Buss Channel with Noise after wrong soldered bypass Cap. - diyAudio
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Old 11th July 2012, 12:11 PM   #1
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Default Buss Channel with Noise after wrong soldered bypass Cap.

hello guys,

i have an problem and i dont know what to do, maybe some one of you can give me an advice. Problem is as this:

I recapped and changed the opamps on 2 of the 4 summing busses. 1 channel is fine after soldering a 33pf cap from the inverted input to the output, the oscillation is gone.

The same on the other channel. But at this channel i did a mistake. I soldered an bypass cap for all elkos on the summing busses. But i soldered 1 bypass cap to the wrong point. instead soldering it to the pins of the elko i soldered on pin on the elko and the other on the pin of the resistor. I got a bad smell after powering up the Desk. And 2 of the resistors are gone to the other world. Today i bought new resistors and changed them the bussing works but if i put the volume fader all the way up i get an noise it rises with the fader. the other summing buss has no noise and is very quiet better than before the upgrade.

I changed all the opamps and elkos, also desoldered the bypass caps and changed the burned resistors. But i still have this noise. There are 2 Transistors in the buss could it be that they are damaged? Or that other resistors are gone? i have no clue at the moment. so i ordered replacements for all the resistors and the transistors, which are not changed yet hope to fix it this way. If someone from you can give me tips or an advice its welcome.

thanks and regards
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Old 11th July 2012, 02:26 PM   #2
DF96 is offline DF96  England
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The best advice is "Don't do it", but it is too late for that this time. Remember it next time you are tempted to 'upgrade' a working system.

Proper fault tracing is the answer now, but I assume if you knew how to do that and had the right equipment you would not need to ask. Two options: carry on changing everything until it works again, or hand it over to someone who can repair it properly.
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Old 11th July 2012, 03:31 PM   #3
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i ordered everything from the pcb that is not replaced yet. it will arrive tomorrow. i will also go and buy an multimeter. i think i could measure the resistors and it has an transistor checker, also i can check the elkos. i think this is the right way.

i changed all the elkos and opamps again. also desoldered the bypass caps. but still noise in this channel. so i will get the multimeter and the other parts tomorrow and then i can check for the damaged parts and replace them.

this was my first attempt to upgrade a mixing desk. the channels which had no problem are sounding fine now, the noise went down a big step. when i dont messed up the channel all would be fine but, so i learn more from it, i hope that i can fix it.... maybe i overlooked something? this i why i am asking you professionals...

Last edited by suchenderxxx; 11th July 2012 at 03:55 PM.
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Old 11th July 2012, 05:58 PM   #4
DF96 is offline DF96  England
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I am not a professional, although there are some on here. The best way to start fault tracing is to compare DC voltages between a working channel and a broken one, and post the results on here (with a circuit diagram, if possible). Naming the equipment might help too! Be very careful when working on live equipment, as mains voltage may be exposed. Also, make sure you don't let a probe slip and short something out.
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Old 11th July 2012, 06:09 PM   #5
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my plan was to check all resistors and the 2 transistors, cause everything else is changed 2 times... so if any value is different to the good channel then this part must be the one which is damaged and needs to be change. or i am wrong?
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Old 11th July 2012, 06:22 PM   #6
M Gregg is offline M Gregg  United Kingdom
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Hi,
You have made the first mistake..
You are assuming that everyone on here knows what you are doing and what you are working on..
This is the information you need to post..
What are you working on?
What were you doing and why?
If you have upgraded Op amps what were they and what are they now?
Do you have a circuit schematic that you can link to or that you can post?
Transistor types and numbers?
If you post a schematic draw any mods on it i.e. additional components you have fitted..
And post it on here..


Regards
M. Gregg
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Old 11th July 2012, 07:09 PM   #7
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you are right gregg. i have forgotten to post that it is an studiomaster 16-4 from the year 1979 with the big vu meters.

i have an schematic but it is useless cause many parts are not drawn in the schematic and some are wrong.

i was working on the desk to recapp it and modify it so i have lesser noise and if possible a better sound.

i recapped the channels 10 of the 20 channels, i also changed the opamp in the mic pre in the channels to an ne 5534, the goal was to minimize the noise and sonically a better performance. the original opamp was an tl 081. the opamps in the eq are the originals.

in the master section i changed the 741 opamps to opa 134, first i got some oscillation but i found an thread, a guy posted that it could be stabilized with and cap from inverted input to the output. i did this with 33pf ceramic caps and the oscillation is gone. i also bypassed the elkos in the channels and the master section but i did a mistake and soldered 1 bypass cap on the wrong point. i soldered it to an resistor. after this i powered the desk on and i got a bad smell. i checked everything and sound my mistake corrected it. then i recognized that 2 of the resistors are gone... looks like they were grilled. then i changed them. now the summing channel works but when i rise the fader i get a rising hf noise.

then i changed all the opamps and elkos again. but the noise is still there. the transistors are the 2n4401 i ordered the bc 337-40 and all the other resistors that were not changed yet, hope that i could fix it this way. if everything is changed it must be fixed, right?

is it right that i have to desolder the resistors to check them with the multimeter?

should i compensate the ne 5534 or not? is this compensating a regulation of the volume or what is this doing acutally?
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Old 11th July 2012, 07:36 PM   #8
DF96 is offline DF96  England
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As you have found, simply swapping opamps for new ones with different performance and different technology (FET vs. BJT) does not always work. You need to think about stability. Slapping on a cap afterwards might stop oscillation, but could also damage frequency response.

Noise could be a sign of oscillation, or a fried transistor. Difficult to diagnose from a distance with no diagram and no measurements.
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Old 11th July 2012, 07:45 PM   #9
M Gregg is offline M Gregg  United Kingdom
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This is not the normal way forward...

Let’s make a few assumptions,
Some of the channels you have modified are “working “in a fashion.
So how many channels are not working?
What were the connections on each side of the cap when it was connected incorrectly?
Board layout may be different on some of the channels and cause instability (in a different way to the other channels)
Often the way forward is the way back..ie put back the original components in the bad channel and see if it works..
Once you have the channel working you have a starting point..remember that it may be a fault some where else and by trying to "mod" an unknown system you have to many variables.
Get back to known ground and then move forward again..post what you find. (without a schematic its the best way forward)

Where in the circuit are the burnt out resistors?

Regards
M. Gregg
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Last edited by M Gregg; 11th July 2012 at 08:13 PM.
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Old 11th July 2012, 08:19 PM   #10
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there are 20 line in channels and 4 buss channels. the line in channels work. i was modding the buss channels 1+2. i did a mistake on buss channel 1, buss channel 2 works like a charm. so its not the components.... cause they are the same as in the buss channel 2 which is also modded.

on buss channel 1 i did a mistake i soldered an 0.1uf film bypasscap wrong. i soldered 1 pin from the bypass cap to an pin at the elko and one pin to an resistor. after that the 2 resistors fried. they was visibly damaged so i changed them the channel works but on buss channel 1 the one with the damaged resistors i have an hf noise when i rise the volume fader up if i put it to zero then no noise. on channel buss 2 which has the same mods like the other channel everything is fine. when i rise the volume fader there is like no noise.
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