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Post by unit7 on Mar 13, 2015 11:52:16 GMT -6
Hi! I'm having a problem with my Shep made PSU for my Neve 10 series modules. I had this problem a few years ago and a tech helped me back then. The problem is that the modules has this 'frying' sound now and then. It comes from all modules at the same time so I believe it has to be a problem with the PSU and not the modules. When this happened a while ago I contaced one of the ace techs here and he made me replace the voltage regulator, but it didn't fix the problem. Then we noticed that the 'frying' came more and more after a while so the tech suggested it was heat related. He used cooling spray and used a small pipe to be able to spray on one component at a time and to our surprise the failing component was a resistor (the modules gave a rattling noise when he sprayed on it) which was replaced and the modules have been silent until now. After seeing how he troubleshooted I could easily do the same, but I've been thinking because this thing is probably 20-30 yrs old, and with my new DIY experience, I might as well replace all components this time. It's like 15-20 components and I guess I'd be looking at like $30-40 for everything. What would you do? And how about the xformer, would you replace that too? Attachments:
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Post by svart on Mar 13, 2015 12:46:08 GMT -6
Really you should only have to replace the electrolytic caps.
Resistors, transistors, film caps, diodes and transformers should not wear out. If they do then you have an issue with part quality/selection or the design is bad somehow.
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Post by unit7 on Mar 13, 2015 13:08:06 GMT -6
Really you should only have to replace the electrolytic caps. Resistors, transistors, film caps, diodes and transformers should not wear out. If they do then you have an issue with part quality/selection or the design is bad somehow. Thanks svart, that's some good insight. Before starting to consider replacing the whole PSU/rack, do you you know if a bad electrolytic cap could break other components in the circuit? |
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Post by svart on Mar 13, 2015 14:25:14 GMT -6
Being a passive component, it cannot necessarily break other parts. However, there could be situations where a failed cap can allow other parts to be broken.
In some situations it's possible that a failed cap allows the raw rectified DC to exist on the regulation circuit. Rectifiers don't turn AC directly into DC, but they turn AC into "pulsating DC" which varies between 0VDC and the peak voltage of the circuit.
If the regulator's maximum input voltage is too close to the peak pulsating DC then it can be damaged by repetitive transients of the un-smoothed pulsating DC.
Also of concern is that capacitors have ripple current maximums, and if your primary bulk smoothing cap goes bad, then the other caps might not handle the increased ripple without heating up and drying out faster.
It's impossible to tell these things without doing a bunch of measurements and some datasheet reading, but in general if you aren't having constant failures, then it's likely you just had a bad resistor that one time.
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Post by unit7 on Mar 13, 2015 14:48:37 GMT -6
Being a passive component, it cannot necessarily break other parts. However, there could be situations where a failed cap can allow other parts to be broken. In some situations it's possible that a failed cap allows the raw rectified DC to exist on the regulation circuit. Rectifiers don't turn AC directly into DC, but they turn AC into "pulsating DC" which varies between 0VDC and the peak voltage of the circuit. If the regulator's maximum input voltage is too close to the peak pulsating DC then it can be damaged by repetitive transients of the un-smoothed pulsating DC. Also of concern is that capacitors have ripple current maximums, and if your primary bulk smoothing cap goes bad, then the other caps might not handle the increased ripple without heating up and drying out faster. It's impossible to tell these things without doing a bunch of measurements and some datasheet reading, but in general if you aren't having constant failures, then it's likely you just had a bad resistor that one time. Thanks again! Now that you mention it, I don't really know what ripple is but I remember that the tech measured and checked for a lot of stuff including ripple and didn't see anything bad, so I don't know.. I'll have to figure something out. |
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Post by svart on Mar 13, 2015 14:54:15 GMT -6
Is there something wrong with it now?
Can you take a high-resolution picture of the board and components and put it up so I can see it?
