service where to start?

[the pilot]

Hello! I would like some help to learn the profession of service, I go third year in high school where I study electricity so I have not read much about electronics yet, will be more when I get into university.

Anyway, my dad has a couple of old mixing desks that i need som help fixing. My teacher is more into math than handson stuff, anyway the consoles have not been used in aprox 15 years. I would like to get then running, according to my dad they where working okey with minor issues back in the days. I have powered both of them up but both consoles have the same problem, many channels  (line inputs)loses sound at low volume but will return if you raise the volume (gain and sometimes even when maxing the fader) I have done the easy stuff, I have cleaned connectors, patch, switches and pots, I have taken out a few caps and had my teacher messure them and he told me they should work, about 20% diff from a new one (100uf where 80uf and so on) good enough to pass signal 100%?

The bigger is an amek and the smaller one is a thore seem, my father says it might have to do with heating but the problem is more or less static over time. Also it might change if I turn the console off and put it on again but sooner or later the problem comes back.

Im 99% sure its not the switches that is the problem becoz I have cleaned them well and they sound good pressing them....well they sound nothing

I use a CD and a behringer mixer to line input, when I send a lower signal from the CD the more problem I get. I listen on the masterbuss and headphones, same problem.
How do I troubleshoot this problem ??

 

[mikefatom]

From past experience, "no signal unless the fader is raised significantly" means that the fader carbon traces (if that's the case) need to be cleaned/replaced or you have an dusty/oxidized connection somewhere in your signal path.

Also, switches that mechanically work does not mean that they are electrically in good shape. I would check them out with a multimeter.

Good luck with getting them back up and running, that should be a great learning experience right there!

cheers

mike

 

[JohnRoberts]

Symptom sounds like dirty switch contacts, like in insert jacks, or switches. Good general troubleshooting advice involves tracing the signal to see how far it gets through the circuit... before getting weak or stopping.

JR

 

[jensenmann]

First you should learn what the console does when it´s working properly. Read the manual. If you know that then you should send signals from everywhere to everywhere to see which of the soundpaths do work and which are corrupted. Then take a block diagram of the desk and try to find where the signals disappear or get corrupted.
Most troublesome for older desks is where mechanics come into the game: switches, jumpers, connectors, pots, faders, etc.,  you get the picture.
There could be written an entire book about what to do then. Too much for here. Go for it and ask specific questions when they appear.

 

[the pilot]

Thanks for all the replies!

Jensenman<--  I have more or less autofire all the switches when problem occurs and found the bad switch that way. Now I have come to the point where I dont find the bad one. The only way to get the signal to pass is to increase the gain and it suddenly works. I have checked the modules for internal switches as well. Im looking at the

Mike How excaclty do you messure if a switch is bad ?

Is there a good way of cleaning the insert jacks ? I have taken a patchcable and sprayed it with alcohol and worked the patch with it.

Also a questions regarding caps. If you look at the specs, what is most important for for audio? Leakage current, Ripple current or tolerance.
If its to complicated maybe someone could direct me to a source on the net where I can find information on caps in audio. 

 

[the pilot]

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eager to know how to use a multimeter detecting a bad switch 

 

[jensenmann]

replace the insertjacks - all of them. The first bad one is a sign that all the others will follow soon. Usually they go bad because the switch-contacts get dirty or oxidized. No chemical on a jack will be able to reach the switch-contacts and hence won´t clean it. You could open the desk, insert a jack to open the contacts and then spray some cleaner (TESLANOL D6, DEOXIT, Kontakt60) between the open contacts. Then remove and reinsert the jack 20x and spray again to remove the dirt. This procedure only works with mechanical dirt. Oxidized contacts will stay oxidized. Replacing is the better solution.

Get some Low ESR 105deg electrolytics. Most people like Panasonic FC or FM. Everything you buy today has better specs than 20 years ago.
For a good read look for the Cyril Bateman article: Capacitor sound

 

[the pilot]

Hmm I will look into the inserts and see what I can do...is it possible to exchange the insert jacks without exchanging the entire 19" panel, sounds expensive.


However I will read the article now, thanks!

Is there a good source on the net that covers electronics for dummies more like hands on stuff, no math. Like how to use a multimeter, how to use a oscilloscope for beginners that is ?

What Im most interested in is how to spot faulty components, how to messure a cap, how to detect a broken IC, how will I know if a diod is broken and so on. Any help is appreciated....Oh and how to spot a broken switch with a multimeter

Im aware that Im a n00b so bare with me please

 

[the pilot]

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[the pilot]

topping again =(