Soundcraft 600 fixer upper

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FarisElek

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Good to know about ESR! I'm kind of under the assumption that any electrolytic over 20 years old should just be changed just in case, but knowing ESR doesn't drastically affect audio signal is noted.

Anyway, anyone have any insights into these CON3 connectors???

As for of the 12 channels I've cleaned, only 3 of them have it. They all connect together.

At the end of this cable that connects these three channels has been CUT!!
 

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Nbtone

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Good to know about ESR! I'm kind of under the assumption that any electrolytic over 20 years old should just be changed just in case, but knowing ESR doesn't drastically affect audio signal is noted.

Anyway, anyone have any insights into these CON3 connectors???

As for of the 12 channels I've cleaned, only 3 of them have it. They all connect together.

At the end of this cable that connects these three channels has been CUT!!
Maybe it went to a connector for an external power supply?
 

FarisElek

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Looks like also the CON4 connectors up by the CON1 connector is also gone and shows signs of being removed with a solder sucker.

So far nothing seems to be working incorrectly besides the missing 48v Phantom. I imagine those come through the big CON2 connector in the middle of the channel strip, but I'll double-check.
 

Ike Zimbel

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WRT ESR testers/testing:
I routinely use ESR testing, in circuit, as a way of determining overall capacitor health and condition. This is partly because it is a convenient first step (as ESR testing in circuit is more likely to work than testing for capacitance value in circuit), and partly because that it's been my experience that high ESR readings are a very good indication of capacitors that are in need of replacement. I am not talking about 1 or 2 ohms here, but readings that are considerably higher, say 5 to >40 ohms. Regardless of whether or not you feel that ESR is an important factor in sound quality, if you have a given make / model of capacitor and you know that they typically read, say, >2 ohms ESR when new, then if you encounter that same cap in a device and it's reading higher ESR, I think you can safely infer that that cap is, at the very least, off spec. Typically, on removal, these caps tend to read low in uf value too.
As a real world example, I recently had a client bring me a Focusrite ISA-220 which he had purchased used, "in mint condition". Externally, the unit was spotless, but it was exhibiting a hum issue when 48v was engaged, and amplitude sweeps looked like the HPF and LPF were both engaged when they weren't (and, the signal just plain dropped 20 dB when the Limiter circuit was engaged...and not from GR). A quick poke around with an ESR meter showed many caps reading high, and, when those caps were removed and tested for value, many of them (like around 30%), were pretty much empty cans reading 0-1uf for 10, 47 and 100uf caps. That was the case with the nine caps in the limiter circuit, and the filter cap in the 48v circuit and a good many others. All of these failures were heat driven. I suspect this unit came from a professional studio where it had been left on 24/7-365 since new, thus allowing the umpteen 5532's in there to cook the life out of the caps. I have certainly seen devices that would simply not pass audio because the capacitors had completely dried out (like the GB8 mentioned earlier in this thread, and sections of a Neve V3 I have worked on), and in most, if not all cases, those caps read high ESR in circuit. After replacing 100+ capacitors, the ISA 220 was as good (or perhaps better, since I used longer lasting caps) as new.
And, to bring this back on to the OP's topic: At one point Soundcraft had what they simply called the "Small", "Medium" and "Large" console power supplies. Your's looks like the "Medium".
 

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Nbtone

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Looks like also the CON4 connectors up by the CON1 connector is also gone and shows signs of being removed with a solder sucker.

So far nothing seems to be working incorrectly besides the missing 48v Phantom. I imagine those come through the big CON2 connector in the middle of the channel strip, but I'll double-check.
You should find your 48v right off one of the connectors on the power supply board. I had no power at all on mine, until I replaced the bad caps. Then everything shows the correct voltage coming off one of the connectors on the power supply board. Be careful working around the power supply. Lots of folks have been found dead after working on switching power supplies without proper knowledge or guidance.
 

FarisElek

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I've built tens of power supplies including those that use 500vdc. I'm quite capable, I just have no time and I'm phishing for advice from people that have already worked on these to save time 😅 .

Guess I'll have to do some more schematic reading on work breaks 🥺

I'll try to save sometime by testing continuity between 48v and the big main idc connector. That will give me some peace of mind about those weird mini connectors that are going nowhere.
 