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Post by unit7 on Mar 13, 2015 15:21:18 GMT -6
Is there something wrong with it now? Can you take a high-resolution picture of the board and components and put it up so I can see it? Hey thanks svart! I'm not in the studio right now, but I took this pic too. Believe that's the best I can do for now. Attachments:
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Post by unit7 on Mar 14, 2015 7:29:50 GMT -6
Some progress here! I bought cooling spray and sprayed on the components. I got some reaction on the trim resistor for 24V. Then when I moved the trim screw I got the same type of noise but not so random but a steady noise. Sounded just like oxide so I fiddled with it back and forth and then trimmed it to 24V and it's been quiet after that. So svart, I guess I'm having oxide in the trimming resistor, or what do you think? |
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Post by unit7 on Mar 15, 2015 11:50:13 GMT -6
Is there something wrong with it now? Can you take a high-resolution picture of the board and components and put it up so I can see it? I forgot to reply to your question, yes there is/was something wrong with it. I had frying noise from all modules simultaneously, and also strange noises when engaging phantom power. Now I have twisted/reconditioned both trim resistors (for 24V and 48V) and the problems seem to be gone. I totally understand if you don't have the time to look at this, but I'd like to update anyway. And of course anyone is welcome! Today while working those trim resistors I noticed that one of them (48V trim) is deformed. To me this looks like a case of overheating..  When I started to look for replacements (10KΩ Bourns 3296) I noticed that they seem to be 0.5W components. I think that is also the case with the two small resistors in the pic, as mentioned a tech replaced one of them a few years back. I know it's probably hard to answer, but could this be the problem? I never had these problems until I got 6+ modules in my rack, so I was thinking the resistors are affected by the load. Either way, how about just replacing all four with 1W or 2W? |
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Post by svart on Mar 15, 2015 12:04:55 GMT -6
I was going to mention that if a pot starts getting noisy, it's likely that there is too much current going through it for it's rating.
I would suggest replacing with higher value pots if you can fit them the spots.
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Post by unit7 on Mar 21, 2015 9:34:14 GMT -6
I've installed two new resistors and two new trim resistors, all 1W. When engaging 48V on all modules there's not nearly as much noise as before. And also, when trimming the trim resistors there's much less noise that previously. So clearly some progress. The PSU has been powered on for a while now. I'm listening in my cans occasionally but, knock on wood, nothing unusual yet. Initially I believe I smelled something burnt, but when sticking my nose close to the new components there's only a smell of warm components. The new trimmer for the 48V is almost burning hot, but specs say operating temp is up to 150C so that's not to worry about, or? BTW, the old 0.5W 48V trimmer was the one that had a melted bubble on the side. The 24V trimmer is only luke warm. svart or someone else - Any comments? |
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Post by tonycamphd on Mar 21, 2015 9:54:13 GMT -6
I've installed two new resistors and two new trim resistors, all 1W. When engaging 48V on all modules there's not nearly as much noise as before. And also, when trimming the trim resistors there's much less noise that previously. So clearly some progress. The PSU has been powered on for a while now. I'm listening in my cans occasionally but, knock on wood, nothing unusual yet. Initially I believe I smelled something burnt, but when sticking my nose close to the new components there's only a smell of warm components. The new trimmer for the 48V is almost burning hot, but specs say operating temp is up to 150C so that's not to worry about, or? BTW, the old 0.5W 48V trimmer was the one that had a melted bubble on the side. The 24V trimmer is only luke warm. svart or someone else - Any comments?Yes,.... there is nothing like the smell of warm components in the morning 8) |
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Post by jimwilliams on Mar 31, 2015 12:48:22 GMT -6
Besides the global psu, any local decoupling caps on any module can leak and cause noise to be shared across all the modules as they are not decoupled from each other in those designs.
As for that PSU, I would ditch it for a Power One 24 volt 3 amp and a 1/2 amp 48 for phantom. Then those psu problems go away.
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