Some basic questions about refurbing an old mixer.

[Sporticus]

Hi guys, I recently picked up a cheap Sony MX-650 mixer and thought I'd try my hand at recapping the thing.  Previously I've messed around with the innards of video game consoles, so I have a basic understanding of what is what and have done a fair bit of fiddly soldering.

My plan is to replace all the electrolytic caps, and this is where I have a couple of questions.  I've gone through and listed all the existing cap values:

10uf/16v X4
3.3uf/25v X6
4.7uf/25v X6
3.3uf/25v
33uf/25v X6
470uf/16
0.47uf/50v X4
10uf/16v X18
1uf/50v X8
47uf/16v X6
100uf/10v
110uf/16v
47uf/10v X4

I understand I can use caps that are of higher value than the existing ones, so does this mean for the 3.3 & 4.7uf 25v caps I would be fine to replace all of those with 5uf/50v caps for example?

What sort of caps should I use? type/brand?

Is there anything else I should be taking into account before I dive in?

I have a couple of other specific questions, but will wait until i have access to a better camera.

thanks guys, I know this very basic stuff and I appreciate any help.

 

[gemini86]

Higher voltage and uf value = physically bigger. So make sure the space is there before you buy a bunch of caps you can't use.

 

[pucho812]

Sometimes the same value can have a different physical size. I had that happen with the Neve vr eq's I recapped. Caused it to oscillate in the HF band because it was making contact with another part.  why are you planning to recap, does the unit need it? A cap job like thaty is best done with an electric desolder vacuum. It will give you the best results and makes it easy and faster. I have done solder wick and the hand held pump and they for me have caused more harm then good. Also I move mroe slowly with those.

Here is an well priced, hard working desolder system.

http://www.howardelectronics.com/xytronic/lf8800.html

It's cheaper then the 1500.00 dollar weller ones we use at work and I think works way better too. I have it in my personal aresenal...

 

[JohnRoberts]

Rather than blindly replacing parts, why don't you consider making some basic audio measurement and look for problems?

Frequency response will tell you a bunch... measure and compare the channels to each other. If you identify some bad parts that way, replace all of the same value as the bad ones.

JR

 

[amorris]

+1 on the test first advice. You should do a benchmark test series. Identify differences and replace suspect caps first. Then see if your changes made a difference. Then try caps of higher value and then barely lower value. Remeber in some places a lower uf value will give better low end response.  Do one trhing at a time and keep old caps in case it gets worse. Also remember that brand new caps could be bad, wildly different value, leaky, open, a myriad of problems from the same batch of caps. Blindly recapping WILL give you problems. A measured approach will not surprise you. Or at least you can go back and see what happened.

 

[Sporticus]

Thanks for the info guys. 

It was manufactured in '73 and i'm assuming these are the original caps, they are all ELNA, though none of them seem to be leaking or swollen so perhaps it has been maintained in the past.  I bought it for $30 off a guy who has been storing it for 5-10 years.

I was actually quite surprised at how good it sounded when it arrived, A/B'd with my A&H the two were almost indistinguishable.  The only problems I've noticed are a low frequency hum on the master fader, and a tendency for freqs under about 60Hz to start breaking up a bit earlier than I'd expect them to.

John - That's not something I had considered but am going to look into that now.  I would like to actually use the thing, so the less work done, the less chance of making a mistake. 

Another question I had was about these two transformers and whether switching them out for something better would drastically improve the sound.  I'm assuming I'm going to have to take measurements to determine what to replace them with...

amorris - I just caught your post, thanks makes perfect sense.

 

[Gareth Connor]

Then try caps of higher value and then barely lower value.

Remeber in some places a lower uf value will give better low end response. 

Also remember that brand new caps could be bad, wildly different value, leaky, open, a myriad of problems from the same batch of caps.

Amorris, I am somewhat confused by the above 3 points from your post.

I don't see how reducing the cap value will be of benefit. Product designers will often specify (by calculation) the lowest value that will be reliable for a particular product lifetime, coupled with the manufacturing plant's stock-standard range of values. Why reduce the designed value further?

How and where specifically can a lower capacitance value improve low-end response?

Which brands of brand new caps have been as abysmally poor with the problems you have noted? "Wildly different values, leaky, open........ from the same batch" sound like fundamental reasons to avoid those brands. What is an "open cap"?