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Nbtone

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As I recall there is one connector with I think 8 or ten pins that is the output on my power supply. There is 48 v , and + and minus 17 volts. I’d unplug all the connectors except your mains and ground connectors and look for output voltages at the pins for the output connectors. You might have a short somewhere else on the board that would bring down the 48 volt supply. Otherwise it’s in the power supply.
 

FarisElek

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Well, no, like I said the 48v rectifier has been completely pulled out as well as one of the transistors in the 48v circuit on the psu.

The only thing I'm worried about as I clean the channels is that there is also connectors pulled out that look desoldered, so I'm worried some hairbrained tech also pulled out the connectors pins instead of just removing the cable(???? So weird).

It's probably fine though, I just don't have time to look over this schematic and follow signal path right now. But I'll probably just have to at some point
 

FarisElek

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I appreciate the advice. I don't really think anything is wrong with my circuit boards though. They look great.

Really just wanted to know if anyone knew off the top of their heads what's good with that weird connector being attached only to 3 of my channels and led to nowhere, and what TR2 and RECT4 is because it doesn't specifically say in the schematic, but honestly a small amount of research when I have more than 5 minutes of freetime sometime soon should yield the results I'm looking for.
 

FarisElek

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Just an update:

I recapped my first channel strip today. Took about 45 minutes to clean the faceplate, recap the electrolytics, inject deoxit with a syringe into all the switches and pots and work them, and finally, take apart the fader and clean it with isopropyl and then use fader deoxit on it.

It wasn't very fun.

Also, realized there is an after-market transformer in PSU. I'm fairly positive. It's way bigger than the ones I've seen in pictures. So I'm going to have to test all the secondaries to see if one of them a 50v so I can eventually re-hook up the phantom.


BIG QUESTIONS:
1. Do y'all think this would be a suitable replacement for the missing rectifier (REC4)?: https://www.mouser.com/ProductDetail/625-KBU4A-E4

2. What do you think TR2 is supposed to be if TR3 and TR4 are both 2SC2240, and TR1 is 2SA970GR?


Thank you!
 

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abbey road d enfer

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FarisElek

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DEFINITELY figured out why 48v was removed.

Previous owner tried to replace all the rectifiers and lifted a bunch of trace. When they turned it on the 48v rectifier probably caught on fire and they just clipped it out and called it a day. I think once I wire these rectifiers and replace the caps it's going to be good as new.
 

FarisElek

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Too late! But it's working great now. I have a perfect 48v on the dot coming through, and it's going through the board with no problems. I know how a bridge rectifier works so I don't know why I didn't think to just max it out for hell of it. I've never worked on a mixer before and all my previous knowledge has gone out the window because of my nerves with taking on this project. But it's been pretty straight forward so far.
 

FarisElek

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Okay, so the I recapped the master and channel one and am working my way up. Noise is very low, I'm quite pleased.

One thing that isn't horrible but a little not to my liking is how warm these preamps are. Does anyone know anything I can do to bring out the high end in this circuit. I've read some stuff about increasing cap size in some places.

Any thoughts?

Another thing:
Meter lights up when powered on and off but doesn't respond to input on each channel. Is there typically something in particular wrong in that case? I'm going yo begin with the caps, but I'd be surprised if every single caps was bad.
 

abbey road d enfer

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Okay, so the I recapped the master and channel one and am working my way up. Noise is very low, I'm quite pleased.

One thing that isn't horrible but a little not to my liking is how warm these preamps are. Does anyone know anything I can do to bring out the high end in this circuit. I've read some stuff about increasing cap size in some places.

Any thoughts?
In order to provide advice, it's necessary to analyze the situation. Set up a preamp that you like, connect it to the insert return. Does it sound better than the Soundcraft preamp?
Meter lights up when powered on and off but doesn't respond to input on each channel. Is there typically something in particular wrong in that case? I'm going yo begin with the caps, but I'd be surprised if every single caps was bad.
You know that the meters follow the "Tape return" switch on the group module, do you?
 

FarisElek

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Things are progressing well thank to everyone's comments. Especially Abbey. So thanks a lot y'all.

A problem i'm having on a recently recapped channel is that the peak light next to the fader is stuck on. It's volume also sounds a little lower than it should. Could this be a problem in the TR3/TR4 area?

UPDATE: It was one of those transistors. I changed both and the peak light turned off and is behaving normally.
 
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