foremost, keep all the parts.
Second, your best servicing instrument is your observation.
Third, who takes it apart, reassembles it.
Forth, write things down.
| Philosophy of Servicing | Working with another tech | Newly built Gear | Divide and Conquer | Intermittant faults | Witness observations | Service ethics |
I'm nominally an electronics technician, but I've done faultfinding on mechanisms from the tiny to the huge; from the mundain to the insanely complex; across electronic, electrical, hydraulic, pneumatic, optical, magnetic, and mechanical as required by the problem.
Starting as supposedly electronic problems, they led me into many strange corners from quarries to intensive care, so I ended up calling myself a 'technical generalist'.
On site:“What does it do?”
“Huh? How can you fix it if you don't know what it does!?”
“If it works on known physical principles, I can fix it.”
A good understanding of basic physics including electricity (with a bit of chemistry and biology thrown in) are required, and a methodical approach. There is a bit of forensic detective work about it that I particularly enjoy, the thrill of the chase after the elusive quarry.
Nobody is born knowing. Libraries are magic. University and College libraries even more so. Find like-minded people at local clubs and societies. Read the magazines that serve your particular interest or speciality. Now we have the web. Ask the right question.
The central problem with fault-finding anything is not the thing being fixed, but you, the person doing the fixing!
It has to do with recognition. Specifically re meaning “again”, and cogito, latin to think or “understand”. Loosely; to arrive at a new understanding.
This presumes you already understand the correct working “law of the machine” (1); what it's supposed to do normally.
The task is to come to an understanding of the new law of the machine, the faulty state, so you know how to restore the desired state.
The thing itself is inanimate, it just is what it is. A stationary target.
You have to shift your view of it, your perspective, and most of all, question your assumptions. Almost by definition, the fault will be found where you initially assume things to be normal.
Follow the anomaly, the bit that doesn't fit, the measurement that doesn't make sense. You will go up false trails and around in circles, all the more if you have never seen on of “these” before. Make notes, draw diagrammes and sketches, write down measurements, record results.
Methodologies are as exciting as a Russian truck, but I'd hate to admit the time I've lost to blown fuses by jumping in at the middle and not doing some basic checks of the obvious first.
A good technician can explain exactly what was wrong in plain language, and reproduce the fault if required. This has some implications for your method of approach to a repair.
Consider the blunderbuss approach of the TV serviceman in Silicon Chip. He loves diving right in and resoldering dubious joints all over the place.
If the fault should happen to disappear he has no idea what he did to fix it, indeed if it is really fixed at all, or just disturbed into a working state only to reappear back in the client's home. This produces bouncers, and bouncers are not good for your reputation as a repairer.
The easy and obvious teaches you little, but “dogs” that take a long time to faultfind or the “bouncers” you didn't get right first time will be the jobs that teach you the most.
The blunderbuss approach does work and is economic on average, which is fine for non-critical consumer devices, but areas such as bio-medical, industrial, and stage/theater require a higher standard.
I've found servicing by committee to be obstructive. It's a solo activity. Until you get stuck. Then you really need someone to bounce it off.
Did you ever wonder why Mad Professors mutter to themselves while they are doing stuff?
This is because only a few people would understand, and the rest don't care.
But that doesn't change the fact that explaining your problem, having to form it into words to describe and explain it to somebody else (anybody else, really) often suddenly exposes the fault to view.
Sleeping on it sounds corny but it also works. Particularly if you're getting tired, frustrated and angry.
Bringing up a new bit of gear is different to servicing something that has been in use for some time.
In old gear it is likely that only one thing has failed.
But in newly built gear anything and everything can be wrong, and multiple faults don't just add, they multiply confusion.
Pick a mid-point and see if the fault is before or after there. Continue like this until complexity is reduced to managable proportions.
In an amplifier the obvious point to start is the volume control because it is at the preamp output and main amp input.