Thanks,

Gareth.

 

[Sporticus]

Ok, I tried running this thing off 8 C batteries and both the hum and low freq distortion have gone.  So I'm not sure if it was the wallwart or the circuit in the actual unit, but both issues I had with this thing have disappeared.  I guess the next move is to try another plug.

I'm still interested in finding out whether it would be worth replacing those transformers. 

 

[AP carrier]

Hhhmm Sony 427-359 tx. Couldn't find much on those transformers. Seems to be out of stock everywhere.

Is that the input channels in the pic for balancing the inputs or the output channel for balancing the outputs? It looks like the output but if it sounds good already then it probably won't make a lot of difference changing them...unless...they distort at much lower levels than a higher quality transformer in which case you'll loose the mojo sound but get a cleaner/more extended sound depending on the transformer.

I would fix it up first then maybe change one the outputs (or inputs) with another tx and compare. Do you have much spare space inside for bigger tx's? It may even sound best with no tx but it's a matter of taste.

cheers,
Andrew

 

[Rob Flinn]

You can't just up the value of caps, without understanding what they are doing in the circuit.    For example I recapped my 1978 console at the beginning of the year. 

In doing so I kept the local PSU decoupling caps the same uF value.  In my experiences testing caps pulled from consoles with an ESR meter the local PSU decoupling caps are seldom out of spec.  It tends to be more the smaller value signal decoupling caps that have high ESR with increased age.

With the signal decoupling caps I doubled the uF (roughly).  This extends the low end response, but not necessarily affecting anything you can hear, because if the roll off is at 20Hz (-3dB point) to begin with you are pretty subsonic when you "potentially" extend it to 10Hz by doubling the cap.

On my console there is 1 electrolytic that determines the low eq frequencies, which obviously I didn't change.

So my advice to you is to look at what people say above about testing the caps. Then get a copy of the circuit diagram so you know which caps are doing what job. This will enable you to make educated decisions about any value changes you decide to make.

 

[thermionic]

It doesn't appear, from what I'm making out, to be a large mixer, so re-capping as a preventative measure is probably a good idea.

If you want to work out where the hum's coming from (it might well be the PSU reservoir caps, in which case a re-cap will cure it) you need to get probing around with a scope. A good place to start is the PSU +ve / -ve rails.

To reiterate Rob's point - although your console isn't too big, I'd advise caution in replacing bypass caps with larger values. The inductance on switch-on can be such that you might just see your rectifier go flying across the room... Once those babies ignite they can be difficult to extinguish, and will make a nasty mark on the floor / carpet - not to mention the fire risk and general fright value. On an 8-ch mixer I doubt you'll be able to cause this to happen by upping caps by a few uF, but on a larger console it's a real danger. Been there, bought the t-shirt. Not a recommended experience.

Justin

 

[Sporticus]

Thanks for the reply Rob, I am going to sit on this one for a while and keep reading I think.  As I said I'm a bit of a noob when it comes to this stuff, but genuinely interested in learning.

AP Carrier - Yes, they are on the outputs.  They sound good to me,  like I said, it was very hard to tell the difference between this and my A&H board until you start pushing them.  I guess I'd describe the A&H as sounding meaty at that point whereas the Sony sounds fizzy.

Thermionic - Yeah it's small, 6 channels.  I think I'll keep trying to track down a schematic and hold off until I have a better understanding of what I'm dealing with and electronics in general.  Thanks.

Anyway, thanks for all the replies, I appreciate it.  Having a good time browsing the forum, some of the stuff you guys are building is absolutely amazing.

 

[ruffrecords]

You asked for recommended makes. For electrolytics I would recommend Rubycon - more expensive than others but much longer life.

Cheers

Ian

 

[Rob Flinn]

It was manufactured in '73 and i'm assuming these are the original caps, they are all ELNA, though none of them seem to be leaking or swollen so perhaps it has been maintained in the past.  I bought it for $30 off a guy who has been storing it for 5-10 years.

In my experience visual condition of caps doesn't mean a great deal.  As I said before I tested a lot of caps pulled from a large MCI console.  Some that looked great were shot to pieces, & others that looked shot & were swollen & leaking tested fine.    Although if I see one that is swollen & leaking I do generally replace them.

Especially if they are accompanied by a fire underneath them ... see attached pic