But watch out for feedback. A fault in a feedback system is reflected throughout the loop and cannot be isolated with the feedback active. It must be broken for faultfinding and this can give rise to considerable problems.
A modern transistor power amp may not tolerate having its feedback fully disconnected.
Here is an example of what can happen if you break the feedback loop in a high power system.
These are just the most maddening and hated thing by any service person - the fault that only occurs “sometimes”. (of which Windoze is a shining example).
The first thing to remember is that it isn't out to get you.
Nothing personal.
It just seems that way.Relax.
Remember: You can't find a fault that isn't there.You either wait, or you try and cause it to happen. There actually is some logic in the old saying “When in doubt, give it a clout” - but please, not in front of the client.
One of the many cases of Murphy's Law states that a fault will only appear when the last of 28 retaining bolts is tightend down.
Well I once had exactly that on a refinery controller. With the lid off and a long lever we warped the casting slightly until the contact touching the side was located.
Intermittants are a frequent source of client friction and call for careful explanation and you need to be clear about the difference between “fixed” and “no fault found”. It's similar to the difference between “there is no bomb” and “we found no bomb”.
Try not to muddy the water for others who may follow if you can't solve the problem.
“My car won't start.”
This seemingly straightforward statement means a wide range of things to different people.
Very few people seem to be good observers.
- A few give you a lot of significant and consistant information about the breakdown.
- Most will bury important clues in attempts to do the faultfinding and leaping to conclusions.
- And a few will give you contradictory and even impossible symptoms.
“Any lights on the dash? Engine crank?”
“No, nothing. Totally dead.”
In fact it cranked briskly and was out of petrol.
Another person brought in a valve amp for repair.
“This isn't valves, it's fully solid-state.”
“Are you sure?”
This is one of those moments that call for all the diplomacy you can muster, when you try not to yell at the client that you're not a @#$%ing idiot like them, and that as a professional tech you know the difference between a valve and a transistor without a guidebook. Just fix the amp and bill them for a new set of valves. ;-)
There are a never ending trickle of “not switched on” type faults, user mis-operation or misunderstanding of operation. The fault is the nut that holds the wheel, the operator. In many of these cases you are justified in charging your minimum service fee, but you generally won't out of goodwill. If you suspect this sort of problem try to be clear about charging before you start or do more than simple diagnostic work.
Then there is the client who actually doesn't want their gear fixed. They only brought it to you so you would say “It's beyond repair” and allow them to buy a new one without guilt.
These are the people who get grumpy and cheezed off, rather than happy, when you tell them it'll be simple and cheap to fix. The simpler and cheaper, the less happy they are.
And some clients turn out to be dishonest, unethical, carpetbaggers, or just plain nuts, and better to avoid.
Assemble a mental list of possible faults from quoted symptoms, but try not to jump to conclusions or spend too much time on detailed conjecture until you have actually examined the thing in question yourself. You can waste a lot of time and form misleading preconceptions.
In electronics, to quote a repair job you have to fix it first. Some TV/Video repair shops now charge for a couple of hours tech time for a quote, refundable on the full repair.
Do no harm. Always take care not to make matters worse. Try to keep the option of at least returning to the original condition you found it in.
Don't set up “death traps” for yourself or others.
Don't exploit the client. Many clients are suspicious about service people because they are experts and the client isn't.
Sometimes a client will offer to sell or give you something you have diagnosed as beyond economic repair (b.e.r.). This can be very tempting, and while you might actually be doing the client a favour, it is unwise because there will always be a lingering suspicion that you were not a fair judge in your own interests.
Try to know and not exceed the limits of your skills. Expand your skills by self-training and study when you encounter new problems.
(1) Wieznebaum, J. “Computer Power and Human Reason”
Off a workshop wall:
Repair work................$20/hr If you started the job.....$30/hr If you want to watch.......$40/hr If you want to help........$50/hr
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<<<OzValveAmps
http://www.ozvalveamps.org/howtofix.html | Last update: 19:57 24/09/05 |